Question 1

What is carcassing timber?

Accepted Answer

Carcassing timber is the structural softwood used to construct the internal framework (the carcass) of a building - the skeleton of joists, rafters, studs, plates, and noggins that provides the structural support for floors, roofs, and partition walls. It is a relatively low-cost, mass-produced product supplied in standard sizes and typically not intended to be left visible in the finished building. The most common carcassing species used in the UK are European Whitewood (Picea abies, commonly called 'whitewood spruce') and European Redwood (Pinus sylvestris, commonly called 'redwood pine'). Both are typically mixed together and sold simply as 'whitewood' or 'carcassing' without species distinction. Carcassing is supplied rough-sawn (as cut from the log, with a rough surface and less consistent dimensions) or regularised (machined on two faces to achieve consistent depth, making it much easier to construct flat, level floors and walls).

Question 2

What are C16 and C24 strength grades?

Accepted Answer

C16 and C24 are strength grades for structural softwood defined by BS EN 338. The 'C' stands for 'conifer' (softwood) and the number represents the characteristic bending strength in N/mm2. C16 is the lower grade, adequate for most standard domestic applications including floor joists, rafters, and wall studs at normal spans. C24 is a higher grade with greater bending strength and stiffness, required for longer spans, more heavily loaded applications, or where deflection must be minimised. In practice, C16 is the most widely stocked carcassing grade at builders' merchants and is suitable for the vast majority of domestic construction. C24 must be specifically ordered and is more expensive. Span tables in Approved Document A (Structure) and TRADA's guidance documents specify which grade is required for given joist sizes and spans - checking these tables is essential when specifying floor and roof timbers.

Question 3

What are standard carcassing timber sizes?

Accepted Answer

Carcassing timber is supplied in standard cross-section sizes (in millimetres, width x depth): 38x89 and 38x140 (stud work and light framing); 47x97 (floor joists and rafters for shorter spans); 47x145 (floor joists for medium spans, common rafter size); 47x195 (floor joists for longer spans, typical in modern housing); 47x220 and 47x245 (longer span floor joists and rafters). These are the standard nominal sizes from which specific sections are chosen based on span table calculations. The most widely available sizes at standard builders' merchants are 47x97, 47x145, and 47x195 in 4.8m lengths (with 3.6m and 6.0m also available). Less common sizes may need to be ordered specially. Note that these are regularised (finished) sizes - the timber was cut slightly larger and machined down. Older buildings used imperial dimensions (2x4, 2x6 inch etc.) which do not correspond exactly to metric sizes.

Question 4

Does carcassing timber need to be treated?

Accepted Answer

Whether carcassing timber needs preservative treatment depends on its location and exposure conditions. Building Regulations (Approved Document A) and BS 8103-3 require that timber in locations where it is at risk of dampness or fungal decay must be preservative treated. Specific applications where treatment is required or strongly recommended include: timber built into or close to external masonry walls (which can be damp); timber in ground-floor constructions over unventilated voids; timber roof members in poorly ventilated roof spaces; timber used in bathrooms, kitchens, or other wet areas; timber in contact with concrete or masonry. Treatment is not generally required for internal carcassing in dry conditions (internal first-floor joists in a properly constructed building, internal wall studs, etc.). Pre-treated (green-tinged) carcassing is widely available and only marginally more expensive than untreated - specifying pre-treated timber for all structural carcassing is good practice that eliminates the need to judge case-by-case.

Carcassing Timber

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