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 today’s video, Catie explains how to calculate birdsmouth cuts by focusing on the height-above-plate measurement. Watch her tutorial in the video above or read on for a summary.
What is a birdsmouth cut?
The birdsmouth is a triangle-shaped indentation cut into a rafter so it can rest on the top plate of the outside walls.
Using height-above-plate to calculate the birdsmouth
The challenge with finding the birdsmouth on hip rafters is that often, a larger beam is used for a hip rafter than a common rafter—meaning you’re not comparing apples to apples. This is where the height-above-plate (HAP) comes in.
“Height-above-plate” is the distance between the tip of the birdsmouth cut to the top of the rafter. Remember that the goal with rafters is to create a continuous plane; HAP, therefore, should be the same no matter the size of the rafter beam.
In this example, we find the birdsmouth on the hip rafter by measuring the birdsmouth on your common rafters. Then, use that measurement to determine the HAP for the hip.
For more from Catie on how to frame hip roofs, check out the anatomy of a hip roof and her tutorial on planing in on the ridge and fascia.