UK Timber Sizing Reference Guide
No allowable span values are provided. This reference guide introduces terms and questions to discuss with a qualified designer. Floor joist, ceiling joist, rafter, loft joist and trussed rafter sizing for the UK's most-stocked softwood strength classes — referenced to Eurocode 5 (BS EN 1995-1-1), NHBC Standards Chapter 6.4 and Part A of the Building Regulations.
Building Control sign-off required
Span values on these pages are a planning starting point. UK Building Control approval under Part A of the Building Regulations requires either accepted span tables (TRADA / NHBC) or an engineer's calculation. Loft conversions, open-plan extensions and any unusual loading typically need a structural engineer.
UK timber strength classes covered
C16 is the most widely used softwood strength class in UK residential construction (BS EN 338). Stocked at every UK builder's merchant — typical in CLS-equivalent stud, joist and rafter sections. Lower characteristic bending strength than C24, but suits most common domestic spans — check the exact figures in the published TRADA Eurocode 5 tables.
C24 is a higher-strength softwood class (BS EN 338) used for longer-span joists, rafters and beams. Kiln-dried to ≤20% moisture and machine-stress-graded. Widely available through UK timber merchants — particularly used for floor joists in loft conversions and open-plan extensions.
TR26 is a UK-specific machine-stress-graded softwood class historically defined under BS 5268-3 for prefabricated trussed rafter manufacture (now produced under BS EN 14250). Supplied to TRA-member truss fabricators rather than direct to site — TR26 stock is generally not available at builders' merchants. Note: TR26 is not part of the BS EN 338 strength-class system.
UK span table standards explained
UK timber design uses Eurocode 5 (BS EN 1995-1-1) for new structural calculation, with strength classes defined in BS EN 338. The legacy code BS 5268-2 is still referenced in older span tables but is no longer the basis for new design work in England, Wales, Scotland or Northern Ireland.
For new-build housing, NHBC Standards Chapter 6.4 (Timber and joinery) is the practical specification baseline accepted by NHBC, Premier Guarantee, LABC Warranty and other UK structural warranty providers. Loft conversions, extensions and any structural alteration require Building Control approval under Part A (Structure) of the Building Regulations.