Floor Joist Span TableBS 5268 & Eurocode 5 Reference
Comprehensive floor joist span tables for UK residential construction. Find maximum allowable spans for every common timber size, spacing, grade, and load condition per British Standards and Eurocode 5.
Quick Answer
A 47x200 mm C24 floor joist at 400 mm centres can span up to approximately 3.87 m for a domestic floor with 1.5 kN/m² imposed load. A 47x250 mm C24 joist at 400 mm centres spans up to approximately 4.82 m. Always verify spans against TRADA span tables and your local Building Control requirements.
Understanding Floor Joist Spans
A floor joist span is the horizontal distance a joist can safely bridge between supports without excessive deflection or structural failure. In the UK, span tables are published by TRADA (Timber Research and Development Association) and referenced in Approved Document A of the Building Regulations, based on BS 5268 (permissible stress design) and Eurocode 5 / BS EN 1995 (limit state design).
Floor joist spans are calculated for two load conditions: imposed load (people, furniture, movable weight — typically 1.5 kN/m² for domestic floors) and dead load (the permanent weight of the structure — typically 0.25 to 0.50 kN/m² for standard framing with plasterboard ceiling below). The combined total load determines the required joist size and spacing.
Deflection limits also control span lengths. Eurocode 5 requires floor joists to limit deflection to span/300 for imposed load and span/250 for total load in domestic applications. Stiffer floors feel more solid underfoot and prevent cracking in plasterboard ceilings below and tile finishes above.
Floor Joist Span Tables by Timber Grade
The following span tables are based on TRADA guidance for domestic floors with 1.5 kN/m² imposed load and 0.25 kN/m² dead load (plasterboard ceiling, no heavy finishes). Spans are listed as maximum clear span in metres. Always use the grade stamped on your timber.
C16 Graded Softwood
C16 — Fb = 5.3 N/mm², E = 8,000 N/mm²
| Joist Size | 400 mm c/c | 450 mm c/c | 600 mm c/c |
|---|---|---|---|
| 47Ă—150 mm | 2.71 m | 2.60 m | 2.38 m |
| 47Ă—175 mm | 3.19 m | 3.07 m | 2.81 m |
| 47Ă—200 mm | 3.49 m | 3.36 m | 3.07 m |
| 47Ă—250 mm | 4.35 m | 4.18 m | 3.83 m |
C24 Graded Softwood
C24 — Fb = 7.5 N/mm², E = 11,000 N/mm²
| Joist Size | 400 mm c/c | 450 mm c/c | 600 mm c/c |
|---|---|---|---|
| 47Ă—150 mm | 3.07 m | 2.95 m | 2.71 m |
| 47Ă—175 mm | 3.61 m | 3.47 m | 3.19 m |
| 47Ă—200 mm | 3.87 m | 3.73 m | 3.49 m |
| 47Ă—250 mm | 4.82 m | 4.64 m | 4.35 m |
C16 — Doubled (twin joists)
C16 — twin 47 mm joists bolted together, effective 94 mm width
| Joist Size | 400 mm c/c | 450 mm c/c | 600 mm c/c |
|---|---|---|---|
| 2Ă—47Ă—200 mm | 4.35 m | 4.18 m | 3.83 m |
| 2Ă—47Ă—250 mm | 5.42 m | 5.22 m | 4.78 m |
C24 — Doubled (twin joists)
C24 — twin 47 mm joists bolted together, effective 94 mm width
| Joist Size | 400 mm c/c | 450 mm c/c | 600 mm c/c |
|---|---|---|---|
| 2Ă—47Ă—200 mm | 4.82 m | 4.64 m | 4.35 m |
| 2Ă—47Ă—250 mm | 6.02 m | 5.79 m | 5.42 m |
Floor Load Requirements (Building Regulations)
Building Regulations Approved Document A and BS EN 1991-1-1 specify minimum design loads for residential floors. These loads determine which span table to use.
- Domestic floors (all habitable rooms): 1.5 kN/m² imposed load + 0.25 kN/m² dead load (plasterboard ceiling) = 1.75 kN/m² total
- Loft floors used for storage: 0.75 kN/m² imposed load (limited storage) or 1.5 kN/m² if the loft is habitable
- Dead load includes joist self-weight, decking, finish flooring, and any ceiling below. If using heavy finishes like tiles on cement board, increase dead load to 0.75–1.0 kN/m²
- Partition loading: an additional 0.5 kN/m² is added where lightweight partitions may be placed on the floor
- Concentrated loads: floors must also support a 1.4 kN concentrated load on a 50 mm square area, which is typically satisfied when deflection limits are met
For most UK domestic floors, use the 1.5 kN/m² imposed load span table. Add 0.5 kN/m² if the floor may carry lightweight partitions. When in doubt, design for the higher load — the cost difference is minimal.
Factors That Affect Joist Span
Beyond the basic span table values, several real-world factors can increase or decrease your required joist size.
- Timber grade: C24 allows longer spans than C16. Always check the grade stamp on your timber — unmarked timber cannot be relied upon for structural use.
- Joist spacing: 400 mm centres allow the longest spans. 600 mm centres are economical but limit span length and require thicker decking (22 mm minimum).
- Bearing length: Joists must bear at least 50 mm on timber or steel and 90 mm on masonry per Approved Document A.
- Notching and drilling: Notches must be in the first or last quarter of the span and no deeper than 0.125 times the joist depth. Holes must be on the neutral axis, between 0.25 and 0.4 of the span, and no larger than 0.25 times the joist depth, per TRADA guidance.
- Heavy finishes: Tile, stone, or thick timber flooring adds significant dead load. When dead load exceeds 0.5 kN/m², use the appropriate adjusted span table or consult an engineer.
- Cantilevers: Joists extending beyond a bearing wall are limited to one-quarter of the backspan.
Always use the actual grade stamped on your timber. When multiple factors overlap (heavy finishes + wide spacing + long span), consider upsizing joists by one depth increment for extra safety margin and a stiffer floor.
Building Regulations & Standards for Floor Joists
The following standards and regulations govern floor joist design in UK residential construction.
- Approved Document A (Structure) — references span tables for timber floor joists in domestic buildings
- BS 5268 Part 2 — permissible stress design of structural timber (being superseded by Eurocode 5 but still widely referenced)
- BS EN 1995-1-1 (Eurocode 5) — limit state design of timber structures, the current European standard for structural timber design
- BS EN 338 — strength classes for structural timber (C16, C24, etc.)
- TRADA Span Tables — the primary reference for UK builders, providing ready-made span tables for common domestic loading conditions
- BS EN 1991-1-1 — imposed loads on buildings, specifying the 1.5 kN/m² domestic floor load
- BS 8103-3 — structural design of low-rise buildings, specifically timber floors
TRADA span tables are the practical reference used by most UK builders and Building Control officers. They are derived from Eurocode 5 calculations and cover all common domestic loading conditions.
Calculate Your Floor Framing Materials
Once you know your joist size and spacing, use our calculators to work out exactly how many joists, hangers, and fixings you need.