plasterboard on the underside of the rafters the effects of deflection under concentrated (point) load do not need to be considered as per guidance given in BS 5268-7.5 clause 4.3. 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.Īs there are no brittle finishes e.g. Rafters are to have minimum end bearing of 35mm. These span tables are the equivalent of other industry-standard span tables and are calculated to the same standards. These spans were calculated using our rafter calculation engine. This span table is to the format and methodology given in BS 5268-7.5. Wane as allowed in BS 4978:2007+A2:2017 is permitted in all sections included in these span tables. Click on a span to generate a full calculation (you can adjust the exact span and slope). Timber to be covered, these span tables do not apply to timber which is fully exposed to the elements. Holes and notches cannot be drilled or cut into the rafters unless proved adequate by specialist calculations. The calculations on which these span tables are based conservatively assume that the rafters are not continuous over the purlins, but they can be continuous over the supporting purlin if required. These span tables are based on the assumption that the tiling battens fastened to the top of the rafters will provide adequate lateral restraint and also ensure that lateral loads are distributed. 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 only apply to roof systems with four or more rafters. These span tables do not apply to trussed rafter roofs. The dead loads shown at the top of span table above do not include the self weight of the rafters, however, the rafter self weights are included (in addition to the dead loads) in the calculations used to determine permissble clear spans. 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 5268-7.5:1990 Structural use of timber - Section 7.5 Domestic rafters. With the purlin/bracing installed, the long rafter span would be broken into two shorter spans, thus allowing the rafter to carry additional load.īefore you decide on lumber sizing and span, consult your local building code officials to determine design loads for your area.Dead load per square metre (in kN/m²) supported by raftersĬentre-to-centre spacing of rafters (in mm) You can break up a long span of rafters by installing a purlin and bracing on the underside. When calculating the span of joists or rafters, use the horizontal distance between vertical supports. The number 1 grade material is more expensive than the #2 grade, but if you can use fewer or smaller number 1 grade joists or rafters in your design, you may actually save money.ĭepending on the availability and cost of various size framing materials, it may be more economical to use larger joists or rafters in your design because you can typically use fewer of them. For example, you can span a #1 grade (no knots) southern yellow pine joist a greater distance than a #2 grade of the same species (with knots). The lengths and sizes of joists vary depending on the species and quality of the lumber used. To calculate maximum spans of additional species of lumber, use the Span Calculator or the Span Tables for Joists and Rafters on the American Wood Council website.Ĭonsiderations when using the Rafter and Joist Span Tables Contact your local building code officials to determine the building code for your area. Weigh all of the elements when designing wood structures. Some areas have wind and seismic conditions. For example, some localities get large amounts of snow, whereas others get none. Before determining the maximum spans for joists and rafters, you must know the load requirements for your region.
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