![]() In other words, how much a joist or rafter bends under the maximum expected load. Stiffness of structural members is limited by maximum allowable deflection. Perhaps the joists were strong enough if they didn’t break! But lack of stiffness leads to costly problems. ![]() For example, first-floor ceiling plaster would crack as occupants walked across a second-floor bedroom that was framed with bouncy floor joists. Strength and stiffness are equally important. Beams, studs, joists and rafters act as a structural skeleton and must be strong enough and stiff enough to resist these loads. The house acts as a structural system resisting dead loads (weight of materials), live loads (weights imposed by use and occupancy), like snow loads and wind loads. ![]() This article will focus on how simple beams like joists and rafters react to loading. If, when the loads of the house are combined, the house weighs more than the soil can support – the house will sink until it reaches a point at which the soil can support the load. Remember when your science teacher said: every action has an opposite and equal reaction? Well every building load has an equal “reaction load”. The structural goal of a house is to safely transfer building loads (weights) through the foundation to the supporting soil. A complete analysis of wood’s mechanical properties is complex, but understanding a few basics of lumber strength will allow you to size joists and rafters with the use of span tables. Wood is naturally engineered to serve as a structural material: The stem of a tree is fastened to the earth at its base (foundation), supports the weight of its branches (column) and bends as it is loaded by the wind (cantilever beam). Using span tables to size joists and rafters is a straight-forward process when you understand the structural principles that govern their use. Some information contained in it may be outdated. Where access to the roof is provided in addition to that necessary for cleaning and repair, the imposed uniformly distributed load should be taken as a minimum of 1.50 kN/m².Please note: This older article by our former faculty member remains available on our site for archival purposes. 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. Joists are to have minimum end bearing of 40mm. Wane as allowed in BS 4978:2007+A2:2017 is permitted in all sections included in these span tables. Holes drilled at the neutral axis (centre line) are not to exceed 0.25 of the depth of a joist and are not to be less than three diameters (centre to centre) apart and are to be located between 0.25 and 0.4 of the span from the support. Notches are not to exceed 0.125 of the depth of the joist and are to be located between 0.07 and 0.25 of the span from the support. Timber to be covered, these span tables do not apply to timber which is fully exposed to the elements. These span tables apply only to flat roof and roof with a slope of up to 10 degrees. ![]() If insulation is placed on top of the flat roof joists and as such the joists are not fastened to the boarding above, the depth to breadth ratio of the joists should not exceed a ratio of 4 to 1. The dead loads shown at the top of span table above do not include the self weight of the joists, however, the joist 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 5268-7.2:1989 Structural use of timber - Section 7.2 Joists for flat roofs. * Boarding to be fastened on top of flat roof joists with a depth to width ratio greater than 4 Dead load per square metre (in kN/m²) supported by joistsĬentre-to-centre spacing of joists (in mm) ![]()
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