Bay-Window Roof: How to Cut and Fit Rafters
The first rafter I’m going to cut is right in the middle—the common rafter. But before I take a measurement, I’m going to make sure the rafter cleat in the back here is a 90° angle to the base—which it is. If it wasn’t, I’d temporarily brace it back or forth, whatever is needed, until it was. Now I need the length of the rafter. I’m going to measure from the outside edge of the cleat here, and the outside edge of the subfascia; I’m measuring 31 in. The rafter length is 31 in. The rafter angles are going to be 45° on the top and 45° on the bottom.
Because this is a 12:12 pitch, all the common rafters will have 45° angles on both ends. But the hips are longer than the common rafters, so they’ll have slightly different angles. The common rafter was 31 in. The hip rafter will be 32-3/8 in. Because it’s longer, the angles will change a little bit at the top and the bottom. Whereas the top and bottom angles on the common rafter were 45°, on the hip rafter they’ll be 43° on the top and 47° on the bottom. They both add up to 90°.
You can trust me on this, but if you want to figure it out on a construction calculator, here’s how: First, measure the height of the cleat. Enter the number and press “rise.” Next, measure the rafter length, enter that number, and press “diagonal.” Finally, press the “pitch” button, and the result is the top angle. Round it up, for 43°. Subtract it from 90 for 47°—which is the bottom angle.
Because we’re at angle, I have to set the bevel cut for the top at 22½°. This fits perfectly from point to point, so I’ll take it and use it as a template for the other side. This is now the template for the next rafter. Line it up and mark the bottom, then mark the bevel end. Keep it oriented like that to cut it. Now I have a right and a left bevel.
We’ll now cut our last common rafter. Just like the first one, it’ll go from the middle here up to the top. It’ll have 45° angles on both sides. The only difference is the top right here will have a 45° bevel. Down here, I don’t have to measure the exact center; I’ll just eyeball it. I run the tape up and get 20½ in.; that’s my rafter length. This is the bottom cut—45°. Since I know the sister on the other side will be the same length, I’ll use this one as a template and cut it. At the top, I’m keeping the saw at a 45° angle. For the bottom cut, it’s a 90° angle. Now we have one for each side.