Some Irregular Roof Geometry

I recently had a roof with a snub hip layover valley. Normally I would frame broken hips from ridge to ridge, but in this case the engineer wanted each roof fully sheathed. This is maybe the 3rd time in 20+ years I’ve had to do this. Here is how I figured they angles. They worked, but I’m not 100% sure I’m doing this right. They did fit and everything planed out. If someone wants to model it, that would be awesome.

First off, while I know the trig to figure the angles, I don’t bother. Instead I use the BuildCalc (available for IOS and Android) app on my smartphone. Here are a few basics to get us started.

The upper ridge in the picture is a 6-12, and the lower roof is a 7-12. I enter the information in that order. The 6-12 is the MAJOR roof and the 7-12 is the MINOR roof. This is important so we can keep track of which angle in BuildCalc corresponds to the right roof.

I pick an arbitrary RUN of 20’ and then enter 6 INCH PITCH 7 INCH CONV Ir/Pitch. Then click Hip/V and it’ll return the results.

I want to know the plumb cut of the hip and the bevel and the backing angle of the MINOR roof. The plumb cut is 20.79 (using a speed square) and the bevel is 40.60 (on the saw). This is the angle the hip makes with the upper ridge. These are the angles on a speed square and correspond to your saw. They both start at 90, not 0.

To figure the angle for where the hip lands on the sleeper, here is the math. 90 - (20.79x2). 90-41.58=48.42. This is the miter you need to cut the hip. The bevel is found with the MINOR roof backing angle 22.49.

Let me know in the comments if you have something different.

*Edit Below is a drawing by SKABuilders1. His feed on Instagram is worth checking out.