Permissible Clear Spans for Purlin Supporting Rafters (to BS 5268-7.6)

Slope Imposed load Strength class

Find the column for your dead load and spacing, and the row for your timber size to find the permissible clear span on slope in metres.
Click on a span to generate a full calculation (you can adjust the exact span and slope).
This span table is to the format and methodology given in BS 5268-7.6.
These spans were calculated using our purlin calculation engine.
These span tables are the equivalent of other industry-standard span tables and are calculated to the same standards.
This table uses commonly available UK regularised timber depths.
Read the notes at the bottom of the table for assumptions, references and related standards.

BACK PURLIN CALCULATOR Equivalent rafter span table

C16	Dead load per square metre (in kN/m²) supported by purlin
	Not more than 0.5						More than 0.5 but not more than 0.75						More than 0.75 but not more than 1
	Purlin spacing (in mm)
Purlin size Width × depth (mm)	1500	1800	2100	2400	2700	3000	1500	1800	2100	2400	2700	3000	1500	1800	2100	2400	2700	3000
Purlin size Width × depth (mm)	Permissible clear span (metres)

72 × 120
72 × 145	2.164	2.030	1.921	1.831			2.030	1.902					1.922	1.800
72 × 170	2.535	2.378	2.251	2.145	2.056	1.978	2.378	2.229	2.108	2.008	1.923	1.849	2.252	2.109	1.993	1.897	1.816
72 × 195	2.905	2.725	2.580	2.459	2.357	2.268	2.726	2.555	2.417	2.302	2.205	2.120	2.581	2.417	2.285	2.175	2.082	1.993
72 × 220	3.274	3.072	2.909	2.773	2.657	2.557	3.073	2.880	2.725	2.596	2.486	2.391	2.910	2.726	2.577	2.453	2.348	2.233
72 × 245	3.643	3.418	3.237	3.086	2.958	2.846	3.419	3.206	3.033	2.890	2.767	2.661	3.238	3.034	2.869	2.731	2.608	2.471

95 × 120	1.974	1.853					1.853
95 × 145	2.382	2.237	2.120	2.022	1.939	1.867	2.237	2.099	1.988	1.895	1.816		2.120	1.988	1.881
95 × 170	2.790	2.620	2.483	2.369	2.272	2.188	2.620	2.459	2.329	2.220	2.128	2.048	2.484	2.329	2.204	2.101	2.012	1.936
95 × 195	3.196	3.002	2.845	2.715	2.604	2.508	3.003	2.818	2.669	2.545	2.440	2.348	2.846	2.670	2.527	2.408	2.307	2.220
95 × 220	3.601	3.383	3.207	3.061	2.936	2.828	3.384	3.177	3.009	2.870	2.751	2.648	3.208	3.010	2.849	2.716	2.602	2.503
95 × 245	4.006	3.764	3.569	3.406	3.267	3.147	3.765	3.534	3.349	3.194	3.062	2.947	3.570	3.349	3.171	3.022	2.896	2.786

The allowable clear spans have been calculated in accordance with BS 5268-2:2002 Structural use of timber - Part 2: Code of practice for permissible stress design, materials and workmanship and BS 5268-7.6:1990 Structural use of timber - Section 7.6 Purlins supporting rafters. The dead loads shown at the top of span table above do not include the self weight of the purlin, however, the purlin self weight is included (in addition to the dead loads) in the calculations used to determine permissble clear spans. These span tables do not apply to trussed rafter roofs. The major axis of the purlin is perpendicular to the rafter slope. It is also assumed ceiling joists will be used to transmit the horizontal component of thrust occurring at eaves level to complementary rafters. These span tables are based on purlins for a single span, if the purlins are two span continuous or more over intermediate supports you can conservatively consider the longest span between supports. The rafters may be continuous or may consist of shorter lengths joined at the purlin. Since continuous rafters load the purlin more severely, this is the case considered. The calculations used for these span tables are based on rafters having two equal spans. Holes and notches cannot be drilled or cut into the purlins unless proved adequate by specialist calculations. Timber to be covered, these span tables do not apply to timber which is fully exposed to the elements. Wane as allowed in BS 4978:2007+A2:2017 is permitted in all sections included in these span tables. Purlins are to have minimum end bearing of 80mm. The imposed load should be calculated in accordance with BS 6399:Part 3:1988 Code of practice for imposed roof loads, as a rule of thumb for altitudes not exceeding 100m you can use a uniformly distributed load of 0.75 kN/m² and for most other areas exceeding 100m but not exceeding 200m you can use 1 kN/m², for parts of Scotland, Pennines, North East England and heights over 200m you will need to refer to BS 6399 Part 3.

Permissible Clear Spans for Purlin Supporting Rafters (to BS 5268-7.6)

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