This document provides tables with details on allowable spans and spacing for common timber sizes used in traditional roof construction. The tables give maximum clear spans in meters for rafters and purlins with various sizes, roof pitches, and spacing. They note the figures are based on typical roofing materials and loading conditions, and other sources should be consulted for heavier loads or unusual conditions. Diagrams illustrate a typical rafter and purlin roof arrangement and proper installation of straps and clips.

1. 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
(http://www.planningportal.gov.uk/buildingregulations/approveddocuments/parta/documenta)
ask your Building Control Officer (/planning­and­
building/building­control/contact­building­control/) 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
This diagram illustrates the typical arrangement of a rafter and
purlin roof and shows the dimensions referred to in the tables above.
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.
Gable walls need to be strapped to the roof at a maximum 2m centres using
30 x 5mm galvanised mild steel straps.
Purlin and Rafter Roofs

2. 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
At eaves level, vertical straps 1m long should be fixed as shown in
the diagram opposite at a maximum of 2m centres.
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.
Updated 08 January 2016