Layout Pattern Rafter

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Roof Framing Math Made SIMPLE!

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What happens here

Once the calculations are complete, the next step is to lay out the cuts on a pattern rafter. Begin by marking the 'plum cut' at the top end of the rafter board. This cut will sit against the ridge board. Using a speed square set to the roof's pitch (e.g., 3 on the common rafter scale for a 3/12 pitch), align the pivot point of the square with the top edge of the rafter and mark the angle. Then, from the plum cut, measure out the 'run' (e.g., 10 feet 2 5/16 inches) along the rafter board. At this point, mark the 'bird's mouth' cut, which is a notch that allows the rafter to sit flush on the top plate or beam. The bird's mouth consists of a 'seat cut' (horizontal) and a 'heel cut' (vertical). The depth of the bird's mouth is determined by the 'Height Above Plate' (H.A.P.) measurement (e.g., 6 1/4 inches), which is the vertical distance from the bottom of the rafter to the top of the beam. A second speed square or rafter square can be used for precise marking of the bird's mouth. Finally, mark the 'soffit' or overhang. For a 12-inch overhang, measure 12 inches past the bird's mouth, then subtract the thickness of the sub-fascia (e.g., 1.5 inches for a 2x material) to mark the final plum cut for the fascia board.

Goal: Accurately mark all cuts on a single rafter board to serve as a template.

Tools and materials

Measuring Tape
Pencil/Marker
Rafter Board
Speed Square

What they say

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laying out my rafter. So, the first thing I need to do is I'm going to lay out my plum cut, which is at the top of my ridge. And with doing so, you just need a simple speed square. In this case, I have a 12in square. So, I'm going to line it up at the edge. And if you notice, there's a word called common. And we're dealing with a common rafter. And all these numbers that are on there signify that that's what the pitch is. And we're dealing with a three pitch. So hold it. You have your pivot point, which is tight. You line up the top of your rafter with the three because it's a three pitch. And then that's your plum cut. Now I know my run is 10 foot two and 516. Again, you're going to put your speed square on there. Pivot. Move it to three to where it's lined up. Draw a line. Now, those two plum cuts are the inside of my ridge and the outside of my wall. The next step is the bird's mouth cut, which has a seat cut and a plum cut. But this is where the hat comes along, the height above plate. I know mine's 6 and a/4. So I'll measure down 6 and 1/4 and square that across. And what this does is this is what's going to sit on top of our 5 and 1/2 inch beam. And this is the outside of my wall. All right. Now, that is the end of my wall. Now, we have to make our sophet, which we're going to go with a 12 in overhang. So, all you have to do is measure over 12 in. You want to subtract an inch and a half because you're going to be putting a subfacial on. And again, set your speed square to three. Hit your mark. We're going to use a 2 by six fascia. So, what I'll do is I'll measure down 5 and 1/2, but I'll come up maybe five and a quarter so it doesn't come into play.

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