What timber grade do I need for rafters?

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.

What loads are these span tables based on?

Domestic rafter span tables typically assume: imposed load of 0.75 kN/m² (snow plus maintenance access) on the slope, dead load of the roof covering and ceiling below. The tables below assume a medium-weight roof covering (concrete tiles, approximately 0.50-0.75 kN/m² on the slope). Lighter coverings (slates, metal) allow longer spans; heavier coverings (clay tiles) may reduce them.

Does roof pitch affect maximum span?

Yes. Steeper pitches generally allow slightly longer rafter spans because the load per metre of rafter length is reduced (the same horizontal area is spread over a longer rafter). However, the difference is relatively small for typical UK domestic pitches (22.5° to 45°). Wind loads become more significant at steeper pitches and can offset this advantage in exposed locations.

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).

Rafter Span Tables — C16 and C24 Timber

Maximum rafter spans for C16 and C24 timber at common roof pitches (15°-45°) for domestic pitched roofs, referencing TRADA span tables and Eurocode 5.

C16 Rafters — Medium Weight Covering (Concrete Tiles)

Dead load 0.50-0.75 kN/m² on slope, imposed load 0.75 kN/m². No purlin support (common rafters only).

Rafter Size (mm)	400mm Centres (m)	600mm Centres (m)
47 x 100	1.81	1.58
47 x 125	2.34	2.06
47 x 150	2.87	2.55
47 x 175	3.35	2.98
47 x 200	3.80	3.38
47 x 225	4.22	3.72

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	2.14	1.88
47 x 125	2.74	2.42
47 x 150	3.32	2.96
47 x 175	3.86	3.44
47 x 200	4.36	3.90
47 x 225	4.84	4.30

Purlin Sizes — C16 Timber

Purlins supporting rafters at mid-span. Maximum purlin span between supports.

Purlin Size (mm)	Max Span (m)	Notes
47 x 150	1.80	Light coverings only
47 x 175	2.10	—
75 x 150	2.40	—
75 x 200	3.00	Most common domestic purlin
75 x 225	3.30	Heavier coverings, wider rafter spacing

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

Note: Span values are indicative. Always refer to current TRADA Span Tables or obtain an engineer's calculation for definitive sizing. Wind loading, snow loading, and altitude must all be considered per BS EN 1991-1-1 and BS EN 1991-1-3/1-4.

See the Joist Span Tables, the UK Timber Sizes reference, and the Rafter Calculator.