I bought a copy of Goss's Roofing Ready Reckoner that a friend recommended for around £20. There are a few good ready reckoners available. Each page has a different table depending on the pitch (angle) of your roof with all the figures you need A ready reckoner is pretty much a 'rafter length calculator' - a book full of tables for working out the rafter lengths quickly and easily. For larger roofs though, I use a roofing ready reckoner.When I pitch a small roof like a shed or dormer, I often just draw and set the rafter lengths out on a sheet of ply - no need for complicated calculations, just a straight edge and a good strong bevel.Go old school & use a scientific calculator and trigonometry (complicated unless you are a maths wiz).Some of the different methods you can use to work out your rafter lengths Tools needed to work out Roof Rafter lengths There are minimum roof pitches many tiles like slates for example must be laid on in order to stay put and not leak. If you are deciding yourself on the pitch make sure it is suitable for your roof covering. The roof pitch should be specified on your technical drawings for the job, along with the sizes of the roof timbers. The other thing you'll need is the roof pitch/angle. (Total span - ridge) / 2 = individual rafter span You might need a long tape measure to get the span, then The first thing to do when working out the actual length of a common rafter is determine the horizontal span of the rafter as discussed in the previous page.Įach common rafter span is calculated by subtracting the thickness of the ridge from the total span, then dividing the answer by two. We'll cover the purple overhang separately and deal initially with the green triangle in the picture that takes the rafter to the wall plate. That way you can get a perfect straight line for the wall and your fascias will look right even if the brickwork is less than perfect. If the brickwork is not so great and the walls are not perfect, it may be safer to cut the rafters with longer overhangs at the bottom, and then mark the tails for cutting later. If your walls and wall plate are all perfectly straight and square, you are probably safe to cut the complete rafters in one go on the ground and then take them all up and assemble the roof. The difference between the green span and the purple span below is the addition of the horizontal overhang shown above to allow a fixing for the fascia and soffit boards. You can see in the picture above where the two right angle triangles are that we use to make the calculations for our rafters. One for the actual length from plate to ridge, the other for the added overhang that will take the fascia and Soffit boards if your roof has these. All of these factors are critical for ensuring the structural integrity of any timber-based construction project.There are two quick calculations to determine the two lengths as shown in the picture above. Additionally, the tables specify the minimum spacing between rafters, as well as the maximum depth that can be safely used in a given application. The tables also provide information on the minimum number of nails that must be used when connecting the rafter to other components. Depending on the type of timber being used, the span will vary. The tables provide detailed information on the maximum rafter length that can be safely used in a given application. The Mgp10 Rafter Span Tables are essential for any builder or professional working with timber in Australia. The tables have been developed to make it easier for professionals to accurately determine the span of a rafter – or joist – when building a structure. The tables are based on the Australian Standard AS/NZS 1684-2010, which outlines the structural design of timber members and their connection to other structural components. This set of tables is designed to provide builders and other professionals with the necessary information needed to determine the safe span for a rafter – or joist – when used in construction. When it comes to construction and the use of timber in Australia, the Mgp10 Rafter Span Tables are an integral part of the process.
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