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). Have you already found the information you're looking for? Then check the roof shingle calculator that is dedicated to a slightly different topic of roofing.Using span tables to size joists and rafters is a straight-forward process when you understand the structural principles that govern their use. It can also be written down as 25% or 14°. Rafter² = rise² + run² = 1.5² + 6² = 2.25 + 36 = 38.25Ĭalculate the roof pitch as the proportion of rise and run:Īngle = arctan(pitch) = arctan(0.25) = 14°įinally, you can find the roof pitch in the form of x:12. Let's say it is equal to 1.5 m.Ĭalculate the rafter length, substituting these values into the following formula: Measure the run length – it is the horizontal distance between the roof ridge and the wall of the building. They have a pitch that can be as high as 21:12 (175%).īefore you start making proper calculations, you might want to open the angle conversion tool to convert between radians and angles. High-pitched roofs often require additional fasteners.They are the easiest ones to construct, and they are safe to walk on. Conventional roofs have the pitch ranging from 4:12 to 9:12 (33.3% to 75%).These are generally difficult to maintain, as they require special materials to avoid leaks. Low pitched roofs have the pitch below 4:12 (33.3%).Generally, these roofs have a pitch from 1/2:12 to 2:12 (from 4.2% to 16.7%). Flat roofs are not perfectly flat in reality – they need a small slope for water runoff. We can divide roofs into the following categories: For example, a pitch of 1:12 means that for every twelve yards of building length, the rise will be equal to one yard. Roof pitch is often expressed as a ratio between rise and run in the form of x:12. It can be assessed in two ways – either as the angle the rafter makes with the horizontal or the proportion between the rise and the run of the roof. Roof pitch is simply the slope created by the rafter.
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