A homeowner’s guide to how the timber in your new home should be protected against fungal decay and insect attack.
Timber is used throughout a typical home: floor joists, roof rafters, sole plates, window frames, cladding, decking, and internal joinery. Each of these applications places very different demands on the wood, and without proper protection, timber is vulnerable to fungal decay and insect attack, both of which can cause serious problems over time.
The NHBC Standards set out the technical requirements that builders must meet for homes covered by NHBC warranties, and Chapter 3.3 of the 2026 edition is dedicated to timber preservation. The chapter explains how natural solid timber should either be selected for its natural durability or treated with a preservative, depending on where and how it will be used. This article walks through each section of the chapter to help you understand what should have been considered during the construction of your home.
Compliance (3.3.1)
The compliance section sets out the underlying principle: any timber used in a new home must either have sufficient natural durability against fungal decay and insect attack, or it must be treated with a preservative in accordance with BS 8417. The treatment specification needs to identify the component type, the Use Class (which describes the environment the timber will sit in), and the Desired Service Life expected of that component.
Preservatives used must also comply with the relevant biocidal regulations, and treatments should follow recognised British Standards or equivalent independent assessments. In practice, this means responsibility for compliance sits firmly with the developer and their suppliers, not with the homeowner.
Durability (3.3.2)
Durability is the heart of the chapter, and where most of the technical detail sits. Different parts of a home are exposed to very different conditions, so the standard categorises timber components into Use Classes ranging from 1 (internal, dry) to 4 (in direct contact with ground or fresh water). Each component is also assigned a Desired Service Life, which is typically 60 years for structural elements like floor joists and roof timbers, 30 years for external joinery and cladding, and 15 years for decking.
Table 1 of the chapter sets out exactly which components require preservative treatment and which can rely on naturally durable heartwood instead. Internal joinery and standard pitched roof timbers in dry conditions don’t require treatment, but ground floor joists, sole plates, tiling battens, external cladding, and decking all do. Table 2 then ranks common species from “very durable” (such as jarrah, greenheart, and teak) down to “not durable” (sycamore, alder, ash, and European beech). Sapwood, the outer layer of any species, is always treated as non-durable unless test evidence shows otherwise.
There are also specific geometric requirements worth knowing about. The base of support posts for porches, canopies, and similar external structures should sit at least 150mm above the adjacent ground level, on a free-draining post base, to prevent moisture wicking up from below. This is exactly the sort of detail an inspector can check externally.
Sitework (3.3.3)
When treated timber arrives on site, it should be accompanied by paperwork, either a delivery note, invoice, or treatment certificate, confirming that the specified treatment has been applied and identifying the Use Class it has been treated to. This is the developer’s record-keeping responsibility, but it matters because treated timber that wasn’t actually treated to the right specification is a defect that may only show itself years later.
Protection and Storage (3.3.4)
Even properly treated timber can be compromised if it’s mishandled before installation. The standard requires that timber and joinery on site are protected from damage as soon as they’re delivered, kept off the ground, protected from rain and weather, stacked in a way that prevents distortion, and arranged to allow air to circulate. Timber that has been left out in standing water or stacked tight against damp ground can absorb moisture and warp, and that distortion may not be obvious until joinery is installed and doors or windows fail to close properly.
Treatment of Cut Surfaces (3.3.5)
Preservative treatment penetrates the outer layers of timber, so any cut, planed, or resawn surface exposes untreated wood underneath. The standard is clear that, ideally, treated timber shouldn’t be ripped, resawn, or planed at all after treatment. Cross-cutting to length is the only reworking generally accepted, and any cut ends must be re-treated with a brush-applied preservative recommended by the original treatment manufacturer.
To make this easier to verify, the standard recommends using a colour-tinted preservative wherever possible, so it’s visible at a glance whether cut ends have been re-coated. Untreated cut ends sitting against masonry or in damp positions are a common source of localised decay.
Compatibility with Metal (3.3.6)
Many modern preservatives contain copper compounds, which are highly effective against fungi and insects but can also accelerate corrosion of mild steel and aluminium fittings when moisture is present. The standard therefore requires that, where dampness is occasional, galvanised fittings should be used in contact with treated timber, and where wetting is more likely, austenitic stainless steel fittings are required. Treated timber should also be re-dried to a moisture content of 22% or less before metal fittings are attached, to reduce the corrosion risk further.
Further Information (3.3.7)
The chapter closes with references to the supporting British Standards and Wood Protection Association documents that underpin the guidance, including BS 8417 (the code of practice for the preservation of wood), BS EN 350 (durability classification), BS EN 335 (Use Classes), and BS EN 599-1 (preservative efficacy). Builders, treatment manufacturers, and timber suppliers all refer back to these documents in their day-to-day work, so the names are useful to be aware of if you ever need to query a specification.
How a Snagging Inspection Fits In
A snagging inspection is not a full structural survey. We don’t open up walls, lift floor coverings, or test the chemical composition of treated timber, and much of what NHBC Chapter 3.3 governs sits behind the scenes: the treatment certificates issued at the timber merchant, the storage discipline on site, and the choice of species for a particular component. Those decisions are made long before the home is finished and aren’t visible in a completed property.
What a thorough snagging inspection can do, however, is identify the visible warning signs that something may not be right. Our inspectors look for issues such as deck posts sitting too close to or directly on the ground, untreated cut ends visible in exposed locations, signs of moisture damage or fungal growth, distorted or swollen external joinery, corrosion staining around metal fittings in contact with timber, and external timber components positioned in ways that don’t appear to comply with the standard. Where we spot any of these, we report them clearly so they can be raised with your developer and put right, ideally well before any longer-term damage develops.
