Light steel frame (LSF) construction is increasingly common in new build homes across the UK, offering benefits such as speed of construction, design flexibility and excellent thermal performance. However, this modern construction method comes with specific technical requirements that builders must follow to ensure your home is built to the correct standards.

The NHBC (National House Building Council) publishes detailed technical standards in Chapter 6.10 that govern how light steel framed walls and floors should be designed and constructed. These standards apply specifically to ‘warm frame’ and ‘hybrid construction’ using steel framing between 0.45mm and 4.0mm thick. Understanding these requirements can help you appreciate what should be in place when you purchase your new home, and what our snagging inspections look for when examining the visible aspects of your property.

6.10.1 – Compliance

The fundamental requirement is straightforward: all light steel frame structures must comply with NHBC’s Technical Requirements. This means that your new home’s LSF elements, whether they form entire walls, roofs, floors, or extensions, must be built to meet these established standards.

LSF structures can serve different purposes in your home. They may be structurally independent, forming whole buildings or additional storeys, or they may be infill walls that don’t carry the main structural loads but resist wind and support cladding. The construction should follow either ‘warm frame’ or ‘hybrid’ principles, where sufficient insulation is placed outside the steel envelope to prevent condensation forming within the steel members themselves.

It’s worth noting that where LSF components cannot be inspected on site, such as closed panels or modular units that arrive fully fitted out, these systems must be treated as proprietary building systems and assessed under NHBC’s System Review service. This additional scrutiny ensures that even factory-built components meet the required standards before they reach your home.

6.10.2 – Provision of Information

Proper documentation is essential for quality construction. The NHBC standards require that designs and specifications must be produced in a clearly understandable format, containing all relevant information, and distributed to all appropriate personnel on site.

This means that site supervisors, specialist subcontractors and suppliers should all have access to detailed drawings showing exactly how the steel frame should be assembled. These documents should include typical wall build-ups showing components like wall ties, breather membranes, sheathing and vapour control layers. They should also specify fixing schedules detailing each connection to be made on site, how the frame connects to other building elements such as roofs and floors, and the specifications for fire stops and cavity barriers.

For you as a homebuyer, this requirement provides assurance that the construction of your home follows a documented plan rather than being left to on-site interpretation. During a snagging inspection, whilst we cannot review the original construction drawings, we can identify visible defects that suggest the work may not have followed proper specifications, such as missing or inadequate fixings, gaps in insulation, or poorly executed connections.

6.10.3 – Structural Certification

One of the most important requirements for light steel frame construction is the two-stage certification process. This ensures both the LSF system itself and its specific application in your home have been properly checked by qualified professionals.

Stage 1 – System Certification involves the manufacturer of the LSF system submitting a system manual to the Steel Construction Institute (SCI) for assessment. This manual must demonstrate that the system has adequate durability (at least 60 years design life), appropriate structural design, suitable steel grades and corrosion protection, and satisfactory performance in fire and acoustic tests. Upon successful assessment, SCI issues a numbered system certificate that validates the manufacturer’s approach.

Stage 2 – Project Certification requires that the specific design of your home be checked by an NHBC registered LSF certifier. This certifier must be a suitably qualified civil or structural engineer who is independent of the original designer. They verify that the proposals comply with the manufacturer’s Stage 1 certificate and NHBC standards, and issue a project certificate confirming the structural adequacy of your specific home.

Your builder should ensure this completed Stage 2 certificate is available on site for inspection by NHBC. Whilst you won’t typically see these certificates during a snagging inspection, their existence is a crucial safeguard ensuring that qualified independent engineers have reviewed the structural design of your home. External infill walls, which don’t form part of the main structure, don’t require this two-stage certification, though they must still meet design requirements.

6.10.4 – Structural Design of Load-Bearing Floors and Walls

The structural floors and walls in your LSF home must be designed to support and transfer loads safely without excessive movement. This section covers three key aspects: structural floors, structural walls, and overall stability.

Structural Floors must be of the correct type and fitted in the specified locations, with appropriately sized trimmers around floor openings. Light steel joists are typically spaced at maximum 600mm centres, though greater spacings may be permitted when specifically designed by an engineer. The joists should be fixed to supporting walls using approved methods such as web cleats, hangers, track connections, or by bearing directly onto the supporting structure with stiffeners where required.

The standards specify strict limits on floor deflection to prevent issues like bouncy floors or cracked ceilings. For a single joist under imposed load, deflection is limited to span divided by 450, whilst under combined dead and imposed loads it’s limited to the lesser of span divided by 350 or 15mm. There are also dynamic performance criteria requiring floors to have a natural frequency of at least 8Hz and limiting deflection under a 1kN point load to ensure the floor doesn’t feel springy underfoot.

Structural Walls must be designed to resist all expected loadings including dead loads, imposed loads, wind loads and snow loads. Individual studs are generally sized to meet structural requirements with maximum spacing of 600mm. Lintels must be provided over any opening where studs are cut or displaced, and these must be securely fixed to supporting studs to ensure loads transfer properly. The design should account for movement in the structure, including elastic shortening of the steel frame and movement of any panels or cladding.

For ground floor LSF construction, specific provisions are needed to prevent ground moisture affecting the floors. This typically involves covering the ground with either concrete or fine aggregate on a polyethylene membrane, and ensuring a minimum 150mm void below the floor with adequate ventilation. During a snagging inspection, we can identify visible signs that these structural requirements may not have been met, such as doors that don’t close properly (suggesting excessive deflection), visible floor bounce, cracks around openings (suggesting inadequate lintels), or signs of inadequate support or moisture protection at ground level.

6.10.5 – Structural Design of Infill Walls

Infill walls are secondary structural components built between the floors of steel or concrete frames. Unlike load-bearing walls, they don’t provide stability to the building or resist floor loading, but they must still be properly designed to resist wind loads, support the weight of cladding and wall components, and handle loads from windows and doors within the panels.

These walls are particularly important in buildings with a primary frame of a different material, where the LSF panels fill in between the structural elements. The design must be in accordance with British and European standards (specifically BS EN 1993-1-3) and should account for load concentrations that result from the presence of openings.

Although infill walls aren’t carrying the main loads of the building, they still play an important role in protecting your home from the elements and maintaining its weathertightness. During a snagging inspection, we examine how these walls interface with windows and doors, checking for gaps, poor sealing, or signs of movement that could lead to draughts or water ingress.

6.10.6 – Roofs

When roofs are supported by light steel frame construction, they must be designed to support the roof coverings and transfer loads safely without undue movement. The connections between LSF walls and timber or LSF pitched roofs require particularly careful consideration in the design.

The NHBC standards specify that LSF pitched or flat roofs should only be used in ‘warm roof’ or ‘hybrid construction’, meaning the insulation must be placed over the rafters (or joists in flat roofs) rather than between them. This approach helps prevent condensation problems that could damage the steel structure. Condensation risk must be carefully analysed in accordance with British Standard BS 5250.

The interface between the LSF walls and roof requires proper detailing to ensure loads transfer correctly. Where roof trusses sit directly on the top track of LSF walls, the design must consider all loads including wind uplift, lateral support requirements and vertical loading. Timber wall plates, where used, should be fixed to the head rail of wall panels and sized to permit trusses to be positioned at any point between studs.

During a snagging inspection, visible signs of roof problems might include sagging roof lines, cracks around the wall-to-roof junction, or evidence of water ingress that could suggest inadequate weather protection or structural movement. However, it’s important to note that snagging inspections do not include detailed structural surveys of roofs, and cannot verify that hidden elements like connections and fixings have been properly installed.

6.10.7 – Steel and Fixings

The quality and protection of the steel used in your home’s frame, along with the fixings that hold it together, are crucial for long-term durability. This section covers four key aspects: steel grade, corrosion protection, connections and fixings, and rules about holes and notches.

Steel Grade: All steel used must conform to European standards (BS EN 10346) and be of specified grades ranging from S280 to S450. These designations relate to the yield strength of the steel, ensuring it has adequate strength for its intended purpose.

Corrosion Protection: All structural steel must be pre-galvanised with a minimum coating of either 275g/m² zinc coating or 150g/m² aluminium-zinc alloy coating. This galvanised layer protects the steel from rust and corrosion over the design life of the building (60 years). Where the LSF floor is located near ground level (150mm or more above external ground level), specific enhanced protection is required for floor joists and ring beams, either through heavier galvanising (450g/m²) or additional protective coatings. Where steel is used less than 150mm above ground level, even more stringent protection measures apply due to the increased moisture risk.

Connections and Fixings: Where two metals are joined, they must either be compatible (to prevent bimetallic corrosion) or isolated from each other. Connections can be made through various approved methods including cleats, bolts, hot-dip galvanised fasteners, rivets, screws or welding, with each type having relevant British or European standards. Holding-down devices, which secure the frame to the foundation, should be manufactured from either zinc-coated mild steel or stainless steel to ensure they remain durable in their exposed environment.

Holes and Notches: This is a particularly important requirement. Joists and studs should not be altered without approval from the steel frame designer. The drilling, cutting or punching through of members must only be undertaken to an engineer’s design. To prevent damage to services, holes and penetrations should be fitted with grommets or swaged under factory conditions. End notching of joists may be required for connections but notches elsewhere in the span are not acceptable.

During a snagging inspection, we can identify visible signs of corrosion, poor fixings, or unauthorised modifications to steel members. However, we cannot verify the grade of steel used, inspect galvanising that’s hidden behind cladding, or examine connections concealed within the structure. These aspects should have been checked during construction by building control and NHBC inspectors.

6.10.8 – Detailing of Steel Joists

Proper detailing of steel joists is essential for ensuring floors perform as intended without excessive movement or noise. The standards address two main concerns: installation details and prevention of roll.

Installation Details: Joists or floor beams must be spaced exactly as shown in the design and accurately cut to length in accordance with the manufacturer’s recommendations to ensure a tight fit. They should be joined with the correct type, size and number of fixings. Where light steel joists are supported by other steel joists, cleats or web stiffeners must be used in accordance with the design. Joists may be doubled up to support partitions or to form trimmers around openings. Where joists run continuously over load-bearing intermediate walls, they should be reinforced as required by the design, and where they overlap on these walls, they must be fixed together with bolts or screws to prevent the cantilevered part of the joist moving upwards and pushing up floor decking or cracking ceilings.

Prevention of Roll: Floors constructed using joists with an asymmetric web profile (such as C-section or Sigma-section joists) can be prone to rolling sideways, which would make the floor unstable. To prevent this, bridging and blocking must be provided in accordance with the design. Unless otherwise specified, where the span exceeds 3.5m for C joists or 4.2m for Sigma joists, one of several bracing methods must be used. These include continuous lines of proprietary steel herringbone struts between joists, solid blocking provided to alternate pairs of joists with ties between them, or joists alternately reversed and tied together in pairs.

During a snagging inspection, we can observe the spacing of visible joists (where accessible), check for obvious signs of inadequate support or poor connections, and identify floor performance issues such as excessive bounce or squeaking that might indicate problems with joist detailing. However, once floors are covered with decking and ceilings are in place, we cannot verify that hidden elements like web stiffeners, blocking or bracing have been correctly installed.

6.10.9 – Restraint

Restraint strapping is a critical safety feature that prevents walls from moving or collapsing, particularly in high winds or other extreme loading conditions. The requirement is straightforward: restraint strapping must be provided in accordance with the design.

Where external walls that are not constructed from LSF (such as masonry walls) need to be stabilised by connection to an LSF floor, restraint straps may be required. These straps are generally fixed to the web of the joist in positions that align with the masonry courses. Where joists run parallel to the wall, the straps should be supported on noggings fixed between the joists, with straps placed at a maximum of 2m apart and carried over three joists. Packing should be provided between the wall and the first joist to ensure proper contact and load transfer.

The straps themselves should be fixed with suitable bolts, screws or rivets in accordance with the design, and they must bear on the centre of bricks or blocks rather than across mortar joints to ensure they provide effective restraint. This seemingly small detail is important because a strap bearing on a mortar joint could pull through the joint under load, rendering the restraint ineffective.

During a snagging inspection, we may be able to observe restraint straps where they’re visible, such as in loft spaces or where ceilings haven’t yet been installed. We can check that they appear to be properly installed and fixed. However, once construction is complete and finishes are applied, these straps become hidden and we cannot verify their presence or correct installation. This is another aspect that should be checked during construction by building control inspectors.

6.10.10 – Construction of Load-Bearing Walls and External Infill Walls

The construction of load-bearing LSF walls and external infill walls must ensure adequate stability through proper preparation, anchoring and accommodation of deflection.

Preparation: The setting out of the structure onto which the LSF is erected and the transfer of loads from the LSF must be in accordance with the design. The supporting structure (such as a concrete foundation or floor slab) may have local deviations in level along its length, and packing will be required to achieve the required tolerances and provide for effective load transfer. Where gaps exist under the base rail, specific packing methods apply depending on the size of the gap. For gaps less than 10mm, shims should be provided under each stud position. For gaps of 10-20mm, shims should be provided and the entire length of the base rail grouted with cement-sand mortar. For gaps exceeding 20mm, advice should be sought from the frame designer or manufacturer, as remedial work to the substructure may be required. Shims should be of pre-galvanised steel or other suitable material, not timber or plastic which could compress or degrade over time.

The LSF should be correctly positioned, square and plumb. The vertical position of members should be within plus or minus 5mm per storey relative to the base, and the horizontal position of base rails should not vary in alignment by more than 5mm in 10m. These tight tolerances ensure the frame is stable and that subsequent elements like cladding and internal finishes can be properly installed.

Anchoring: The frame must be anchored to resist both lateral movement and uplift in accordance with the design, including bolt-down brackets where required. The anchoring must ensure that appropriate edge details are provided and minimum edge distances specified by the fixing supplier are maintained to avoid spalling (breaking away) of masonry or concrete. Where fixings go into masonry, they should go into solid concrete blocks with a minimum crushing strength of 7.3N/mm² positioned to receive fixings. Where the design incorporates gas membranes (for methane or radon protection), fixings should not puncture them, but where unavoidable, the penetration should be sealed to maintain the membrane’s integrity.

Accommodation of Deflection: Infill walls (secondary structural components) should accommodate anticipated deflection within the primary frame in accordance with the structural design. All structural frames will deflect to some degree under load, and infill walls must be detailed to allow this movement without being damaged or losing their weathertightness.

During a snagging inspection, we can identify visible signs of poor construction such as walls that are noticeably out of plumb, gaps between the base rail and the supporting structure, signs of inadequate anchoring, or cracking that suggests the structure isn’t accommodating movement properly. However, we cannot verify the presence of hidden fixings, measure precise tolerances, or confirm that gas membranes have been properly protected. These aspects require inspection during the construction phase.

6.10.11 – Interfaces with Staircases

The interface between your LSF floors and walls and any staircases must be properly designed and constructed to avoid compromising the performance of either element. This seemingly straightforward requirement actually involves several important considerations.

Wall linings should be continuous behind the string (the side piece) of staircases rather than being cut to fit around them. This continuity is important for maintaining fire protection and preventing gaps that could allow sound to travel between floors or rooms. The fixing connections between the staircase and the LSF structure must be coordinated to ensure both fire protection continuity and structural adequacy.

Staircases impose concentrated loads on the floor structure where they’re attached, and these loads must be properly transferred to the LSF frame. The connections must be strong enough to support the weight of the staircase and the people using it, whilst also maintaining the fire resistance of the floor and the acoustic separation between storeys.

During a snagging inspection, we can identify visible problems at staircase interfaces such as gaps between the staircase and walls, signs of movement or deflection when the stairs are used, or defects in finishes around the staircase that might indicate poor installation. We can also check for obvious issues with fire protection, such as large gaps that haven’t been properly sealed. However, we cannot verify hidden fixings or assess whether the structural connections meet the design requirements, as these are typically concealed within the construction. As always, proper inspection during construction by building control is essential for these hidden elements.

6.10.12 – Fixing Floor Decking and Ceilings

The materials used for floor decking and ceilings must be adequately fixed using materials of adequate strength and moisture resistance. This section specifies the acceptable materials and fixing methods to ensure your floors and ceilings perform properly.

Floor Decking Materials: The joist spacing and decking thickness must be compatible. For domestic loads (1.5kN/m²), the minimum thicknesses are specified for different materials. For joists at 400mm centres, moisture-resistant chipboard (type P5) should be at least 18mm thick, plywood at least 15mm thick, and oriented strand board (OSB3) at least 15mm thick. Where joists are spaced at 600mm centres, these minimum thicknesses increase to 22mm for chipboard, 18mm for plywood, and 18mm for oriented strand board. It’s worth noting that in England and Wales, these minimum thicknesses may not achieve the 15kg/m² mass required to meet sound insulation requirements under building regulations, so additional measures may be needed.

Flooring should be fixed at maximum 300mm centres using self-tapping screws or fixings recommended by the LSF manufacturer. This frequent fixing pattern ensures the floor decking doesn’t lift, creak or move when walked upon. The screws must be of adequate length to penetrate through the decking and into the steel joist properly, creating a secure connection.

Ceiling Finishes: Plasterboard should be fixed in accordance with NHBC Chapter 9.2 (Wall and Ceiling Finishes), using self-drilling, self-tapping screws. These specialised screws are designed to drill through the steel without the need for pre-drilling holes, and then tap their own threads into the steel to create a strong connection. Standard wood screws would not be appropriate for fixing to steel joists.

During a snagging inspection, we can identify floor decking problems such as squeaking, bouncing, loose boards, or inadequate fixing. We can also spot ceiling defects like sagging, cracks, or signs that the plasterboard hasn’t been properly secured. However, we cannot verify the thickness or type of decking materials once they’re covered with floor finishes, or measure fixing centres once the construction is complete. Issues with floor performance (such as excessive noise transmission between floors) may become apparent during occupation and could indicate that the wrong materials or thicknesses were used.

6.10.13 – Other Design Issues

Beyond the specific requirements covered in individual sections, the home must be designed to adequately address all critical performance issues. The NHBC standards identify several key topics that the designer must consider and address, even though detailed guidance for some of these issues may be covered in other chapters or included in the Stage 1 system certificate.

Behaviour in Relation to Fire: The design must ensure compliance with relevant building regulations regarding internal linings, fire stops and cavity barriers, and penetrations through fire-resisting elements. Fire safety is paramount and the LSF construction must not compromise the required fire resistance.

Acoustic Performance: The design must ensure compliance with relevant building regulations for sound insulation. This includes preventing sound transmission both through walls and floors (airborne sound) and from impacts on floors being transmitted to rooms below (impact sound).

Moisture Control: This encompasses several related issues including thermal performance, condensation risk and water ingress. The design must specify the type, thickness and location of insulation material to achieve the required thermal performance and prevent condensation. It must also include protection from water ingress at low levels where the structure is closer to the ground or to roof waterproofing layers.

Wall Construction: The designer must consider acceptable claddings (cross-referenced to NHBC Chapter 6.9), provision of adequate cavity widths, type of wall ties to be used, sheathing requirements, and provision for attachments such as hung boilers or kitchen units which impose concentrated loads on the walls.

Balconies, Terraces and Parapets: Where these features are incorporated, specific design considerations apply including structural design for the loads they must support, durability in exposed conditions, and weathertightness to prevent water ingress that could damage the structure.

This holistic approach ensures that the designer doesn’t focus on individual elements in isolation but considers how all the components work together to create a home that is structurally sound, thermally efficient, fire safe, acoustically private, and protected from moisture damage.

During a snagging inspection, we cannot verify that all these design issues have been properly addressed, as many involve hidden elements or require specialist testing equipment. However, we can identify visible symptoms that might indicate problems, such as signs of condensation or moisture penetration, cold spots that might indicate thermal bridging, or construction details around balconies and terraces that appear inadequate. Significant issues with acoustic performance or fire safety typically only become apparent after occupation, though obvious defects like gaps in fire-stopping may be visible during inspection.

6.10.14 – Behaviour in Relation to Fire

LSF walls and floors must comply with all relevant building regulations regarding fire safety. Fire performance is a critical aspect of construction that can literally be a matter of life and death, so the requirements are stringent and detailed.

The guidance within supporting documents to the relevant building regulations must be fully considered in the design and construction of LSF walls, floors and roofs. The detailing and specification of components should follow the steel frame manufacturer’s recommendations and guidance from the Steel Construction Institute, supported by representative test evidence to appropriate standards. These standards include BS 476-21 or BS EN 1365-1 for load-bearing walls, BS 476-22 or BS EN 1364-1 for infill walls, and BS EN 1365-2 for floors.

Specific Details Requiring Attention: Several specific details must be properly addressed to maintain fire safety. Fire protection to the structure around openings such as doors and windows must be maintained to prevent fire spreading through these vulnerable points. Cavity barriers must be correctly detailed and installed, including appropriate moisture protection to prevent the barrier material deteriorating over time. Service penetrations (where pipes, cables or ducts pass through fire-resisting walls or floors) must be properly fire-stopped to prevent fire and smoke spreading through these openings. Compartmentation, including interfaces with fire doors, must be maintained to ensure fire is contained within defined areas for the required period.

Fire Resistance of Materials: The materials used in LSF construction, including the plasterboard linings, insulation, and other components, must be selected to provide the required period of fire resistance. In a domestic setting, walls and floors typically need to resist fire for 30 or 60 minutes depending on their location and purpose, giving occupants time to escape and allowing the fire service to tackle the blaze.

During a snagging inspection, we can identify visible fire safety defects such as gaps around fire doors, missing or incomplete fire-stopping around service penetrations, obvious gaps in cavity barriers (where visible), or the use of materials that appear unsuitable for their location. We can also check that escape routes are unobstructed. However, we cannot conduct fire resistance testing or verify that hidden elements of the construction meet the required fire resistance periods. We’re also unable to confirm that the specific products used have appropriate fire certification. These aspects should be verified during construction by building control inspectors who can review test certificates and check hidden elements before they’re covered up.

6.10.15 – Acoustic Performance

LSF walls and floors must have adequate resistance to the passage of sound to comply with building regulations. In England and Wales, these regulations set specific performance standards for separating walls and floors between different dwellings, and for walls and floors separating habitable rooms from internal communal areas like corridors and stairwells.

Separating Walls: Walls that separate different dwellings (such as between semi-detached houses or between flats) must be constructed in accordance with the design, which should meet the acoustic performance requirements. Particular care is needed to avoid gaps which would allow sound to pass through. Gaps can occur between mineral wool quilt or batts if they’re not tightly butted together, between internal lining board layers if boards are not properly fitted, between cavity barriers if they’re not continuous, or around openings for services if they’re not properly sealed.

Even small gaps can significantly compromise acoustic performance because sound will find the path of least resistance. A wall that would otherwise provide excellent sound insulation can perform very poorly if there are gaps that allow sound to bypass the main construction. This is why attention to detail during construction is so important.

Separating Floors: The floating part of a floor (where used in the design) must be separated from the main structure and surrounding walls by a resilient layer. This isolation prevents impact sounds (such as footsteps) from being transmitted through the structure to the rooms below. Where boards are laid loose over insulation without battens, joints should be glued to prevent movement that could cause squeaking and compromise the acoustic performance.

During a snagging inspection, we cannot test acoustic performance, as this requires specialist equipment and methodology. However, we can identify visible defects that are likely to compromise sound insulation, such as obvious gaps in wall linings, poorly fitted insulation (where visible), gaps around service penetrations, or incorrectly installed resilient layers in floors (where accessible). Acoustic problems typically only become apparent once you’re living in the property, when you notice that you can hear neighbours more than you should be able to. If you experience acoustic issues after moving in, it’s important to raise these with your developer during the warranty period, as remedying poor sound insulation after construction is complete can be very difficult and expensive.

6.10.16 – Moisture Control

The structure must be adequately protected from the effects of moisture, which could otherwise cause corrosion of the steel frame, deterioration of insulation and other materials, and create conditions for mould growth. Details for LSF at low level (close to ground) must fully consider the durability of materials, protection of the building from moisture ingress, and thermal bridging which could cause cold spots and condensation.

Cavities in External Walls: A clear cavity must be provided between the cladding (such as brickwork) and the insulation. The minimum cavity width depends on the type of cladding: 50mm for masonry, 25mm for render on board background, and 15mm for other cladding types. For vertical tile hanging with a breather membrane, the cavity width depends on the batten support layout. The cavity must extend at least 150mm below the damp-proof course (DPC), be kept clear to allow any water that enters to drain away, and be provided with weepholes or other suitable means of drainage at its base.

Protection of Steel at Low Level: This is a particularly important requirement because steel located close to ground level is at much greater risk from moisture. The base rail of LSF should be kept a minimum of 150mm above the external ground level (or waterproofing layer of a flat roof, balcony or terrace) and above cavity fill. At this height, the LSF may be protected against corrosion with standard galvanising as specified earlier. Wall insulation should overlap the base rail by a minimum of 150mm to prevent thermal bridging at this vulnerable junction.

There is some flexibility for locally raised ground levels (up to the internal floor finish) covering less than 15% of the external perimeter to accommodate level thresholds (important for accessibility). In these areas, the cavity should be kept clear and allow drainage, and insulation should overlap the base rail by 150mm.

Where the base rail or lowest steel is less than 150mm above ground level, more stringent measures are required. The steel frame must have factory-applied protection to achieve a 60-year design life, which may be galvanising to 600g/m² or galvanising to 275g/m² with additional heavy-duty bituminous paint. Sheathing or backing boards used below 150mm should be suitable for severe moisture exposure (service class 3). The cavity drainage must be designed considering ground conditions, particularly where it discharges below ground level. Insulation must be specified to limit thermal bridging and interstitial condensation, potentially requiring thermal modelling to demonstrate adequacy.

DPCs, DPMs and Cavity Trays: Damp-proof courses, damp-proof membranes and cavity trays must be provided at openings to prevent rain penetration and should be installed underneath the full width of the base rail, lapping with any damp-proof membrane in the floor where present. Acceptable materials include polyethylene to BS 6515 or bitumen to BS 6398.

During a snagging inspection, we can identify visible moisture-related defects such as signs of dampness or water ingress, blocked or missing weepholes, inadequate cavity widths (where measurable), or missing DPCs at thresholds. We can also look for signs of condensation or mould growth that might indicate moisture control problems. However, we cannot verify that hidden elements like DPMs, cavity trays, or the full extent of steel protection at low level have been correctly installed, or that the specified materials have been used. These aspects require inspection during construction before they’re covered up. If moisture problems develop after you move in, they can be very difficult to remedy and may indicate that these requirements weren’t properly followed during construction.

6.10.17 – Insulation

Insulation must be correctly installed, be of a suitable material and thickness to comply with relevant building regulations, and reduce the risk of interstitial condensation (condensation forming within the structure rather than on visible surfaces).

Insulation Requirements: The insulation should meet several key criteria. It must be inert, durable, rot and vermin proof, and should not be adversely affected by moisture or vapour. This ensures it continues to perform effectively throughout the life of the building. The insulation should cover the whole external face of the wall and be complete within the frame, with no gaps that would create cold spots. Importantly, it should extend 150mm below the base rail to minimise thermal bridging (cold spots where heat can escape through the structure) and maintain a “warm frame” where the steel stays above the dew point temperature to prevent condensation forming on the steel itself.

For rigid board insulation, boards should be tightly butted with joints taped where required by the design. This prevents gaps opening up that would compromise thermal performance. Foil-faced insulation boards with an integral facing on one side only should be fixed with the foil face on the cavity side (facing outwards towards the external cladding).

Acceptable Insulation Materials: The standards list acceptable insulation materials with their relevant standards, including mineral wool (BS EN 13162), flame retardant expanded polystyrene (BS EN 13163), flame retardant extruded polystyrene (BS EN 13164), rigid polyurethane foam and polyisocyanurate (BS EN 13165), phenolic foam (BS EN 13166), and cellular glass (BS EN 13167). Other insulation materials may be used if assessed in accordance with Technical Requirement R3.

The choice of insulation type should take into account the British Research Establishment document BR 135 regarding fire performance of external thermal insulation for walls of multi-storey buildings. This is particularly important following the increased focus on fire safety in buildings after recent incidents involving external insulation systems.

During a snagging inspection, we can identify visible insulation defects where insulation is exposed or visible, such as gaps between insulation boards or batts, compressed or damaged insulation, or insulation that doesn’t extend properly around the base rail. We can also identify symptoms that might indicate insulation problems, such as cold spots, condensation on internal surfaces, or higher than expected heating costs (though the latter only becomes apparent after occupation). However, once insulation is covered by internal linings and external cladding, we cannot verify that it has been correctly installed throughout the property, that the specified type and thickness have been used, or that it extends the full 150mm below the base rail. We also cannot measure thermal performance. These aspects should be checked during construction, and thermal performance can be verified by post-construction air tightness and thermographic testing where required.

6.10.18 – Air and Vapour Control Layers

Air and vapour control layers (AVCLs) restrict the passage of warm, moist air from within the home to the cold steel frame where it could condense, causing moisture problems and potentially corroding the steel. These layers must be correctly installed to be effective.

When AVCLs Are Required: An AVCL should be provided unless a condensation risk analysis shows it’s not necessary. The analysis is typically conducted using the Glaser method in accordance with BS EN ISO 13788, using specific boundary conditions (more than 60% internal relative humidity, at 21°C internally and -2°C externally). These conditions represent a reasonably worst-case scenario for UK dwellings. Split layers of AVCL-type material should be avoided unless condensation risk analysis shows this to be acceptable, as splits create potential weak points where moisture could penetrate.

Installation Requirements: Where AVCLs are provided, they should be 500-gauge polyethylene sheet, vapour control plasterboard, or material assessed in accordance with Technical Requirement R3. The AVCL must be fixed on the warm side (internal side) of the wall insulation and frame, in accordance with the design. It should cover the entire external wall, including base rails, head rails, studs, lintels and window reveals, with overlapping at the base rail. Crucially, the AVCL must be fully sealed and any punctures made good, as even small holes can allow significant amounts of moisture to pass through.

Installation Details for Different AVCL Types: Where polyethylene sheet is used, each joint should be located on studs or noggings and lapped by a minimum of 100mm. Double-sided tape or adhesive should be used as a temporary fixing to hold the lapping in place before the wall board is fixed, ensuring the joint remains sealed. Where vapour control plasterboard is used, joints between sheets should be positioned on studs or noggings. Care must be taken not to displace the vapour control material when cutting vapour control plasterboard, as this could create gaps in the barrier.

During a snagging inspection, we cannot verify that AVCLs have been correctly installed, as they’re completely hidden behind internal wall linings by the time we inspect. We might identify symptoms of AVCL problems, such as condensation forming on internal surfaces, mould growth, or a musty smell that could indicate moisture is penetrating into the structure. However, these symptoms may only become apparent after the property has been occupied for some time, particularly during the first winter when heating is used extensively. If you notice signs of condensation or moisture problems after moving in, particularly if they seem to be occurring within the structure rather than just on surface, this should be reported to your developer during the warranty period, as it could indicate that the AVCL wasn’t correctly installed. Remedying AVCL problems after construction is complete can be extremely difficult and expensive, as it typically requires removing internal linings.

6.10.19 – Breather Membranes

Breather membranes are installed on the outside of the insulation layer (between the insulation and the cladding) to protect the sheathing and frame from external moisture whilst allowing any vapour that does reach the cavity to pass into the ventilated cavity space where it can be carried away by air movement. This “breathing” function is critical to the health of the structure.

Technical Requirements: Breather membranes must meet several specific performance criteria. They should be vapour resistant to less than 0.6MNs/g (or 0.12 Sd, which is a measure of water vapour resistance) when tested according to specific standards. This relatively low resistance allows water vapour to pass through readily. They must be capable of resisting water penetration (preventing liquid water from entering whilst allowing vapour to exit), be self-extinguishing (to limit fire spread), and be durable enough to last the lifetime of the building.

The membranes must be at least Class W2 to BS EN 13859-2, with no water leakage during testing. However, in areas of very severe exposure to wind-driven rain (such as exposed coastal or hilltop locations), or where liquid water penetration of the cladding is anticipated (for example with open-jointed cladding where rain can pass through gaps), Class W1 should be used as it provides better water resistance. Where membranes are likely to be left exposed during construction for longer than normally expected, or where open-jointed claddings are used, performance should be based on artificially aged behaviour to ensure the membrane will remain effective after exposure to UV light and weathering.

Installation Requirements: Breather membranes should be installed so that each joint is protected and moisture drains outwards. They should be lapped to a minimum of 100mm at horizontal joints and a minimum of 150mm at vertical joints, with the upper layer always overlapping the lower layer so water runs down the face and cannot penetrate through the joints. This installation pattern is sometimes referred to as “tile fashion” because it mimics how roof tiles overlap.

When Breather Membranes Can Be Omitted: Breathable membranes should be used to protect sheathing board and insulation in the majority of cases. However, breather membranes may be omitted where water-resistant insulation boards with taped joints are used. The tape must be of a type recommended by the insulation manufacturer, be breathable to allow water vapour to move freely, and resist water penetration. Suitable taping should also be applied at lintel interfaces and other penetrations to direct water outside rather than allowing it to penetrate behind the insulation.

During a snagging inspection, we cannot verify that breather membranes have been correctly installed, as they’re completely hidden behind the external cladding by the time the property is complete. We might identify symptoms of problems, such as dampness penetrating to internal surfaces (particularly after periods of driving rain), staining on external walls, or deterioration of the cladding that could suggest water is being trapped behind it rather than being able to drain and evaporate. However, these symptoms may only become apparent over time, particularly after severe weather events. The correct installation of breather membranes is another aspect that must be checked and verified during construction before the cladding is applied, as there is no practical way to inspect them afterwards without removing cladding.

6.10.20 – Cladding, Lining and Sheathing Boards

Cladding panels, lining and sheathing boards must be suitable for their intended purpose. These elements form the visible surfaces and protective layers of your home, and their correct selection and installation is crucial for appearance, weatherproofing, and structural performance.

External Cladding: The design and construction of external walls should fully consider cavity drainage, differential movement (the cladding and frame may expand and contract at different rates), restraint (how the cladding is held in place), and fire resistance.

For masonry cladding, the construction should follow NHBC Chapter 6.1 (External Masonry Walls). The masonry should not be supported by the LSF walls unless specifically designed by an engineer, as the steel frame may not have been designed to carry the weight of the cladding. The masonry should be tied to the LSF walls with flexible wall ties fixed through to the studs. Importantly, movement joints must be provided as appropriate, with approximately 1mm gap per continuous metre of vertical clay masonry at openings and soffits to allow for differential movement. Without these movement joints, thermal expansion of the masonry (particularly clay bricks which expand when they absorb moisture) could cause cracking or buckling.

For lightweight cladding such as render, tile hanging, or cladding boards, construction should follow NHBC Chapter 6.9 (Curtain Walling and Cladding). The cladding must be compatible with the LSF system construction and supported by systems assessed under Technical Requirement R3, ensuring cladding design loads are effectively transferred to the building structure.

Sheathing Boards: Sheathing boards are fixed to the outside of the steel frame (before the insulation and breather membrane) and serve multiple purposes including providing racking resistance (preventing the frame distorting), providing a fixing surface for insulation and cladding, and contributing to fire and acoustic performance. Because they contribute to meeting many critical performance issues and cannot be easily replaced, they must be specified for the design life of the building (60 years).

Sheathing boards should be appropriate for the exposure of the building and suitable for use in humid conditions. The minimum thicknesses are specified for different materials: cement-bonded particle board thickness is determined by design, oriented strand board (OSB3 required) should be at least 8.0mm thick, and plywood should be at least 5.5mm thick. Fixings used to apply sheathing boards should be selected according to the board manufacturer’s instructions and be suitably specified for strength and long-term durability.

Sheathing boards must be adequately protected from weather during construction through a combination of using water-resistant boards with accredited proof of performance, using sealed jointed water-resistant insulation to reduce water penetration, applying a breathable membrane to the sheathing board, and sequencing construction to minimise daily exposure with fully waterproof temporary coverings overnight and during inclement weather. For all sheathing board types, junctions between adjacent boards and at interfaces with other building elements should be sealed or taped in accordance with the manufacturer’s recommendations. A breather membrane should be used to provide protection to the building during and after construction in areas of very severe exposure to wind-driven rain.

Internal Lining Boards: Internal lining boards should be fixed in accordance with the design and manufacturer’s recommendations, attached to light steel studs using self-drilling, self-tapping screws at maximum 300mm centres. For plasterboard specifically, it should comply with BS EN 520 and be a minimum of 9.5mm thick for stud spacing up to 450mm or 12.5mm thick for stud spacing up to 600mm. The plasterboard must also be shown to provide adequate fire resistance where required.

During a snagging inspection, we can examine the condition and installation of visible cladding, looking for defects such as cracked or damaged bricks or render, poorly finished joints, missing movement joints, or signs of water penetration. We can check that weepholes are present and unblocked, and that external finishes are properly completed. For internal lining boards, we can identify defects such as poorly finished joints, cracks, or damage to the plasterboard surface. However, we cannot verify the type, thickness or quality of hidden sheathing boards, confirm that they’ve been adequately protected during construction, or check that all taping and sealing has been correctly completed at hidden junctions. These aspects should be verified during construction before they’re covered up. We also cannot verify fixing centres for internal lining unless defects like sagging or loose boards make inadequate fixing apparent.

6.10.21 – Wall Ties

Wall ties connect the steel frame to the external cladding (typically masonry) and must be suitable to transfer loads whilst accommodating the differential movement between the frame and cladding. Wall ties are critical components that prevent the cladding from pulling away from the building or collapsing.

General Requirements: Wall ties should comply with BS 845-1 (the standard for wall ties), be fixed to the studs rather than to the sheathing (which wouldn’t provide adequate support), be inclined away from the LSF so that any water running down the tie drips off before reaching the steel frame, and be manufactured from austenitic stainless steel (or other material assessed under Technical Requirement R3) of a type which accommodates the differential movement between the LSF and the cladding.

The differential movement requirement is particularly important because the steel frame and masonry cladding respond differently to temperature changes and moisture. The steel may expand and contract with temperature, whilst clay masonry expands irreversibly over time as it absorbs moisture. If rigid wall ties were used, this differential movement could cause either the ties to fail or the cladding to crack. Flexible ties accommodate this movement whilst still providing adequate support.

Wall Ties for Masonry Cladding: Wall ties for masonry should be installed according to the design, typically at a minimum density of 3.7 ties per square metre. A common spacing pattern meeting this requirement would be maximum 600mm horizontally and 450mm vertically. However, alternative densities may be used where they’ve been demonstrated by building-specific calculation and accepted under the Stage 2 certificate.

Additional ties are required at jambs (sides) of openings, with spacing of maximum 300mm vertically within 225mm of the masonry reveal. This increased density of ties around openings ensures the cladding is adequately supported where it’s been cut and is therefore potentially weaker. Additional studs may be needed in the LSF frame to provide fixing points for these additional ties.

Wall ties must be kept clean and free from mortar droppings during installation. Mortar droppings on ties can create a path for water to travel from the outer leaf of masonry back to the steel frame, potentially causing corrosion and compromising the structure.

During a snagging inspection, we can identify visible defects related to wall ties where any ties are visible (such as at reveals around windows or doors before finishes are applied). However, once construction is complete, wall ties are almost entirely hidden within the cavity and cannot be inspected. We might identify symptoms of wall tie problems, such as cracks in masonry, bulging of walls, or gaps opening up between the cladding and the building, but these symptoms typically only develop over time if ties are absent, inadequate, or have failed. The correct installation of wall ties is another critical aspect that must be verified during construction by building control inspectors before the cavity is closed and the cladding is completed. Problems with wall ties can be very serious, potentially leading to structural issues with the cladding, but they’re also very difficult to detect or remedy after construction is complete.

6.10.22 – Services

Services such as electrical cables, water pipes, heating pipes and ventilation ducts must be adequately protected from damage, and their installation must not compromise the performance of the building structure.

Design Requirements: Service mains and service outlets should be designed to ensure the fire resistance of walls and floors is not impaired. Services passing through fire-resisting elements create potential weak points where fire could spread, so these penetrations must be properly fire-stopped using appropriate materials. The design should also ensure that the required sound insulation of walls and floors is maintained. Services can create flanking paths for sound to travel between rooms or dwellings if they’re not properly detailed. Services should be installed in accordance with the design and be located on the warm side of the insulation (the internal side) where they’re less likely to be affected by cold temperatures or condensation.

Restrictions on Structural Alterations: This is a critical requirement. Light steel joists or studs should not be notched to accommodate services, as notching would significantly weaken the structural member. Holing of structural light steel members should only be carried out in accordance with this chapter and the manufacturer’s recommendations. On-site hole cutting should be avoided wherever possible, as badly cut edges can have an adverse effect on the durability of the frame (by damaging the protective galvanised coating) and may cause damage to pipes and cables (due to sharp edges or burrs).

Where on-site adaptation of the frame is unavoidable, it should be undertaken by the manufacturer (or their approved representative), with prior notification to NHBC, and completed in line with the steel frame designer’s remedial details. All cut edges must be treated with protective coating, and badly cut edges must be avoided entirely. Significant adaptations should be overseen by the design engineer to ensure the structural adequacy of the frame is maintained.

Protection of Services and Structure: Grommets should be used around the edge of service holes to protect electrical cables and reduce the risk of bimetallic corrosion between the LSF and copper pipes. When different metals are in contact in the presence of moisture, an electrical current can flow between them, causing corrosion of one of the metals. Swaged holes (where the steel is formed into a smooth-edged collar around the hole) for electric cables and plastic piping do not require grommets as the swaging process creates a smooth, protective edge.

Additional Requirements for Scotland: In Scotland, services are not permitted within framed separating walls or separating wall cavities. This stricter requirement aims to ensure better acoustic performance between dwellings by eliminating all potential flanking paths for sound through separating walls.

During a snagging inspection, we can identify visible services-related defects such as exposed cables or pipes that appear inadequately supported, signs of damage to services, gaps around service penetrations that haven’t been fire-stopped, or evidence that structural members have been cut or damaged to accommodate services. However, the majority of services are hidden behind finishes by the time we inspect, and we cannot verify that hidden services have been properly installed, that all penetrations have been correctly fire-stopped and acoustically sealed, or that structural modifications have been properly designed and executed. Issues with services often only become apparent during occupation, such as leaks in plumbing, electrical faults, or sound travelling along service routes between rooms or dwellings. Any concerns about services should be raised with your developer during the warranty period.

6.10.23 – Further Information

The NHBC standards reference several publications from the Steel Construction Institute (SCI) that provide additional detailed guidance on light steel frame construction. These publications are invaluable resources for designers, engineers, and construction professionals working with LSF systems.

The key SCI publications referenced include:

• Building Design Using Cold Formed Steel Sections: Construction Detailing and Practice (P165) – This provides comprehensive guidance on construction details and practical aspects of building with cold-formed steel sections.
• Modular Construction Using Light Steel Framing: Design of Residential Buildings (P302) – Specifically focused on modular and volumetric construction methods using LSF, which are increasingly popular for rapid construction of residential buildings.
• Light Steel Framing in Residential Construction (P402) – A comprehensive guide covering all aspects of LSF in residential buildings, widely considered the primary reference document for LSF construction in the UK.
• Design and Installation of Light Steel External Wall Systems (ED017) – Focused specifically on external wall systems, providing detailed guidance on cladding integration with LSF structures.
• Design of Stability Systems for Light Steel Framing (P437) – Addressing the crucial aspect of how LSF buildings resist lateral loads from wind and achieve overall stability.

These publications, whilst technical in nature and primarily intended for construction professionals, can provide you with additional background information if you wish to understand more about how your LSF home has been (or should have been) designed and constructed. The Steel Construction Institute can be contacted at Silwood Park, Ascot, Berkshire, SL5 7QN.