Before I set my rafters, I like to set the ridge in position first.
That’s why I recorded the Rise measurement of our rafter.
I make this plumb cut at the peak with my saw before marking my seat cut (or “bird’s mouth,” in some vernacular).
This way, I have something to hook my tape measure on, which is very handy for long rafters.
In time, an older carpenter named Rich Murphy took me up on a roof and helped me lay out and cut a reverse gable using a framing square and his roofer’s pocket reference or “bible.” That experience sent me on the quest to master the art of roof cutting.
I can’t say I ever mastered it, but I’ve come a long way since Rich graciously took the time to help me understand what was going on. Today, I use a calculator to find rafter lengths and angles.
mark on the body intersects the parallel plumb line I drew earlier; you’ll see that the line I trace will be exactly 4 in. To me, that’s the quickest way to draw the seat cut.
Measuring from the tip of the rafter, I mark off the diagonal measurement along the top edge of the rafter.
Then, using my framing square (some carpenters choose to use a speed square, but speed squares aren’t as precise, especially on fractional pitches), I draw the parallel plumb line across the rafter, marking along the tongue of the square.
There are also two laudable software versions available as smartphone apps: one from Calculated Industries, and one from Build Calc.
They essentially do the same thing as an actual construction calculator, but I prefer the real thing—if I’m going to drop a calculator in the mud, I’d rather it not be my pricey smartphone! I write this down on my template rafter as the adjusted overall run.