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Pitched roof timber sizes

The following tables give details of the allowable spans and spacing for some of the more common timber sizes used in traditional roofing construction. All the figures are based on roofing tiles or slates laid on timber laths over sarking felt. For heavier roofing materials or unusual loading conditions these tables may not be appropriate and you should refer to the Approved Document to Part A of the Building Regulations or ask your Building Control Officer for advice.

Maximum clear span of rafters in metres for roof pitches of between 15 and 45 degrees

Roof pitch between 15 and 22.5 degrees with rafter spacing

Size of rafters in mm 400mm 450mm 600mm
100 x 50 2.35 2.29 2.09
125 x 50 2.98 2.87 2.61
150 x 50 3.57 3.43 3.12

Roof pitch between 22.5 and 30 degrees with rafter spacing

Size of rafters in mm 400mm 450mm 600mm
100 x 50 2.45 2.36 2.14
125 x 50 3.05 2.93 2.67
150 x 50 3.65 3.51 3.20

Roof pitch between 30 and 45 degrees with rafter spacing

Size of rafters in mm 400mm 450mm 600mm
100 x 50 2.53 2.43 2.21
125 x 50 3.15 3.03 2.76
150 x 50 3.76 3.62 3.30

Maximum clear span of purlins in metres for roof pitches of between 15 and 45 degrees

Roof pitch between 15 and 22.5 degrees with rafter spacing

Size of rafters in mm 1.5m 2.1m 2.7m
175 x 50 2.08 1.84 -
200 x 50 2.38 2.10 1.85
225 x 50 2.67 2.35 2.07
175 x 75 2.41 2.13 1.95
200 x 75 2.75 2.44 2.22
225 x 75 3.09 2.74 2.50

Roof pitch between 22.5 and 30 degrees with rafter spacing

Size of rafters in mm 1.5m 2.1m 2.7m
175 x 50 2.13 1.88 -
200 x 50 2.43 2.15 1.91
225 x 50 2.74 2.42 2.14
175 x 75 2.46 2.19 1.99
200 x 75 2.81 2.50 2.28
225 x 75 3.16 2.80 2.56

Roof pitch between 30 and 45 degrees with rafter spacing

Size of rafters in mm 1.5m 2.1m 2.7m
175 x 50 2.21 1.95 -
200 x 50 2.52 2.23 2.01
225 x 50 2.83 2.50 2.24
175 x 75 2.55 2.26 2.06
200 x 75 2.91 2.58 2.36
225 x 75 3.26 2.90 2.65

Typical arrangement of a rafter and purlin roof

This diagram illustrates the typical arrangement of a rafter and purlin roof and shows the dimensions referred to in the tables above.

Gable wall strapped to the roof with galavanised mild steel straps

Gable walls need to be strapped to the roof at the maximum 2m centres using 30 x 5m galvanised mild steel straps.

Effective strapping at gable wall

Effective strapping at gable wall

Each strap should be fixed as shown in the diagram opposite with noggins between the rafters to support the metal strap along its whole length.

Vertical strapping at eaves pitched roofs

Vertical strapping at eaves pitched roofs

At eaves level, vertical straps 1m long should be fixed as shown in the diagram opposite at a maximum of 2m centres.

Fixing truss clip to connect the rafter and wall plate.

This is the right way to fix truss clips connecting the rafter and wall plate. Note that these are normally only required to trussed rafter roofs and not for traditional purlin and rafter roof construction.