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Rafter Span Tables — C16 and C24 Timber
Maximum rafter spans for C16 and C24 timber at roof pitches of 15°-45° for domestic pitched roofs, calculated to BS EN 1995-1-1 (Eurocode 5) with UK National Annex parameters.
C16 Rafters — Medium Weight Covering (Concrete Tiles)
Dead-load band up to 0.75 kN/m² on the slope including battens and felt, imposed load 0.75 kN/m² on plan. No purlin support (common rafters only). Valid for pitches of 15-45 degrees, computed at 15 degrees. Spans are clear spans measured along the slope.
| Rafter Size (mm) | 400mm Centres (m) | 600mm Centres (m) |
|---|---|---|
| 47 x 100 | 1.53 | 1.42 |
| 47 x 125 | 2.06 | 1.90 |
| 47 x 150 | 2.59 | 2.38 |
| 47 x 175 | 3.13 | 2.86 |
| 47 x 200 | 3.68 | 3.34 |
| 47 x 225 | 4.14 | 3.77 |
C24 Rafters — Medium Weight Covering (Concrete Tiles)
Same loading conditions, higher strength grade.
| Rafter Size (mm) | 400mm Centres (m) | 600mm Centres (m) |
|---|---|---|
| 47 x 100 | 1.76 | 1.63 |
| 47 x 125 | 2.35 | 2.16 |
| 47 x 150 | 2.95 | 2.70 |
| 47 x 175 | 3.56 | 3.24 |
| 47 x 200 | 4.10 | 3.72 |
| 47 x 225 | 4.52 | 4.13 |
Purlin Sizes — C16 Timber
Purlins at mid-slope supporting rafters of up to 2.5 m slope span each side, installed plumb and loaded in major-axis bending. Maximum clear purlin span between supports (struts, gable walls or loadbearing partitions).
| Purlin Size (mm) | Max Span (m) | Notes |
|---|---|---|
| 47 x 150 | 1.32 | Smallest section shown |
| 47 x 175 | 1.55 | |
| 75 x 150 | 1.57 | |
| 75 x 200 | 2.12 | Most common domestic purlin |
| 75 x 225 | 2.40 | Deepest section shown |
Roof Covering Weights
| Covering Type | Weight (kN/m² on slope) | Category |
|---|---|---|
| Natural slate (thin) | 0.30 – 0.45 | Light |
| Fibre cement slate | 0.20 – 0.25 | Light |
| Concrete interlocking tile | 0.50 – 0.65 | Medium |
| Clay plain tile | 0.65 – 0.80 | Heavy |
| Natural slate (thick) | 0.55 – 0.75 | Heavy |
Basis of These Tables
- Valid for roof pitches of 15-45 degrees; computed at 15 degrees, the most onerous case in the band, so conservative for steeper pitches.
- Imposed allowance 0.75 kN/m² on plan, covering a ground snow load sk of up to 0.94 kN/m² (roughly lowland UK below about 200 m altitude). Above that altitude, or in higher snow zones, have the roof checked.
- A separate 0.9 kN concentrated-load check is applied to the rafters.
- Rafters are birdsmouthed onto wall plates and tied at ceiling level. Not valid for raised-tie, collar or room-in-roof designs.
- Rafter spans are clear spans measured along the slope.
- Purlins support rafters of up to 2.5 m slope span each side, are installed plumb, and are checked in major-axis bending.
- Deflection is limited to the stricter of span/333 and 12 mm; a minimum end bearing of 40 mm is assumed; values are rounded down to 0.01 m.
Span values calculated by TradeCalculator to BS EN 1995-1-1 (Eurocode 5) with UK National Annex parameters for the stated load case, and cross-checked against published industry span tables. Always confirm sizing with a structural engineer, particularly for unusual loading or non-standard conditions. Wind loading, snow loading and altitude must all be considered per BS EN 1991-1-1 and BS EN 1991-1-3/1-4.
Related Pages
See the Joist Span Tables, the UK Timber Sizes reference, and the Rafter Length Calculator.
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Last updated: April 2026
Verified against UK standards · estimates only, confirm with your supplier.