C24 Structural Timber: When Your Engineer Specifies the Stronger Grade

Your structural engineer's drawings say C24. Your builder says C16 is fine, it's cheaper and he can get it today. He's wrong to make that call without the engineer's sign-off. When building control inspect the roof structure, they check grade stamps on every timber member against the approved drawings. If the stamps say C16 where the drawings say C24, you're looking at a non-compliance notice, potential removal of completed work, and a build that stops dead.

What it is and what it's for

C24 is a strength grade for structural softwood timber, defined by BS EN 338 (the European standard that classifies timber by its mechanical properties). The "C" stands for coniferous (softwood). The "24" means the timber has a characteristic bending strength of 24 N/mm2, the force per square millimetre it can resist before breaking when loaded across its span.

For comparison, C16 timber has a bending strength of 16 N/mm2. That's the other standard grade used in UK domestic construction. Between them, C16 and C24 cover virtually every structural timber application in a house extension: floor joists, roof rafters, ceiling joists, purlins (horizontal beams that support rafters mid-span), wall plates (the timber that sits on top of the blockwork to receive the rafters), and ridge beams.

C24 timber is predominantly imported from Scandinavia and the Baltic states. The cold, slow-growing conditions produce denser wood with tighter growth rings, which is what gives it the higher strength. UK-grown softwood (mainly Sitka spruce) is typically graded to C16. The UK Government's 2025 Timber in Construction Roadmap acknowledges that C24 is over-specified in many projects where C16 would be adequate, driven by "greater market familiarity with the higher grade of imported timber" rather than engineering necessity.

So why does your engineer specify C24? Three reasons: longer spans, higher loads, or smaller timber sections. A 47x195mm C24 joist at 400mm centres can span 4.55 metres. The same joist in C16 would need to be a size larger, or spaced closer together, to carry the same load. If your extension floor needs a long clear span, C24 may be the only option at that section size. You could use a deeper C16 joist (47x220mm) to achieve a similar span, but that requires the engineer to recalculate and building control to approve the change.

Strength properties

The difference between C16 and C24 is not just bending strength. Every mechanical property is higher in C24, and some of the less obvious ones matter more than you'd expect.

Look at compression perpendicular to grain: 7.9 versus 1.8 N/mm2. That's a 4.4x difference, not the 1.5x you see in bending strength. This property governs how much load timber can take at its bearing points, where a joist sits on a wall or inside a joist hanger. Simpson Strong-Tie, the dominant manufacturer of timber connectors in the UK, rates their joist hanger capacities for C24 timber. If you use C16, you must apply reduction factors: 0.90 for lateral loads, 0.86 for axial loads. On a balcony or deck with cantilever connections, that reduction can push the connection below its required capacity. This is not theoretical. A BuildHub thread documents a case where a structural engineer specified 150x50mm C24 for a balcony specifically because the joist hanger connections required C24's perpendicular compression strength.

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How strength grading works

Every piece of structural timber sold in the UK must carry a grade stamp. No stamp, no structural use. Building control inspectors check these stamps on site, and if a piece of timber in a structural position is unmarked, they can require its removal.

Grading happens two ways. Machine grading feeds each piece through a stress-grading machine that measures stiffness by bending or vibrating the timber and recording its response. This is fast, consistent, and how most imported C24 is graded. Visual grading involves a trained grader assessing each piece by eye: checking knot sizes, slope of grain, rate of growth, and defects against published criteria. Visual grading is more common for UK-grown timber and lower-grade material.

The grade stamp tells you everything you need to verify: the strength class (C24), whether it's kiln dried (KD), the grading body that certified it, the species, and the reference standard (BS EN 338). "KD" means the timber's moisture content was reduced to below 20% in a kiln before grading. Kiln-dried timber is dimensionally more stable than air-dried, meaning less warping and twisting after installation.

Types, sizes, and availability

C24 structural timber comes in standard cross-sections based on the old imperial sizes, now regularised (planed to consistent dimensions). The "47mm" width is what you'll see at every merchant. It was originally 2 inches (50mm) before regularising.

Standard sections and their typical applications:

Those floor joist spans are for 47mm C24 at 400mm centres with a 1.5 kN/m2 imposed load (the standard domestic loading for habitable rooms). At 600mm centres, spans reduce by roughly 10-15%. Your structural engineer's specification will state the exact section, grade, spacing, and span for your project, based on the actual loads and layout.

Lengths run from 2.4m to 7.2m in 600mm increments, though not every length is stocked at every merchant. The 3.0m, 3.6m, and 4.8m lengths are universally available. Longer lengths (5.4m, 6.0m, 7.2m) may need ordering, especially at the 47x195mm and 47x220mm sections.

How to work with C24 timber

Handling and weight

C24 is denser than C16 (420 kg/m3 versus 370 kg/m3), which means it's heavier per metre. A 4.8m length of 47x195mm C24 weighs roughly 19 kg. That's manageable for one person, but when you're unloading a delivery of 40 or 50 pieces, the cumulative weight adds up. Roof rafters in particular need manhandling up scaffolding. Plan for two people on delivery day.

The wet timber problem

This is the single most common practical issue with C24 and it's absent from every competitor guide. Five out of eight forum threads reviewed for this page mention it.

C24 treated timber frequently arrives damp. Sometimes very damp. One forum poster reported building an entire loft roof from C24 that "arrived covered in ice." This is normal trade practice, not a defect. The timber is treated with preservative under pressure, and that process saturates it with liquid. Merchants store treated timber outdoors. If it rained last week, your delivery will be wet.

The problem comes when wet timber is installed into a sealed structure before it has dried. As it dries, it shrinks, warps, and twists. In a floor, this creates bounce and squeaking. In a roof, it causes rafters to bow and potentially pull away from connectors. Wet timber can be 4mm wider than its nominal dimension, meaning joints that were tight at installation loosen as drying occurs.

Cutting and fixing

C24 cuts with the same tools as C16. A circular saw or mitre saw for cross-cuts, a hand saw for on-site trimming. The denser grain means slightly more resistance when cutting, but nothing a sharp blade can't handle.

For connections, C24 works well with all standard fixings: nails, screws, joist hangers, and framing connectors. The higher density actually improves nail and screw holding capacity. When your carpenter nails rafters to a wall plate or fits joist hangers, the connections are stronger in C24 than they would be in C16. This is one of the reasons connector manufacturers rate their products for C24.

Storage on site

Timber warps when stored badly. Keep it off the ground on bearers. Keep it flat. Keep it covered from rain but allow air circulation (a tarpaulin draped over the top, not wrapped around it). Don't lean long pieces against a wall for days; they'll develop a permanent bow. If timber arrives and can't be installed for more than a week, restick it (place thin spacer sticks between layers) so air can circulate and drying happens evenly.

Cost and where to buy

C24 costs more than C16 at every section size, and the premium is larger than most merchant marketing suggests. Several timber retailers claim a "10-15% premium." That may reflect trade account pricing or narrow size ranges, but at retail, the real difference is steeper.

Current retail pricing from major UK merchants (2026):

Specialist timber merchants typically offer the best value. Get quotes from at least three suppliers before ordering.

Where to buy

Specialist timber merchants (Howarth, Armstrong Supplies, local independents) consistently offer the best per-metre rates and hold the widest stock. If your project needs significant volumes (a full roof's worth of timber), these should be your first call.

Builders' merchants (Travis Perkins, Jewson) stock standard C24 sizes and can order larger sections. Trade accounts get better rates than list price. Jewson branches vary in stock, so check availability before relying on next-day collection.

DIY retailers (Wickes) carry C24 but at the top of the price range. Wickes is convenient for collecting a few pieces but expensive for a full order. Their online pricing is transparent, which makes them useful as a benchmark, even if you buy elsewhere.

When C24 is required (and when it isn't)

This is the question your structural engineer answers, and you should not override their specification. But understanding the logic helps you have an informed conversation.

C24 is typically specified for:

• Floor joists spanning more than about 4 metres (the exact threshold depends on section size and spacing)
• Main roof rafters on extensions with wider footprints
• Purlins carrying heavy roof loads (concrete tiles on a wide roof)
• Timber ridge beams (where the engineer has decided timber rather than steel can carry the ridge load)
• Balcony and deck structures where connector capacity is critical

C16 is usually adequate for:

• Ceiling joists (lightly loaded, just supporting the ceiling and loft insulation)
• Short-span floor joists (under 3.5 metres, depending on section)
• Wall plate timber (sits on top of the blockwork, distributes rafter loads)
• Studwork and non-load-bearing partitions
• Noggins (short bracing pieces between joists)

The UK Government's position is clear: C16 is "strong enough for the demands of most construction." The push toward C24 in domestic projects is often habit rather than engineering requirement. But when your engineer does specify C24, there's a reason, and the reason is usually span length or connector loading. Accept the specification. Don't let cost savings on timber create a building control problem that costs ten times more to resolve.

Alternatives

C16 Structural Timber is the lower-grade alternative and costs 30-50% less at retail. For standard domestic floor joists, ceiling joists, and light-load applications, C16 is perfectly adequate and is the default grade for mainstream UK housebuilding. If your spans are short enough and your engineer hasn't specified C24, there's no benefit to paying more for it. C16 at £2.50 – £3.10 (47x100mm) and £4.70 – £6.10 (47x200mm) is the budget-conscious choice where the span tables allow it.

Steel beams are the alternative for primary load-bearing applications where timber can't provide the required span or load capacity. A steel ridge beam handles longer spans and heavier loads than any solid timber option. Steel requires structural engineer calculations, building control approval, and fire protection (double-layer plasterboard to achieve 30 minutes fire resistance). The cost difference is substantial, but so is the capability.

Glulam (glued laminated timber) and LVL (laminated veneer lumber) are engineered timber products that bridge the gap between solid C24 and steel. They can achieve longer spans than solid timber at lower weight than steel. Your structural engineer may specify glulam for a ridge beam where steel feels like overkill but solid C24 can't reach the required span. These products are specialist-ordered, not stocked at general merchants.

Flitch beams (a steel plate sandwiched between two timber members, bolted together) are another intermediate option for moderate-span ridge beams. Less common than steel or glulam but still specified in domestic work.

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The substitution trap

This deserves its own section because it's the single most expensive mistake a homeowner can make with structural timber, and it happens on real extension sites.

The scenario: your builder or carpenter can't source C24 locally, or it's more expensive, or they've always used C16 and never had a problem. They suggest using a larger section of C16 instead. "A 9x2 C16 is stronger than an 8x2 C24, right?" In bending strength, that can be true. But the specification on your approved drawings says C24. And those drawings were submitted to building control. Changing the material without new structural calculations and revised building control approval is non-compliance.

Building control can issue a Section 36 notice requiring removal or alteration of non-compliant work within 28 days. These notices can be issued up to 10 years after completion. Without a completion certificate (which is withheld if work doesn't comply), the non-compliance surfaces on conveyancing searches when you sell the property.

The reverse also matters. Don't over-specify C24 where C16 is adequate. You're paying a 30-50% premium for timber that provides no structural benefit in that position. Ceiling joists that will never carry more than insulation and the occasional person in the loft do not need C24.

Where you'll need this

• Roof structure - rafters, purlins, and timber ridge beams where the structural engineer specifies C24 for longer spans or higher loading
• Insulation - structural framing that also houses insulation, where spans require the higher grade

C24 timber appears in the structural phase of any extension or renovation project where the structural engineer's calculations demand higher-grade timber for specific members.

Common mistakes

Assuming all structural timber is the same grade. When timber arrives on site, check the grade stamps before it's installed. A delivery of mixed C16 and C24 (which is normal if your project uses both) should be separated immediately and stacked on clearly marked pallets. Once timber is cut and installed, it's much harder to verify the grade stamp.

Not checking structural engineer drawings before ordering. The SE specification is not a suggestion. It states the exact section size, grade, spacing, and span for every structural member. Order exactly what's specified. If your builder orders the timber, confirm the order matches the drawings before delivery.

Installing wet timber in a sealed roof. C24 treated timber arrives damp far more often than dry. Enclosing damp timber in a vapour-sealed roof structure traps moisture, promotes mould growth, and causes dimensional movement that weakens connections. Allow timber to acclimatise. If the build programme won't allow drying time, discuss the issue with your builder before proceeding.

Buying C24 for everything. This is the flip side of substitution. Some builders and merchants default to C24 for all structural timber because "it's stronger." On a typical domestic extension, C24 is needed for a fraction of the timber. The ceiling joists, noggins, wall plate, and short-span members can all be C16. Over-specifying C24 across the board inflates your timber cost by 30-50% for no structural benefit.

Cutting graded timber and reusing the offcuts structurally. Offcuts from C24 joists are not C24 members. Once you alter the cross-section by ripping (cutting lengthways), the grade is invalidated. Short offcuts from cross-cutting retain their grade for their remaining length, but ripped pieces are ungraded.