Rafter Span TableComplete UK Roof Framing Reference
Comprehensive rafter span tables for UK residential roof framing. Find maximum allowable spans for every common rafter size, spacing, timber grade, and load condition per British Standards and Eurocode 5.
Quick Answer
A 47x150 mm C24 rafter at 400 mm centres can span up to approximately 3.15 m horizontally for a domestic roof with 0.75 kN/m² imposed load (snow and maintenance). A 47x200 mm C24 rafter at 400 mm centres spans up to approximately 4.20 m. Always verify against TRADA span tables for your specific conditions.
Understanding Rafter Spans
A rafter span is the horizontal projection distance a rafter can cover between its bearing point at the wall plate and the ridge board or ridge beam. In the UK, rafter spans are measured as the horizontal run, not along the slope of the rafter. The actual rafter length is longer than the span because it follows the roof pitch.
Rafter design considers two primary loads: the dead load (tiles, slates, battens, underlay, insulation — typically 0.5 to 1.0 kN/m² depending on covering) and the imposed load, which includes snow load and a minimum 0.6 kN/m² maintenance access load per BS EN 1991-1-3. In snow-prone areas of Scotland and upland England, higher snow loads apply per the UK National Annex.
Eurocode 5 provides the basis for span calculations, with TRADA publishing ready-made span tables for common UK conditions. Rafters must satisfy deflection limits of span/200 for imposed load. Steeper pitches allow slightly longer spans for the same rafter size because the load component perpendicular to the rafter is reduced.
Rafter Span Tables by Timber Grade (0.75 kN/m² Imposed, Light Covering)
The following tables are based on TRADA guidance for rafters with 0.75 kN/m² imposed load (snow + maintenance) and light roof covering (slates or lightweight tiles, dead load ≤0.50 kN/m²). Spans are horizontal projection in metres.
C16 Graded Softwood — Light Covering
C16 — 0.75 kN/m² imposed, ≤0.50 kN/m² dead (slates/lightweight tiles)
| Rafter Size | 400 mm c/c | 450 mm c/c | 600 mm c/c |
|---|---|---|---|
| 47×100 mm | 2.06 m | 1.98 m | 1.82 m |
| 47×125 mm | 2.42 m | 2.33 m | 2.14 m |
| 47×150 mm | 2.82 m | 2.71 m | 2.49 m |
| 47×175 mm | 3.22 m | 3.10 m | 2.84 m |
| 47×200 mm | 3.75 m | 3.61 m | 3.31 m |
C24 Graded Softwood — Light Covering
C24 — 0.75 kN/m² imposed, ≤0.50 kN/m² dead (slates/lightweight tiles)
| Rafter Size | 400 mm c/c | 450 mm c/c | 600 mm c/c |
|---|---|---|---|
| 47×100 mm | 2.34 m | 2.25 m | 2.06 m |
| 47×125 mm | 2.75 m | 2.64 m | 2.42 m |
| 47×150 mm | 3.15 m | 3.03 m | 2.78 m |
| 47×175 mm | 3.66 m | 3.52 m | 3.22 m |
| 47×200 mm | 4.20 m | 4.04 m | 3.70 m |
C16 Graded Softwood — Heavy Covering
C16 — 0.75 kN/m² imposed, 0.50–1.0 kN/m² dead (concrete tiles, heavy slates)
| Rafter Size | 400 mm c/c | 450 mm c/c | 600 mm c/c |
|---|---|---|---|
| 47×100 mm | 1.82 m | 1.75 m | 1.61 m |
| 47×125 mm | 2.14 m | 2.06 m | 1.89 m |
| 47×150 mm | 2.49 m | 2.40 m | 2.20 m |
| 47×175 mm | 2.84 m | 2.73 m | 2.51 m |
| 47×200 mm | 3.31 m | 3.19 m | 2.92 m |
C24 Graded Softwood — Heavy Covering
C24 — 0.75 kN/m² imposed, 0.50–1.0 kN/m² dead (concrete tiles, heavy slates)
| Rafter Size | 400 mm c/c | 450 mm c/c | 600 mm c/c |
|---|---|---|---|
| 47×100 mm | 2.06 m | 1.98 m | 1.82 m |
| 47×125 mm | 2.42 m | 2.33 m | 2.14 m |
| 47×150 mm | 2.82 m | 2.71 m | 2.49 m |
| 47×175 mm | 3.22 m | 3.10 m | 2.84 m |
| 47×200 mm | 3.75 m | 3.61 m | 3.31 m |
Snow Load Impact on Rafter Spans
Snow load is a key consideration for UK roof design, particularly in Scotland, northern England, and upland areas. BS EN 1991-1-3 and the UK National Annex define ground snow loads which are converted to roof snow loads.
- Zone 1 (most of southern England, lowland Wales): ground snow load 0.30–0.40 kN/m² — the lightest zone, standard span tables apply for most locations
- Zone 2 (central England, midlands, southern Scotland lowlands): ground snow load 0.40–0.50 kN/m² — minor span reductions of 5–10% may apply
- Zone 3 (northern England, Scottish Highlands, upland Wales): ground snow load 0.50–0.80 kN/m² — significant span reductions of 10–20% compared to standard tables
- Altitude correction: add 0.1 kN/m² for every 100 m above sea level on top of the ground snow load
- Roof slope reduces snow accumulation: pitches above 30° shed snow more effectively. BS EN 1991-1-3 provides shape coefficients that reduce the roof snow load for steep pitches
- Drifting: valleys, parapets, and changes in roof height can create snow drifts with concentrated loads 2–3 times the uniform snow load
In southern lowland England, standard span tables are generally adequate. In Scotland, northern England, and areas above 200 m altitude, always check the specific snow load for your location and reduce spans accordingly.
How Roof Pitch Affects Rafter Sizing
Roof pitch affects rafter design in multiple ways: actual rafter length, load distribution, snow shedding ability, and the need for collar ties or ridge beams.
- Low pitch (10° to 22°): Rafters carry load almost like a floor joist. The horizontal thrust at the bearing wall is high, requiring adequate ceiling joists or collar ties. Limited choice of roof covering — membrane, metal, or specific low-pitch tiles.
- Medium pitch (22° to 40°): The load component perpendicular to the rafter decreases. Most UK roof coverings (tiles, slates) are suitable in this range. Ceiling joists or collar ties are still required unless a structural ridge beam is used.
- Steep pitch (40° to 60°): Good snow shedding and dramatic appearance. Actual rafter length is much longer than the horizontal span, increasing material cost. Wind uplift becomes a bigger concern — mechanical fixing of tiles is essential per BS 5534.
- Pitch multipliers for rafter length: 20° = 1.064, 25° = 1.103, 30° = 1.155, 35° = 1.221, 40° = 1.305, 45° = 1.414.
- Structural ridge beam: eliminates the need for ceiling joists by supporting the ridge. Required for vaulted ceiling designs. The beam must be engineered by a chartered structural engineer for the specific loads.
- Collar ties vs ceiling joists: Collar ties in the upper third of the rafter length prevent ridge separation. Ceiling joists at the plate level resist outward thrust. Most conventional roofs need ceiling joists; collar ties alone are not a substitute.
For most UK residential roofs (30° to 45° pitch), TRADA span tables can be used directly. For vaulted ceilings without ceiling joists, a structural ridge beam is required and must be designed by an engineer.
Building Regulations & Standards for Rafters
The following standards and regulations govern rafter design in UK residential construction.
- Approved Document A (Structure) — references span tables for timber rafters in domestic buildings
- BS 5268 Part 2 — permissible stress design of structural timber (legacy standard, still widely referenced)
- BS EN 1995-1-1 (Eurocode 5) — limit state design of timber structures
- BS 5534 — code of practice for slating and tiling, governs fixing specifications based on exposure and pitch
- BS EN 1991-1-3 — snow loads on structures, with UK National Annex for regional snow maps
- TRADA Span Tables — the primary builder's reference for rafter sizing under common UK loading conditions
- BS EN 338 — strength classes for structural timber (C16, C24, etc.)
TRADA span tables, derived from Eurocode 5, are the standard reference for UK builders. BS 5534 governs the tile and slate fixing details. For unusual geometries or heavy roof coverings, consult a structural engineer.
Calculate Your Roof Framing Materials
Once you know your rafter size and spacing, use our calculators to determine how many rafters, how much sheathing, and what fixings you need.