C16 Structural Timber: The Standard Grade for UK Domestic Construction

Your builder orders 30 lengths of 47x200mm timber for the roof. The delivery arrives, your carpenter starts cutting, and three days later building control turns up for the roof inspection. The inspector checks the grade stamps. Half the timber is unmarked. It's ungraded. It doesn't comply. Your carpenter has already cut and fixed it. Now you're stripping out three days of work, ordering replacement stock, and paying your carpenter again. That scenario costs you days of wasted labour plus replacement materials. Knowing what C16 timber is, what the stamp means, and what to check on delivery prevents it entirely.

What it is and what it's for

C16 is a strength grade for structural softwood timber, defined by BS EN 338:2016. The "C" stands for coniferous (softwood), and the "16" means the characteristic bending strength is 16 N/mm2 (newtons per square millimetre, the force per unit area the timber can withstand before it fails in bending). That number matters because your structural engineer uses it to calculate how thick and deep your joists and rafters need to be for a given span.

C16 is the standard grade for UK domestic construction. Floor joists, roof rafters, ceiling joists, purlins (the horizontal beams that support rafters mid-span), wall plates (the timber that sits on top of the wall to support the roof), and stud wall framing are all routinely specified in C16. NHBC Standards Chapter 6.4 confirms that solid floor joists must be C16 or C24 minimum.

The timber itself is predominantly UK-grown or Scandinavian spruce and pine. C16 comes from faster-growing trees with slightly wider growth rings and more knots than C24. Those characteristics reduce bending strength compared to the tighter-grained imported timber that typically grades to C24, but for most domestic spans, C16 with the correct cross-section size is the right choice.

Timber Development UK (the industry body for structural timber) puts it clearly: C16 is suitable for most domestic spans when correctly sized. The Homebuilding & Renovating claim that C16 is "rarely considered strong enough for joists or roofing, other than over short spans" is over-cautious and contradicted by both NHBC standards and TDUK guidance. The key is using the right size for the span, which is what span tables are for.

Types, sizes, and specifications

Standard sizes

C16 structural timber comes in a standard set of cross-section sizes. All use a nominal 47mm width (the actual finished width after regularising is approximately 45mm, more on that below). The depth determines what span the timber can cover.

Standard lengths available are 2.4m, 3.0m, 3.6m, 4.2m, and 4.8m. Some merchants stock 6.0m lengths (Travis Perkins carries these in 47x150mm). Longer lengths cost proportionally more per piece but the per-metre rate stays consistent. If your span requires 4.5m of timber, you'll need to buy 4.8m lengths and cut to fit. There's no option to order exact lengths from a standard merchant.

Regularised vs sawn

This distinction confuses most homeowners and causes ordering mistakes.

Regularised timber has been machined to a consistent cross-section after kiln-drying. The edges are smooth and the dimensions are predictable along the full length. Regularised C16 with a 47mm nominal width finishes at approximately 45mm. Wickes lists their C16 at the actual finished dimension (45mm), while most other merchants use the nominal 47mm. The physical timber is the same product described differently.

Sawn timber has a rough surface directly from the sawmill. The dimensions are less precise, with slight variation along the length. Sawn timber is cheaper but less suitable for applications where consistent depth matters (floor joists where you need a flat surface for boarding, for example).

For structural work on a domestic extension, buy regularised. It costs marginally more and saves time on site because every piece is the same depth. Your carpenter won't need to pack or shim joists to get a level floor.

How to read the grade stamp

Every piece of structural timber must carry a grade stamp. Building control inspectors check for this at every joist and rafter inspection. Unmarked timber does not comply, regardless of what the merchant told you it was.

The stamp must appear at least once on a face or edge, at least 600mm from the end of the piece, and contains:

• Strength class (C16 or C24)
• Certification body number (identifies who graded the timber)
• Species or origin code (identifies the wood species and country of origin)
• Moisture condition (DRY or KD, confirming kiln-dried to below 20% moisture content)

NHBC requires that timber for internal structural use be marked "DRY" or "KD" per BS 4978. If the stamp doesn't say DRY or KD, it hasn't been kiln-dried to the required moisture content and shouldn't be used for internal floor joists.

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Span tables: what size do you need?

Your structural engineer specifies the timber size based on the span, the spacing between joists or rafters (called "centres"), and the expected loads. You don't need to do this calculation yourself, but understanding the span tables helps you sanity-check what's been specified and understand why your SE chose a particular size.

Timber Development UK publishes free Eurocode 5 span tables for C16 covering floor joists, ceiling joists, rafters, purlins, and flat roof joists. These are the current reference standard.

For floor joists at 400mm centres (a common domestic spacing) under standard domestic loading (1.5 kN/m2):

These are approximate figures from BS 5268-based tables. Eurocode 5 tables (4th edition) are more conservative by roughly 15-20% for some configurations, particularly rafters. Building control increasingly expects Eurocode compliance, though some BCOs still accept pre-Eurocode tables in practice. Use whichever standard your structural engineer has designed to.

Strutting

For floor joist spans exceeding 2.5m, NHBC requires solid or herringbone strutting (cross-bracing between joists that prevents them twisting under load). One row at mid-span for spans up to 4.5m. Two rows at third-points for spans over 4.5m. Maximum joist spacing is 600mm centres in all cases.

Strutting gets installed before the floor is boarded. Building control checks for it. Skip it and your floor will develop bounce and creaking over time as the joists twist under load.

How to work with it

Weight and handling

C16 structural timber is not heavy compared to masonry, but a 4.8m length of 47x200mm weighs around 20-22 kg. A full delivery of 30-40 lengths is a meaningful amount of material to move around a site. Plan where the delivery is dropped so the timber doesn't need to be carried far.

Cutting

A circular saw or mitre saw handles C16 cleanly. Hand saws work but are slow for structural sections. For notching joists (cutting slots for pipes or cables to pass through), strict rules apply: notches are permitted in either the top or the bottom of the joist (but not both on the same joist), and only within the zone between 0.07 and 0.25 of the span from each support. The maximum notch depth is 0.125 times the joist depth (so 25mm maximum on a 200mm joist). Over-notching weakens the joist and building control will flag it.

Drilling holes through the joist for services is permitted in a specific zone: between 0.25 and 0.4 of the span from each end, through the centre third of the joist depth, with a maximum hole diameter of 0.25 times the joist depth.

Storage on site

Treated C16 arrives pressure-impregnated with water-borne preservative and is often visibly wet (greenish tint from the tanalith treatment). It needs to acclimatise in a dry environment before being cut and fixed. If you fix wet treated timber immediately, it will shrink and twist as it dries in place. That shrinkage causes squeaky floors, gaps between joists and hangers, and uneven surfaces.

Stack timber on bearers at least 150mm off the ground. Place stickers (25mm timber spacers) between each course to allow air circulation on all faces. Cover the top with breathable sheeting that keeps rain off but allows moisture to escape at the sides. Separate treated from untreated timber.

Inspect every delivery before the driver leaves. Check for:

• Grade stamp visible on every piece
• DRY or KD marking present
• No severe bow, twist, or warp (sight along the length)
• Treatment colour visible (green tint for tanalith)
• Correct size matches your order (measure one piece)

Reject bent, twisted, or unmarked stock immediately. Using warped timber as joists creates high and low spots in the floor that are impossible to correct without replacement.

Cost and where to buy

C16 structural timber pricing has stabilised after the volatility of 2021-22. Prices vary by cross-section size and by merchant.

The smallest common structural size, 47x100mm, runs at £2.50 – £3.10 per metre ex-VAT. That's the section you'd use for stud wall framing or short-span ceiling joists.

At the larger end, 47x200mm (the workhorse joist for most domestic floors and roofs) costs £4.70 – £6.10 per metre. The gap between online specialist merchants like UK Timber (lower end) and high-street retailers like Wickes (higher end) is noticeable. Wickes prices include the convenience of click-and-collect but you pay for it.

For a complete roof structure on a single-storey extension (rafters, ceiling joists, purlins, wall plate, ridge board), expect a total timber material cost of £1,200–2,000.

Travis Perkins, Jewson, and Wickes all carry C16 in standard sizes. Jewson offers both "Standard" and "Premium" tiers (Premium has fewer defects and more consistent grain). Travis Perkins stocks the widest range of lengths including 6.0m. Local timber merchants and independent builders' merchants consistently undercut the national chains by 10-20%, and if you're buying 30+ lengths for a roof, that saving adds up to hundreds of pounds.

C24 timber costs approximately 15-25% more than C16 in equivalent sizes. If your structural engineer has specified C16, don't pay the C24 premium. It won't make your floor stiffer in any meaningful way because the size, not just the grade, determines the stiffness.

C16 vs C24: when does it matter?

C24 has a characteristic bending strength of 24 N/mm2 versus C16's 16 N/mm2. The modulus of elasticity (a measure of stiffness) is 10,800 N/mm2 for C24 versus 8,800 N/mm2 for C16. In practical terms, C24 can span further with the same cross-section, or use a smaller section for the same span.

Your structural engineer specifies C24 when:

• The span exceeds what C16 can handle in the largest standard section size
• Deflection must be minimised (exposed beams, long open-plan floor spans)
• The load is higher than standard domestic (heavy equipment, water tanks, specialist applications)
• Building regulations require it for the specific application

For a typical single-storey extension with spans under 4m and standard domestic loading, C16 in the correct size handles everything. C24 becomes necessary for longer spans, deeper open-plan areas, or where the engineer's calculations show C16 at the maximum section can't meet the deflection limit.

C24 timber is predominantly imported from slow-growing northern European forests (Baltic states, Scandinavia). C16 is commonly home-grown (UK and Scandinavian). Specifying C16 with FSC certification supports the UK-grown timber supply chain. C24 also has a known tendency to warp more than C16 in some applications, which is counterintuitive but noted by experienced builders. The tighter grain that gives C24 its strength also makes it more prone to movement during moisture changes.

Where you'll need this

C16 structural timber appears at multiple stages of any extension or renovation project involving timber framing:

• Roof structure - rafters, ceiling joists, purlins, wall plate, and ridge board. Most domestic roofs use C16 unless the structural engineer specifies C24 for specific members.
• Insulation - the timber framework between which insulation is fitted, including rafter spaces and stud wall cavities.
• Flooring - floor joists in timber-framed upper floors or raised ground floors.

These uses span the structure and first-fix phases of any project that involves new floors or roofs.

Common mistakes

Using ungraded timber in structural positions. Sawn timber from a general timber yard, without a grade stamp, is not structural timber regardless of what it looks like. Building control inspectors check every piece. One unstamped joist means the inspection fails and you're pulling up the boarding to replace it. Buy from a merchant that stocks graded, stamped C16.

Ordering C16 when the structural engineer specified C24. Read the drawings. The strength class is written on every structural calculation sheet and on the structural layout drawing. If the spec says C24, that's what you buy. If C24 isn't available locally, the correct response is to call the structural engineer and ask them to redesign using a larger C16 section. The wrong response is to assume C16 will be close enough.

Fixing wet treated timber without acclimatisation. Treated C16 arrives saturated from the pressure-treatment process. Fixing it immediately means the timber shrinks and twists as it dries over subsequent weeks. Floor joists shrink away from joist hangers. Rafter connections loosen. Boards lift and squeak. Stack the delivery in a dry, ventilated area for at least a few days before cutting and fixing. A week is better.

Not re-sealing cut ends. Every cut through treated timber exposes untreated end grain. The preservative only penetrates a few millimetres from the surface. Brush-on end-grain preservative costs very little for a tin that covers hundreds of cuts. Skipping this step is false economy when the timber is being concealed inside a floor or roof structure for the next 50 years.

Storing timber flat on wet ground. Timber stored on wet concrete or muddy ground absorbs moisture from below and warps before you even use it. Stack on bearers, use stickers between courses, and keep the top covered. Five minutes of setup on delivery day saves hundreds of pounds in wasted, warped timber.

Ignoring the nominal vs actual size difference. When your structural engineer's drawing says "47x200mm C16," the actual timber you buy is approximately 45x195mm (regularised). This is normal. The structural calculations account for the actual finished size. But if you're measuring cavity widths or fitting timber into specific gaps, use the actual dimension, not the nominal.