Wood frame construction relies on the lumber design values and allowable span tables published in building codes and standards, and industry technical. Timber to be covered, these span tables do not apply to timber which is fully exposed to the elements. The spans in this table are based on a maximum supported rafter span of 16. Wane as allowed in BS 4978:2007+A2:2017 is permitted in all sections included in these span tables. Rafters 3-7 Roof Lintels 8-18 Timber Flat Roof Joist Span Tables Flat Roof. Rafters are to have minimum end bearing of 35mm. 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. 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.There’s lots of factors that determine the size of wood beams. Wood is somewhat elastic, so it depends on how many allowable factors you care to investigate to determine its maximum span.Įven the same grade and species can have its “allowable stresses” increased based on “duration of load.” That is to say that if you put a heavy short duration load on a beam it will act the same as a smaller long term load. Grade of beam can be upgraded if more of the beam (edges of beam) can be observed during grading. That’s why carpenters like to hand pick beams, (they know they have an extra safety factor so the beams won’t twist, Bend, etc.)īeams are calculated using the “working stress design” or “ultimate stress design”. This is based upon the span of the rafter (horizontal span, not total rafter length), live and dead loads. Working stress leaves a BIG safety factor because they know a home owner will be using it. To size joists and rafters use the Maximum Span Tables for southern yellow pine. This Rafter Span Calculator is species specific for Douglas Fir, hemlock, etc. Ultimate stress design figures exactly when the beam will fail and then uses an allowable safety factor.sometimes 1.5, or 1.8%. I like a very safe size rather than figuring everything too tight. Span Calculator for Wood Joists and Rafters available for the iPhone and iPad. (Maximum spans for Douglas fir-larch, hem-fir, southern pine and spruce-pine-firb and. You never know when the client will call and want to add an air conditioner on the roof, or they’re going to invite you to their 50th anniversary out on their deck. Simple clicks on the drop down boxes will allow variable inputs to calculate a.
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