I could calculate this on my construction calculator, but honestly, I find it easier to draw it out—it’s much safer, since drawing makes it easier to keep track of the numbers. To get to the top of the ridge, I need to measure the rafters HAP, or “Height Above Plate.” Looking at the illustration (below), you can see the triangle that our construction calculator calculated.By drawing it out on a story pole, I find the post elevation, and I can then cut the story pole to post the ridge. The calculator has no idea about the depth of the seat cut, or the size of the rafter material—it’s easiest to measure from the seat cut to the top edge of the rafter I’ve cut, and that is the HAP. In real life (not mathematics), not everything is perfect. more, to allow me to shim the ridge into position perfectly.For instance, I’d also like to know the diagonal measurement, which will help me layout the seat cut.All I have to do is press the Diag key, and the calculator displays the measurement: 3 ft. I write this measurement down on the template rafter, too.There are also two laudable software versions available as smartphone apps: one from Calculated Industries, and one from Build Calc.
Construction calculators are pre-programmed with Pythagorean formulas for finding the values of right triangles—and roofs are all about right triangles.Note: It’s important to remember to press the Inch key when entering the roof pitch—without it, the calculator treats the number entered as ‘degrees of pitch’ instead of the rise/run ratio of the roof.Now the calculator has all the details it needs, and it can provide me with every bit of information about that triangle.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.