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# How to figure the angle to cut a shed type porch roof rafter.

*Submitted by Ray Thornburg on Sat, 09/17/2011 - 21:07*

This is the easy way to figure the angle to cut a porch roof rafter

Here’s a simple formula to help** figure out the angle cut at the top of a shed roof when it lies on the main roof**. First you must know the pitch of the main roof and the porch or shed roof. Usually it can be found on the plans when building a new home. Second you must be able to convert the pitch which is usually expressed as inches of rise per foot of run…. for example a 7/12 roof will be 7” rise to 12” of run. Sometime you can get these numbers from the side of a speed square or from roof slope tables. Of course once you know the mathematical way it’ll free you from your reliance on these charts. A 7/12 roof for instance means 7 divided by 12 which will give you the tangent. In this case it’s .583333. Get a calculator and choose the inverse tangent operation and it’ll show 30.2564˚. Subtract the integer portion and you have .2564, multiply this by 60 (60 minutes in a degree) and you’ll get 15 minutes. So it’ll look like this 7 ÷ 12 = .583333 next enter inv tan .583333 = 30.2564˚. Convert the decimal portion to minutes by multiplying by 60 and you’ll get 15 minutes which will be added to 30˚ for 30˚15̕.

Now that you know how to convert the pitch **simply subtract the high pitch from the low pitch in degrees and the resulting pitch will be the setting to use on the framing square. Simply mark your line with a framing square using the level side(12” side of framing square)** If you don’t have a framing square it’s 90 – the This Angle Pitch because there’s 90˚ between plumb cuts and level cuts. So in the example above 7/12 = 30˚15̕ , 4/12 = 18˚26̕ , so 30˚15̕ - 18˚26̕ = 11˚56̕ which is about a 2 ½ in 12 …. just use the level cut or if you prefer it’s 90˚ - 11˚56̕ = 78˚04̕ if you like using a speed square.

To learn how to do this with a framing square see this page. How to Mark the Top of a Shed Roof Rafter with a Framing Square.

To figure how long to make a shed type porch rafter see this page.