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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. Use the Timber Span Calculator to check a span against section size and grade.
Next up: related pages
Frequently Asked Questions
C16 is the most commonly used grade for domestic rafters in the UK and is adequate for most spans up to about 4m. C24 is used for longer spans, steeper pitches, or heavier roof coverings. All structural timber must be strength-graded to BS EN 14081 and bear a grade stamp. The grade must match what was specified in the structural design or span table used.
The tables below assume an imposed load of 0.75 kN/m² on plan (snow plus maintenance access), which covers a ground snow load sk of up to 0.94 kN/m², roughly lowland UK below about 200 m altitude. A separate 0.9 kN concentrated-load check is also applied. The dead-load band is up to 0.75 kN/m² on the slope, including battens and felt but excluding rafter self-weight, which is added separately: a medium-weight covering such as concrete interlocking tiles. Heavier coverings (clay plain tiles, thick natural slate) can exceed the band and need a separate check.
Yes. Plan loads such as snow produce less bending per metre of rafter as the pitch steepens, so a low pitch is the most demanding case. These tables are valid for pitches of 15° to 45° and are computed at 15°, the most onerous case in that band, so they are conservative for steeper pitches. Wind loads become more significant at steeper pitches and in exposed locations, where the roof should be checked separately.
For rafter spans exceeding the maximum in the span tables (typically over 2.5-4m depending on timber size), mid-span purlins are required. Purlins reduce the effective rafter span by providing intermediate support. A purlin-supported roof is the most common arrangement for traditional UK cut roofs. Purlins themselves must be designed to span between supports (typically gable walls, structural partitions, or struts to load-bearing walls).
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Last updated: April 2026
Verified against UK standards · estimates only, confirm with your supplier.