When purchasing a new build home, the foundations are one of the most critical structural elements, yet they’re often the least visible. If your property uses raft, pile, pier or beam foundations, it’s essential to understand the standards that govern their construction. The NHBC (National House Building Council) has established comprehensive guidelines to ensure these specialised foundation types are designed and built to provide safe, long-lasting support for your home.

This article explains the key requirements set out in NHBC Standards Chapter 4.4, helping you understand what should be in place when these types of foundations are used in your new build property.

4.4.1 Compliance

The fundamental requirement is straightforward: all raft, pile, pier and beam foundations must comply with the Technical Requirements and provide adequate support to the load-bearing elements of your home. This includes external walls, separating (party) walls, internal load-bearing walls, chimney breasts, piers, sleeper walls and internal masonry walls.

The NHBC Standards reference several British Standards and codes of practice that builders must follow, including BS 8004 for foundations, BS EN 1991 for actions on structures, BS EN 1992 for concrete structure design, BS EN 1997-1 for geotechnical design, and BS 10175 for investigating potentially contaminated sites. Whilst these technical documents might seem daunting, they represent decades of engineering knowledge and best practice designed to keep your home safe and stable.

For homeowners, the key point is that foundations complying with the guidance in this chapter will generally be acceptable. This means your builder should be following these established standards, and any deviation should be justified by an engineer’s design specific to your site conditions.

4.4.2 Provision of Information

Proper documentation is crucial for foundations, yet it’s an area where issues can arise if communication breaks down on site. The NHBC requires that designs and specifications must be produced in a clearly understandable format, include all relevant information, and be distributed to all appropriate personnel involved in the construction.

This means that site supervisors, specialist subcontractors and suppliers should all have access to detailed information showing dimensions, foundation types and depths, detailing of ducts, junctions, steps, movement and construction joints, the location of services, and critical sequences of construction. All dimensions and levels should be referenced to at least one benchmark and reference point on site to ensure accuracy.

Importantly, both designers and site operatives need to be fully aware of the ground conditions, any features requiring special attention (such as existing sewers or other services), water table levels, and the presence of any hazardous substances, including sulphates. When this information isn’t properly communicated, mistakes can occur that may compromise the foundation’s performance.

As a homeowner, whilst you won’t have direct access to these technical documents, it’s reassuring to know that robust information management is required. If issues arise during your snagging inspection that relate to foundations, poor documentation or communication on site may be a contributing factor.

4.4.3 Site Conditions

Foundations don’t exist in isolation, they must be carefully designed to suit the specific conditions of your building plot. The NHBC requires that raft, pile, pier and beam foundations take account of several critical site factors.

Firstly, all information relating to the site and its ground conditions that’s necessary for full and proper foundation design should be obtained before work begins. This site and ground appraisal forms the basis for appropriate foundation design. Importantly, building over changes in ground characteristics should be avoided where possible, as this can lead to differential settlement and potential structural issues.

The dwelling design, layout and site levels must also be considered during foundation design. On sloping sites, for example, stepped foundations and suspended floors may be required to accommodate the gradient safely. The shape, size and construction method of your home all influence the foundation solution that’s appropriate.

Trees and hedges deserve special attention. Where the soil is shrinkable clay and nearby trees and hedges are existing, proposed or have been recently removed, foundations must be designed in accordance with Chapter 4.2 of the NHBC Standards on building near trees. This is crucial because tree roots can cause significant ground movement in clay soils, potentially affecting your home’s foundations.

Frost susceptible soils present another consideration. To avoid damage from frost action, the depth to the underside of foundations in frost susceptible ground should be at least 450mm below finished ground level. This ensures that seasonal freezing and thawing doesn’t cause the ground to heave beneath your foundations.

Finally, differential settlement, where different parts of the building settle at different rates, must be avoided through careful design. This is particularly important for terraced homes or those adjoining an existing building, where special precautions may be needed. Foundations for attached bays, porches, garages, conservatories and other structures should generally be a continuation of those for the main home, unless an alternative design specifically accounts for differential movement.

4.4.4 Hazardous Ground

Ground contamination and hazardous conditions are more common than many homebuyers realise, particularly on brownfield sites or former industrial land. The NHBC Standards require that raft, pile, pier and beam foundations take full account of ground conditions and hazards, with specific notification requirements when hazardous ground is identified.

Where hazardous ground has been discovered, the builder must notify NHBC in writing at least eight weeks before work on site begins. This advance notice allows for proper review and ensures that appropriate measures are in place. The design of foundations on hazardous ground must be carried out by a qualified engineer in accordance with Technical Requirement R5.

When toxic materials or those likely to present a health hazard are found, all available information should be supplied to NHBC, together with proposals for remediation. This protects not only the structural integrity of your home but also the health and safety of construction workers and future occupants.

Two particular chemical hazards warrant special attention: sulphates and acids. Sulphates can cause expansion and disruption of concrete over time, gradually weakening the foundations. High acidity, such as that found in peat or permeable soil with acidic groundwater, can also cause damage to concrete. Where sulphates or high acidity are present in the ground or groundwater, specific concrete mixes must be used that are resistant to chemical attack.

The level of sulphate and other chemicals should be determined in terms of the ACEC class (Aggressive Chemical Environment for Concrete class), in accordance with BRE Special Digest 1. This classification system helps engineers select the appropriate concrete specification to resist the specific ground conditions present on your site.

For homeowners, it’s worth understanding whether your site had any previous uses that might indicate potential contamination. Former industrial sites, petrol stations, landfills or agricultural land may present ground chemistry challenges that require specialised foundation designs and concrete specifications.

4.4.5 Services and Drainage

The interaction between foundations and services is a critical consideration that must be properly addressed during construction. New build homes require numerous services, including water supply, electricity, gas, telecommunications and drainage, many of which must pass through, above or under the foundations.

Where services need to penetrate or cross foundations, suitable provision must be made using ducts, sleeves or lintels. These openings must be carefully detailed to ensure they don’t impair structural stability or affect the foundation’s ability to carry loads. Crucially, they must make suitable provision to allow for movement and have sufficient space to maintain the line and gradient of drainage even if some settlement occurs.

Existing services and drainage present different challenges. Any existing services or drains on the site must be properly supported and protected, or bridged to prevent them from carrying any load from the new foundations. They should not be rigidly encased in concrete, masonry or other materials, as this can lead to damage if the foundation settles or the pipe needs to move.

Land drains, which manage groundwater, should be diverted to a suitable outfall to prevent water accumulation beneath or around the foundations. Other drains should be diverted or bridged as appropriate. Failing to properly address existing drainage can lead to water-related foundation problems or damage to drainage infrastructure.

These considerations are particularly important with raft foundations, where services often need to be carefully routed through the floor slab, or with piled foundations where service routes must be planned around the pile locations. Poor coordination between foundation design and service installation is a common source of construction defects that can lead to future problems.

During your snagging inspection, whilst you won’t be able to see the foundations themselves, any issues with service penetrations, drainage falls or evidence of poorly coordinated installations may indicate problems with how these requirements were implemented during construction.

4.4.6 Safe Transmission of Loads

The primary purpose of any foundation is to safely transmit loads from the structure above to the ground below without excessive settlement. For raft, pile, pier and beam foundations, this requirement becomes more complex than for simple strip foundations, requiring careful engineering consideration.

These specialised foundation types must be designed with adequate stiffness to ensure that differential movement doesn’t adversely affect the supported structure. This means the foundation must be rigid enough to distribute loads evenly and prevent parts of the building from settling at different rates, which could cause cracking or structural damage.

The design must also account for the nature and bearing capacity of any fill material placed under the foundation. Not all fill materials are suitable for supporting foundations, and the specification of both the concrete and the cover to reinforcement must be appropriate for the site conditions and the loads being transmitted.

Raft and Semi-Raft Foundations

Raft and semi-raft foundations have specific design considerations. They must be designed to prevent the erosion of ground beneath the raft, which could lead to voids forming and subsequent settlement. Where required, they must accommodate warm air ducts, service ducts or services without any adverse effect upon the foundation’s performance, whilst limiting the risk of these ducts becoming flooded.

A critical requirement is that the foundation must support the building envelope without risk of differential movement between the leaves of cavity walls. This is particularly important because raft foundations support the entire floor slab as well as the walls, and any differential movement could cause the cavity to close or open unevenly.

Any fill used for raft foundations must be engineered fill in accordance with Chapter 4.6 of the NHBC Standards. This ensures the fill material has been properly selected, placed and compacted to provide adequate support.

Semi-Raft Foundations on Engineered Fill

For semi-raft foundations constructed on engineered fill, the NHBC provides detailed guidance that engineers must follow. Whilst these are technical requirements, understanding the principles helps homeowners appreciate the engineering that goes into their foundations.

Sufficient internal beams must be provided to stiffen the slab adequately, with the area between downstand beams not exceeding 35 square metres and the ratio of adjacent sides on plan not exceeding 2:1. The minimum depth of perimeter and party wall beams must be 450mm, and on larger homes, some internal beams should match this depth.

These beams must be sufficiently wide at their base to carry their total loading at the allowable bearing pressure for the site. They’re designed to span 3 metres when simply supported and to cantilever 1.5 metres, providing the structural capacity needed to support your home safely.

Reinforcement must be properly formed in accordance with British Standards, and where mesh is used in beams, it should arrive at the site pre-bent to ensure correct positioning. All beams must be cast on a minimum of 50mm concrete blinding, with minimum cover to reinforcement of 40mm to protect the steel from corrosion.

Floor slabs should be at least 150mm thick, with nominal top face reinforcement as a minimum and anti-crack reinforcement in the bottom face where appropriate. During construction, stools or similar supports should be used to hold the floor slab mesh in the correct position during casting.

A minimum cavity drain of 225mm below the damp proof course (DPC) must be maintained to prevent moisture problems. This is deeper than the typical requirement for strip foundations and reflects the different construction method.

Piled Foundations

For piled foundations, the design must specify appropriate precautions for cohesive soils where volume changes can occur, such as shrinkable clay. This is crucial because piles transfer loads through unstable surface soils to more stable ground below, and the design must account for potential ground movement around the pile shafts.

The bearing capacity and integrity of piles should be confirmed by testing when required. This provides assurance that the piles are performing as designed and can safely support the intended loads.

4.4.7 Construction

Even the best foundation design will fail if construction isn’t carried out correctly. The NHBC requires that raft, pile, pier and beam foundations be constructed strictly in accordance with the design, with particular attention to setting out, excavations, installation quality and load verification.

Setting Out and Excavations

Accuracy in setting out is fundamental to successful foundation construction. The NHBC requires that the accuracy of setting out be checked by control measurements of trenches, including their location relative to site boundaries and adjacent buildings. Levels must be checked against benchmarks where appropriate.

For excavations, builders must verify trench lengths, trench widths and the length of diagonals between external corners. These measurements ensure the foundations are square and correctly positioned. For piled, pier and beam foundations, additional checks are required for spacing, alignment and positions in relation to the proposed superstructure.

A critical requirement is that walls should be located centrally on the foundation unless specifically designed otherwise. This seems simple, but inaccuracy in setting out may prevent walls and piers from being located centrally, resulting in eccentric loading that could cause foundation failure. Even small deviations can create stress concentrations that compromise structural performance.

Any discrepancies in the design of the foundations or variations in ground conditions discovered during excavation should be reported formally to the engineer. This is essential because ground conditions don’t always match the assumptions made during design, and the engineer may need to modify the design accordingly. Variations in design or ground conditions should be recorded and distributed to NHBC and others concerned with the site work to maintain proper documentation.

Foundation excavations must be kept free from water and should not be excessive in depth or width beyond what the design requires. Excavating too deeply can disturb the bearing strata, whilst water in excavations can soften the ground and affect concrete quality.

Localised Effects and Trench Bottoms

Ground conditions at the bottom of excavations can vary significantly, and the NHBC Standards provide clear guidance on how to address different situations. Trench bottoms affected by rainwater, groundwater or drying should be rebottomed to form a sound surface. This means removing any softened or disturbed material to reach stable ground.

Where differences in bearing capacity exist, such as from localised changes in strata, the engineer should be consulted to determine the appropriate response. Soft spots present a particular challenge, in such cases, excavations should be deepened locally to a sound bottom, or the concrete should be reinforced to bridge across the weak area. Conversely, hard spots should be removed to create a uniform bearing surface.

If visible roots are found, especially in clay soils, the engineer should be consulted and the design depth may need to be modified. Tree roots indicate that the ground may be subject to seasonal movement as moisture levels change with the tree’s growth cycle.

Installation of Piles, Piers and Ground Beams

Pile installation requires specialist expertise and must be carried out by an appropriate specialist contractor under the supervision of an engineer. Piles must be vertical unless the design specifically requires a raked (angled) pile for a particular purpose.

Tolerances for pile installation are clearly defined. Where piles are more than 75mm out of position, or out of alignment by more than 1:75 (approximately 0.76 degrees), the engineer should reconsider the adequacy of the foundation design. More significant deviations require corrective action: where piles are misaligned by more than 150mm in any direction, or more than 5 degrees from their specified rake, they should be replaced unless the engineer recommends otherwise. Alternatively, additional piles may be provided in accordance with design modifications from the engineer.

Care must be taken to ensure that the bond between beams and piers or piles is adequate and in accordance with the design. This connection is critical for transferring loads from the ground beams into the piles or piers.

Load Capacity Verification of Piles

Test loading of piles should be undertaken when required to verify they can support their design loads. This typically involves applying loads to sample piles and measuring the resulting settlement to confirm the piles are performing as expected. The builder must obtain written confirmation that the piles are suitable for their design load, providing documented evidence that the foundations meet the required standards.

4.4.8 Engineer Checks

Engineer-designed foundations represent a significant investment in your home’s structural integrity, but the design is only as good as its implementation on site. The NHBC requires that engineer-designed foundations must be inspected by the engineer during construction to verify that the design is being correctly executed.

The engineer should undertake site visits at critical stages to ensure two key aspects are satisfied. Firstly, they must verify that the design of the foundation is suitable for the actual ground conditions encountered during construction. Ground investigations provide valuable information, but they’re based on a limited number of test locations, and conditions can vary across a site. When foundations are excavated, the engineer can directly observe the ground conditions and confirm whether they match the design assumptions or require modifications.

Secondly, the engineer must ensure that the construction is being carried out in accordance with the design. This includes checking that excavations are to the correct depth and dimensions, reinforcement is correctly positioned and specified, concrete is of the appropriate specification, and all critical details are being implemented as designed.

These inspections provide an essential quality control mechanism. Even experienced construction teams can make mistakes or encounter unexpected conditions, and the engineer’s oversight helps identify and correct issues before they become permanently built into the structure. For homeowners, whilst you won’t typically be present for these engineer inspections, they represent an important safeguard in the construction process.

In England and Wales, foundations should also be approved by the person responsible for building control inspections, adding another layer of independent verification before construction proceeds.

4.4.9 Compressible Materials

In certain ground conditions, particularly shrinkable clay soils, the ground beneath foundations can heave (swell upwards) when moisture levels increase. This is the opposite of settlement and can cause equally significant structural damage. To accommodate potential heave forces, compressible materials are sometimes incorporated into foundation designs.

The NHBC requires that compressible materials must be capable of absorbing potential heave forces without transmitting damaging loads to the structure above. These materials work by compressing when heave occurs, essentially providing a cushion that allows the ground to move without pushing the foundation upwards.

Materials used to accommodate heave should be assessed in accordance with Technical Requirement R3 and used following the manufacturer’s recommendations and any independent assessment where applicable. This ensures the materials have been properly tested and are suitable for their intended purpose.

An important consideration is that the thickness of compressible material must account for both the minimum void required (the space needed to accommodate the expected ground movement) and the residual thickness of the material when compressed. If the material compresses too much, it may bottom out and no longer provide protection against heave. Different types of compressible materials behave differently under load, so the design must consider the specific properties of the material being used.

Compressible materials are most commonly used in piled foundations in clay soils, where they’re placed beneath ground beams to prevent heave forces from lifting the beams and the structure they support.

4.4.10 Reinforcement

Reinforcement transforms plain concrete into a structural material capable of resisting tension forces and controlling cracking. For raft, pile, pier and beam foundations, proper reinforcement is essential to ensure safe load transfer and accommodate localised ground conditions.

The NHBC requires that reinforcement be in accordance with the design, sufficient to ensure the safe transfer of loads and suitable for localised ground conditions. This means the reinforcement must be appropriately sized for the forces it needs to resist, placed correctly in accordance with the structural drawings, and properly detailed at all connections and junctions.

Several practical requirements ensure reinforcement performs as intended. The steel must be clean and free from loose rust, whilst some surface rust is acceptable and can even improve bonding with concrete, heavy rust or scale must be removed as it can prevent proper adhesion. The reinforcement must be secured at laps (where bars overlap) and crossings to prevent movement during concrete placement.

Critically, reinforcement must be properly supported to ensure that the cover indicated in the design is maintained. Cover refers to the thickness of concrete between the reinforcement and the surface, and it serves two purposes: protecting the steel from corrosion and ensuring adequate fire resistance. If reinforcement is positioned too close to the surface, it may corrode prematurely, compromising the foundation’s long-term durability.

Reinforcement may be necessary in various situations beyond the basic structural requirements. For example, it’s often required at construction joints where one concrete pour meets another, over small localised soft spots or changes in bearing strata where it can bridge across variations in ground conditions, or in areas of stress concentration such as corners and around openings.

For raft foundations specifically, the NHBC guidance notes that floor slabs should have nominal top face reinforcement as a minimum and anti-crack reinforcement in the bottom face where appropriate. This controls shrinkage cracking as the concrete cures and helps distribute loads evenly across the slab. During construction, stools or similar supports must be used to hold floor slab mesh in the correct position during casting, ensuring the reinforcement ends up where the designer intended rather than settling to the bottom of the slab.

The guidance also notes that where mesh is used in beams, it should be delivered to site pre-bent to the required profile. This ensures the reinforcement can be correctly positioned and reduces the risk of it being bent incorrectly on site. Corners and junctions to beams should be adequately tied using similar reinforcement to the beams themselves, providing continuity of reinforcement around the structure.

For homeowners, whilst you won’t see the reinforcement once concrete is placed, photographic records taken during construction can provide valuable documentation of the reinforcement installation, particularly if any questions arise later about the foundation’s construction.

4.4.11 Concrete

Concrete is the fundamental material in raft, pile, pier and beam foundations, and its quality directly determines the foundation’s performance and longevity. The NHBC requires that concrete for these foundation types must be of a suitable mix design to achieve the required strength and resistance to chemical and frost action, and that it must be correctly mixed, placed and cured.

Suitable Mix Design

The concrete mix must be carefully specified to achieve two critical objectives. Firstly, it must develop sufficient strength to carry the structural loads without impairing the foundation’s performance. Secondly, it must be sufficiently resistant to chemical and frost action that could degrade the concrete over time.

The appropriate mix design depends on the site conditions. Where sulphates or acids are present in the ground or groundwater, as discussed in section 4.4.4, specialist concrete mixes with enhanced resistance to chemical attack must be used. The NHBC Standards reference Chapter 3.1 on Concrete and its Reinforcement for detailed guidance on acceptable concrete mixes for different exposure conditions.

The concept of ACEC classes (Aggressive Chemical Environment for Concrete) provides a systematic approach to matching concrete specification to ground conditions. Ground investigations should identify the chemical composition of soil and groundwater, allowing engineers to specify concrete with appropriate resistance. Using standard concrete in aggressive ground conditions is a recipe for premature deterioration, whilst over-specifying concrete adds unnecessary cost.

Frost resistance is another important consideration. In frost susceptible locations, concrete must be able to withstand repeated freezing and thawing cycles without deterioration. This typically requires concrete with appropriate air entrainment and a sufficiently low water-to-cement ratio.

Correctly Mixed, Placed and Cured

Even the best concrete mix design will fail if the concrete isn’t properly handled during construction. Before concrete is placed, excavations and reinforcement may need to be approved by the engineer or their representative. In England and Wales, foundations must be approved by the person responsible for building control inspections before concreting proceeds. This approval process provides a final check that everything is ready before the permanent works begin.

Concreting should be carried out in one operation as far as possible to avoid weak construction joints. Where this isn’t possible, construction joints should be carefully detailed and reinforced as required by the design. The work must take account of weather conditions and available daylight, as concrete shouldn’t be placed in conditions that could compromise its quality, such as during heavy rain, extreme cold or when lighting is inadequate to ensure proper placement.

Concrete should be placed as soon as possible after the excavation has been completed or after reinforcement has been checked and approved. Delays between excavation and concreting can allow the ground to deteriorate, particularly in wet conditions, and exposed reinforcement can accumulate dirt or rust that affects bonding. The concrete must be placed in trenches that are even, compact and reasonably dry.

The mixing, placing, testing and curing of concrete should be carried out in accordance with Chapter 3.1 of the NHBC Standards on Concrete and its Reinforcement. When work is carried out in cold weather, additional precautions are required as detailed in Chapter 3.2 on Cold Weather Working. These measures might include protecting concrete from freezing, using accelerated mixes, or providing temporary heating.

Proper curing is essential for concrete to achieve its designed strength and durability. Concrete gains strength through a chemical process that requires moisture and appropriate temperatures. If concrete dries too quickly or freezes before it has gained sufficient strength, its performance will be permanently compromised. Curing might involve covering concrete with polythene sheeting, applying curing compounds, or keeping it moist through regular watering, depending on the weather conditions and the specific requirements.

4.4.12 Movement Joints

All structures experience some degree of movement due to thermal expansion and contraction, moisture changes, loading variations and settlement. Movement joints are deliberate breaks in the structure designed to accommodate these movements in a controlled way, preventing uncontrolled cracking and damage.

The NHBC requires that raft, pile, pier and beam foundations must have movement joints suitable for their intended purpose and formed using appropriate materials. These joints should be located strategically to limit the risk of damage caused by movement.

The design of movement joints requires careful consideration of several interrelated factors. The anticipated movement must be estimated based on the structure’s size, the materials used, expected temperature ranges and potential settlement patterns. Different parts of a building may move at different rates, for example, a section built over piled foundations may behave differently from an adjacent section on a raft foundation.

The movement capability of the seal material must match or exceed the anticipated movement. Joint sealants are typically specified with a movement accommodation factor, for example, plus or minus 25% of the joint width. If the joint is expected to open and close by 10mm, the sealant must be capable of stretching and compressing by at least this amount without failure.

The designed joint width and actual joint width must be carefully controlled. Joint width affects how much absolute movement the sealant can accommodate. A wider joint filled with a material capable of 25% movement can accommodate more absolute movement than a narrow joint with the same sealant. However, the actual joint width achieved during construction must match the design assumptions, if joints are formed too narrow or too wide, the sealant may not perform as intended.

Joint depth, surface preparation and backing medium are also critical. The depth of sealant affects its ability to stretch and compress, deeper joints may restrict movement. Surface preparation ensures the sealant bonds properly to the concrete surfaces. A backing medium (typically a compressible foam rod) is often used to control the sealant depth and ensure it bonds only to the two opposite faces of the joint, not to the bottom, allowing it to stretch and compress more effectively.

Finally, the projected lifespan of the joint must be considered. Some sealant materials degrade over time due to ultraviolet exposure, chemical exposure or simply aging. The joint design should consider whether maintenance or replacement will be required during the building’s life and whether this is practical for the joint’s location.

For raft foundations in particular, movement joints may be required where different sections of the building are expected to settle at different rates, at the junctions between the main building and attached structures like garages, or at regular intervals in very large raft slabs to control thermal movement.

4.4.13 Resistance to Moisture

Preventing moisture penetration from the ground is fundamental to creating a habitable and durable home. For raft, pile, pier and beam foundations, which often support suspended ground floors or incorporate the floor slab as part of the foundation system, moisture resistance requires careful detailing.

The NHBC requires that these foundation types must prevent the passage of moisture to the inside of the home and, where necessary, include a drained cavity and damp proof membranes. The specific requirements depend on whether the foundation supports a cavity wall and the type of foundation system used.

Cavity Drainage Requirements

Cavity walls must be designed so that any water penetrating the outer leaf can drain away safely below the damp proof course (DPC) without crossing to the inner leaf or flooding the cavity above the DPC. This drainage requirement is critical because water can penetrate the outer leaf through various means, including wind-driven rain, and must be able to escape.

The cavity should prevent water from crossing from the outside to the inside of the wall. This is achieved through maintaining adequate cavity width, installing cavity trays where required, and ensuring weepholes are provided to allow water to escape. The cavity must also prevent flooding of the space above the DPC, which could lead to moisture problems in the habitable areas of the home.

The minimum cavity dimension below the DPC varies depending on the foundation type. Where strip, trench-fill or ground beams are used, a minimum 225mm clear cavity must be maintained below the DPC. This generous dimension provides good drainage capacity and reduces the risk of mortar droppings or debris bridging the cavity.

However, where other types of foundations are used, such as raft foundations, a reduced minimum of 150mm clear cavity below the DPC is permitted, provided that weepholes and other necessary measures are taken to ensure that the cavity can drain freely. The reduced dimension reflects the different construction sequence and conditions with raft foundations, but the requirement for effective drainage remains paramount.

DPC Detailing for Different Foundation Types

The document provides specific guidance on DPC detailing for different foundation configurations, illustrated in Figures 3 and 4 of the NHBC Standards.

For suspended ground floors over piled foundations (Figure 3), the typical detail shows a minimum 225mm clear cavity maintained below the DPC, with the ground beam positioned to support the wall whilst maintaining this drainage space. This configuration allows any water penetrating the outer leaf to drain down to ground level well below the floor structure.

For raft foundations (Figure 4), the DPC detail differs due to the nature of the construction. The raft forms a continuous slab that extends under the walls, and the minimum 150mm clear cavity is maintained above the raft surface. Critically, a weephole must be provided above the cavity tray to allow water to escape from the cavity. Without this weephole, water could accumulate in the cavity, potentially bridging to the inner leaf or causing damp problems.

Important Restriction on Cavity Trays

The NHBC Standards include an important restriction that’s worth highlighting: DPC cavity trays are not an acceptable waterproofing solution for the edges of specialised foundations, such as rafts and ground beams. This is because cavity trays are designed to manage relatively small amounts of water that penetrate the outer leaf of cavity walls. They’re not designed to act as a primary waterproofing system for foundation edges where water pressure or volume might be significantly higher.

This means that the waterproofing strategy for raft and beam foundations must rely on proper drainage around the foundation perimeter, appropriate DPC membranes in the correct locations, and maintaining the required cavity dimensions, rather than depending on cavity trays to manage moisture at the foundation level.

Implications for Homeowners

For homeowners, moisture resistance measures are largely invisible once construction is complete, but they’re critical for long-term comfort and durability. During a snagging inspection, whilst you cannot see the DPC or cavity construction below ground level, you can look for evidence of proper detailing such as:

• Weepholes in the outer leaf of cavity walls at appropriate intervals and locations
• Proper finishing levels that ensure the external ground level is well below the DPC
• Evidence that cavity trays are correctly positioned (sometimes visible during construction)
• No signs of moisture penetration at the base of walls, which might indicate inadequate moisture resistance measures

Any dampness appearing at the base of walls, particularly in the early months after construction, should be investigated promptly as it may indicate a problem with the moisture resistance detailing of the foundations.

4.4.14 Further Information

For those seeking more detailed technical information about concrete in aggressive ground conditions, the NHBC Standards reference BRE Special Digest 1, titled “Concrete in aggressive ground” (3rd edition). This comprehensive document provides detailed guidance on assessing ground chemistry, classifying aggressive chemical environments, and specifying appropriate concrete mixes.

The NHBC Standards also cross-reference several other chapters that contain relevant information for raft, pile, pier and beam foundations, including Chapter 5.1 on substructure and ground floors, Chapter 5.2 on suspended ground floors, and Chapter 5.4 on drainage below ground. These interconnections reflect the fact that foundations don’t exist in isolation but form part of an integrated building system.

Understanding the NHBC Standards for raft, pile, pier and beam foundations helps new build homeowners appreciate the engineering and care that should go into their property’s structural support. These specialised foundation types are used when ground conditions or building design make simple strip foundations inappropriate, and they require careful design, quality materials and skilled construction.

Whilst much of the foundation work will be complete and covered over before you move in, the standards outlined in Chapter 4.4 provide important benchmarks. If your snagging inspection reveals issues that might relate to foundations, such as uneven floors, cracking in walls, or evidence of settlement, understanding these standards can help you discuss concerns with your builder and, if necessary, seek appropriate expert advice.

At New Build Inspections, our experienced inspectors understand these technical requirements and can identify signs that might indicate foundation-related issues during your snagging inspection. Whilst we cannot see the foundations directly once they’re covered, our expertise allows us to recognise indicators that may warrant further investigation.

If you’re purchasing a new build home with raft, pile, pier or beam foundations, a professional snagging inspection provides valuable peace of mind towards knowing that your home has been constructed to the required standards. Our comprehensive reports document the condition of your property and provide you with the information needed to ensure any issues are addressed before they become more serious concerns.