#RAFTER SPAN CALCULATOR FULL#

To show how easy it is to use, let's imagine the roof has a 3.265m total span from outside to outside of the wall plates, the ridge is 45mm thick and the pitch is 30°.įirst thing to do is to subtract the thickness of the ridge from the span of the roof so that the ridge will fit perfectly between the two common rafters. Setting out Rafter Lengths with a Ready Reckoner You don't need a framing square or rafters to calculate the rafter lengths, you can do it all on paper allowing you to calculate all the materials needed to price the job accurately just by knowing the size of the roof and the centres for the rafters. It is also handy when quoting a price for a roof that you haven't been supplied a technical drawing for. Once you've measured the run of a rafter, simply look up the right table in the ready reckoner for your roof pitch angle and using the measurements relevant to the span of the wall plates it tells you everything from all the common rafter lengths, plumb and seat angles to the length of the hips, the jack rafter cuts and several others are covered too. This saves a ton of time and trips up/down the ladder. I use it with an adjustable stanley quick square and can cut the whole roof on the ground, take all the timbers up and assemble the lot in one go. No sh#t, it has probably paid for itself a hundred times over.Īll the complicated trigonometry calculations are done for you, and there's loads more in there too, like diminishing jack measurements and hip rafter lengths and angles as well. 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

#RAFTER SPAN CALCULATOR FULL#

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. There are two quick calculations to determine the two lengths as shown in the picture above.

Rafter span calculator