Maximum Joist and Beam Spans: Timber Sizing Tables for UK Construction

Summary

Timber span tables tell you the maximum permissible clear span for a solid timber member of a given size, strength class, and spacing under a specified load. In the UK, the authoritative span tables are published by Timber Development UK (formerly TRADA) and are referenced by Approved Document A of the Building Regulations as the accepted method of demonstrating compliance for domestic timber floors, ceilings, and roofs. The tables are calculated to BS EN 1995-1-1 (Eurocode 5) and replace the older BS 5268 permissible stress tables, though many building control departments still accept either basis. Getting the timber size wrong is a notifiable Building Regulations matter — undersized joists will fail inspection, and oversized joists waste money. These tables are the carpenter's and builder's primary tool for sizing structural timber without needing a bespoke engineer's calculation.

Key Facts

• Span tables apply to simply supported solid timber members in domestic dwellings (houses and flats up to three storeys).
• The two standard structural softwood grades in the UK are C16 and C24, defined by BS EN 338. C24 is approximately 50% stronger in bending and 37.5% stiffer than C16.
• Standard domestic floor loading is 1.5 kN/m2 imposed load with a dead load (excluding joist self-weight) of not more than 0.50 kN/m2 — this covers typical chipboard decking and plasterboard ceiling.
• Standard joist widths are 38mm and 47mm. The 47mm width is now the most common for domestic work and is what merchants stock as standard.
• Common joist spacings are 400mm and 600mm centres. 400mm centres give shorter spans but stiffer floors; 600mm centres are more economical on timber but reduce maximum span.
• Joists spanning more than 2.5m require herringbone or solid strutting at mid-span. Joists spanning more than 4.5m require two rows of strutting at the one-third and two-third span positions.
• If your design specifies C24, you must not substitute C16 without recalculating — this is a structural downgrade that will fail building control inspection.
• Span tables do not apply to engineered timber products (I-joists, LVL, glulam) — these require manufacturer-specific design data or a structural engineer's calculation.

Timber Grades Explained

What's the difference between C16 and C24?

C16 and C24 are strength classes for structural softwood (coniferous) timber, defined in BS EN 338:2016. The "C" stands for coniferous and the number is the characteristic bending strength in N/mm2.

Practical guidance: C24 is now the default grade for most structural applications and is what most timber merchants supply as standard. C16 is adequate for non-structural or lightly loaded members (e.g. ceiling joists carrying only insulation and plasterboard, short-span stud walls). If in doubt, specify C24 — the cost premium is modest and it gives you longer spans and a stiffer floor.

Timber must be strength graded and stamped by an accredited grader. The grade stamp on every piece of structural timber confirms the strength class. Ungraded timber must not be used for structural purposes.

Floor Joist Span Tables

Domestic floor joists — maximum clear spans

Based on BS 5268-7.1 / Eurocode 5 span tables. Imposed load 1.5 kN/m2, dead load 0.25-0.50 kN/m2 (typical domestic floor with chipboard deck and plasterboard ceiling).

Notes:

• Spans are permissible clear spans — measured between faces of supports, not centres.
• These assume simply supported joists (i.e. not continuous over intermediate supports).
• Strutting is required for spans over 2.5m.
• For dead loads exceeding 0.50 kN/m2 (e.g. heavy screed, stone tile finish), spans reduce — consult the full TRADA tables.
• The 47 x 170mm and 47 x 195mm sizes are the workhorses of domestic floor construction, covering most room spans.

Quick sizing guide for common room widths

Ceiling Joist Span Tables

Ceiling joists — maximum clear spans (no loft storage)

Based on BS 5268-7.3. Imposed load 0.25 kN/m2 (access for maintenance only, no storage), dead load not exceeding 0.25 kN/m2 (plasterboard and insulation).

Notes:

• These spans are for non-habitable loft space with access for maintenance only.
• If the loft is used for storage, the imposed load increases to 0.75 kN/m2 and spans reduce significantly — treat as a floor joist calculation with the appropriate load.
• If the loft is a habitable room, use the floor joist span tables (1.5 kN/m2 imposed load).
• Ceiling joists also serve as ties in a coupled rafter roof — the span table assumes this tying function.

Roof Rafter Span Tables

Pitched roof rafters — maximum clear spans

Based on BS 5268-7.5. Imposed load 1.00 kN/m2 (snow + wind for most UK locations), dead load not exceeding 0.50 kN/m2 (tiles, felt, battens). Roof pitch 22.5 degrees to less than 30 degrees.

Rafters at steeper pitches (30 degrees to less than 45 degrees)

Steeper roof pitches allow slightly longer spans because the rafter carries less of the load in bending:

Notes:

• Rafter spans are measured along the slope (not the horizontal plan distance).
• These tables assume rafters are restrained by purlins, ceiling ties, or collar ties at appropriate intervals.
• For concrete interlocking tiles (heavier dead load), spans reduce — check against the specific dead load column in the full TRADA tables.
• For lightweight coverings (e.g. metal sheeting with dead load not exceeding 0.25 kN/m2), spans may be slightly longer.

Detailed Guidance

How do I read a span table?

1. Identify your member type — floor joist, ceiling joist, rafter, or purlin. Each has its own table with different assumed loadings.
2. Determine your dead load — this is everything the member carries apart from people and furniture (imposed load). For a typical domestic floor, dead load is 0.25-0.50 kN/m2 for chipboard and plasterboard. Heavier finishes (screed, tiles) increase the dead load.
3. Select your timber grade — C16 or C24. Check the stamp on your timber.
4. Select your spacing — typically 400mm or 600mm centres. Some tables also show 450mm centres.
5. Read across to find the maximum clear span for your chosen joist size. If your required span exceeds the value shown, you need a deeper joist, a higher grade, or closer spacing.

The span shown is the maximum permissible clear span — the distance between the inner faces of the supports. It is not the overall joist length (which must include bearing at each end, typically a minimum of 40-50mm).

When do I need an engineer's calculation?

You need a structural engineer (or competent timber frame designer) when:

• The span exceeds what the standard tables cover (typically around 5-6m for solid timber).
• The loading is non-standard — for example, a bath over 1,000 litres, a grand piano, heavy plant equipment, or commercial loading (exceeding 1.5 kN/m2).
• The member is continuous over intermediate supports (the tables assume simply supported).
• You are using the joist as a trimmer or trimming joist around an opening (stairwell, chimney, loft hatch) — increased loads from the trimmed joists require calculation.
• The roof has non-standard geometry, large openings, or valley or hip configurations.
• You are specifying engineered timber products (I-joists, LVL, glulam) — these are not covered by the standard TRADA span tables.
• Building control specifically requests it — some authorities require engineer's calculations for any structural alteration.

What about engineered timber (I-joists, LVL)?

Engineered timber products offer significant advantages over solid sawn timber for longer spans:

I-joists (e.g. JJI Joists, Easi-Joists) have an OSB or plywood web with solid timber or LVL flanges. They are lightweight, dimensionally stable, and can span 6-8m in domestic applications. They are the standard choice for new-build floors and are increasingly used in renovation work.

LVL (Laminated Veneer Lumber) is made from thin veneers bonded together. It offers high, consistent strength and is used for beams, lintels, and ridge beams. Typical domestic LVL beams span 4-7m depending on section size and loading.

Glulam (Glued Laminated Timber) is made from multiple layers of solid timber bonded together. It can span very long distances (10m+ for domestic applications) and is often left exposed for architectural effect.

All engineered timber products require manufacturer-specific design data or an engineer's calculation. The TRADA span tables do not cover them. Your timber supplier or the manufacturer's technical department can usually provide span tables or a free design service.

Frequently Asked Questions

Can I use C16 instead of C24 to save money?

Only if the span table confirms the C16 size is adequate for your span and loading. C16 is typically 10-15% cheaper than C24, but you will often need to go up one joist depth to compensate, which may negate the saving. For most domestic floor and roof work, C24 is the more economical choice overall because it allows longer spans from the same section size. Never substitute C16 for C24 where C24 has been specified without checking the span tables.

What bearing length do joists need at supports?

A minimum of 40mm bearing on timber wall plates or steel beams. NHBC standards and most building control authorities require at least 40mm. Where joists bear on masonry, a minimum bearing of 75mm is typical. Joist hangers (e.g. Simpson Strong-Tie BAT or BEA types) are an alternative to direct bearing and are required where the joist does not have adequate bearing.

Do I need to notify Building Control for a floor joist replacement?

Replacing joists like for like (same size and grade) in an existing structure does not normally require a Building Regulations application. However, any alteration to the structure (e.g. forming a new opening, changing joist sizes, adding a new floor, or converting a loft) is notifiable under the Building Regulations and requires either a Building Notice or Full Plans application.

What is strutting and when is it required?

Strutting provides lateral restraint to prevent joists from twisting or buckling sideways under load. It is required for:

• Spans over 2.5m — one row of strutting at mid-span.
• Spans over 4.5m — two rows of strutting at one-third and two-third span positions.

Strutting can be herringbone (timber or galvanised steel noggins set at 45 degrees between joists) or solid (blocking of at least 38mm thick, extending to at least three-quarters of the joist depth). Proprietary galvanised steel herringbone struts are the most common choice for speed and ease of installation.

How do I account for notches and holes in joists?

Notches and holes for pipes and cables reduce the effective cross-section and must be positioned within permitted zones to avoid compromising the structural capacity:

• Notches: only in the top of the joist, within the zone 0.07 to 0.25 of the span from each support. Maximum notch depth is 0.125 times the joist depth.
• Holes: drilled on the neutral axis (centre line) of the joist, within the zone 0.25 to 0.4 of the span from each support. Maximum hole diameter is 0.25 times the joist depth. Holes must be at least 3 diameters apart (centre to centre).

Notches and holes outside these zones or exceeding these limits require an engineer's check.

Regulations & Standards

• Building Regulations 2010, Approved Document A: Structure — the overarching regulatory document for structural design in England and Wales. The original timber span tables were included in Appendix A of earlier editions.

• BS EN 1995-1-1 (Eurocode 5) — the current European/British standard for structural timber design. The TRADA span tables are calculated to Eurocode 5.

• BS EN 338:2016 — defines timber strength classes (C16, C24, etc.) and their characteristic mechanical properties.

• BS EN 14081-1 — requirements for strength grading of structural timber, including machine grading and visual grading.

• BS 5268 (Parts 2 and 7) — the former British Standard for timber design (permissible stress method). Officially superseded by Eurocode 5 but still widely referenced and accepted by many building control bodies.

• NHBC Standards Chapter 6.4 — timber and concrete upper floors. Includes span tables in Appendix 6.4-A derived from the TRADA publication.

• Timber Development UK (TDUK) — C16 and C24 Span Tables (free download)

• Timber Beam Calculator — Free UK Span Tables (BS 5268)

• Taylor Maxwell — C16 Span Tables (PDF)

• Taylor Maxwell — C24 Span Tables (PDF)

• BSW Timber — Strength Graded Timber & Span Tables (PDF)

• Lancaster City Council — Building Control Guidance: Domestic Timber Floors (PDF)

• Lancaster City Council — Building Control Guidance: Ceilings and Flat Roofs (PDF)

• NHBC Standards 2025 — Timber Joist Spans (6.4.8)

• Ryedale Timber — C16 vs C24 Timber Guide

• TDUK — C16 and C24 Timber: Your Questions Answered

• Right Survey — Floor Joist Span Tables for Surveyors

• Right Survey — Rafter Span Tables for Surveyors

• first fix second fix — First fix and second fix stages

• building control — When building control sign-off is needed