We’re lucky to have relationships with the expert instructors and craftspeople at Seattle Central College’s Wood Technology Center. In this video series, Catie Chaplan, a veteran instructor, guides us through some of the foundational carpentry concepts and methods for framing a basic equal-pitch hip roof, as taught in the center’s curriculum.
Catie is dedicated to teaching the next generation of highly skilled carpenters. As a professional builder for 31 years, she's worked for general contractors, boat builders, and cabinet shops, and has owned and operated a residential design-build company in Seattle since 2002. She's been an instructor at the Wood Technology Center for the past 25 years, where she currently leads the carpentry program and teaches computer-aided design (CAD) and computer numeric control (CNC) classes.
Throughout the Framing a Hip Roof series, Catie shows us how to calculate theoretical hip roof framing (meaning that the calculations go to the very center of the roof), then how to adjust those calculations for the thickness of the materials used in different parts of the roof where pieces come together (this is called adjusting for "reality").
In this video from Catie’s virtual course curriculum, she explains how understanding triangles—and brushing up on the Pythagorean theorem—will help you make accurate calculations when framing a hip roof. Watch her tutorial above or keep reading for a summary.
With gable rafters, when you know the run (the distance from the center of the building to the outside edge of the top plate) and the slope (or pitch) of the roof, you can calculate the length of the diagonal.
With hip rafters, you use the same building run, but it's a different triangle. Not only does the hip rafter have to extend to the outside of the building, but it also has to rotate to travel along a 45-degree angle—meaning the rafter needs to be longer in order to stretch the length of the plane. For example, the run of a common rafter is always based on 12”, but with a hip rafter, it’s based on 17”. (The pitch stays the same.)
The easiest way to understand this relationship is by looking at the right triangle formed by the common and hip rafters. If two sides of the triangle are 12” each, then you’d simply use the Pythagorean theorem (a^2 + b^2 = c^2) to find the hypotenuse—which would be 17”. Essentially, what this means is that, for every 12” of run in one direction, the hip rafter needs to make 17” of run to reach the same point.
For more from Catie on how to frame hip roofs, check out the anatomy of a hip roof.