|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)|
Width × depth (mm)
|Permissible clear span (metres)|
|72 × 120|
|72 × 145||2.039||1.911||1.808||1.929||1.807||1.838|
|72 × 170||2.389||2.239||2.118||2.018||1.932||1.858||2.261||2.117||2.001||1.905||1.823||2.154||2.016||1.904||1.812|
|72 × 195||2.738||2.567||2.428||2.313||2.215||2.130||2.591||2.427||2.295||2.184||2.090||2.004||2.469||2.311||2.183||2.077||1.974||1.870|
|72 × 220||3.086||2.894||2.738||2.608||2.498||2.402||2.921||2.737||2.587||2.463||2.357||2.245||2.784||2.606||2.462||2.343||2.212||2.094|
|72 × 245||3.434||3.220||3.047||2.903||2.780||2.674||3.251||3.046||2.880||2.742||2.623||2.485||3.099||2.901||2.741||2.600||2.447||2.318|
|95 × 120||1.862|
|95 × 145||2.247||2.109||1.997||1.904||1.825||2.128||1.996||1.889||1.800||2.030||1.902|
|95 × 170||2.632||2.470||2.339||2.231||2.138||2.058||2.493||2.338||2.213||2.109||2.020||1.944||2.378||2.229||2.108||2.008||1.923||1.849|
|95 × 195||3.016||2.831||2.681||2.557||2.451||2.359||2.857||2.680||2.537||2.418||2.316||2.228||2.726||2.555||2.417||2.302||2.205||2.120|
|95 × 220||3.399||3.191||3.023||2.883||2.764||2.660||3.221||3.021||2.860||2.726||2.612||2.513||3.073||2.880||2.725||2.596||2.486||2.391|
|95 × 245||3.781||3.550||3.364||3.208||3.076||2.961||3.583||3.362||3.183||3.034||2.907||2.797||3.419||3.206||3.033||2.890||2.767||2.661|
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.