Angle Cut for Facia on Gable Rake Side

[dkeener]

I have a 3/12 pitch roof (3 inch's of rise for each 12 inch's of run). I am installing Facia Board (1x8 pine) and my question concerns the proper angle cut at the ridge of the roof. The right angle triangle by definition of the 3/12 pitch has a 90degree angle at the top plate center, a 14degree angle at the eave and a 76degree angle at the ridge (for a total of 180degrees in the right triangle)

Vision a gable roof and the two right triangles would fit against each other and form the gable. The two 76degree angles would be together at the ridge of the roof. What would the plumb cut be in degrees for the facia board on the ridge? I thought that with two 76degree angles combined that I would split that in half for the cut, but obviously that is wrong. The angle cut is not 76degrees and my miter saw wouldn't cut that anyway. I think the angle cut may be 14degrees for each of the facia boards joining at the ridge on the two 76degree angles, but I don't know why I think that or how the angle is actually calculated.

Thanks for any assistance with the angle cut.

David

[Pat]

90 minus 76 = 14

[dkeener]

14degrees makes sense, but why am I subtracting from 90degress when the angle is 76 degrees? Guess I should have paid more attention in school huh?

[Pat]

Probably need a blackboard so I can draw you a picture. A miter saw makes a

90 degree angle with the fence. To cut a 76 degree angle on the board you set the saw to 14 degrees.

[marson]

What most carpenters would do is hold their speed squares on the 3/12 mark, and look down and see what angle the plumb cut corresponds to. Mathematically orientated carpenters can use a scientific calculator and enter 3 / 12 INV TAN. Which I guess is technically the angle of the rafter to the ceiling joist, but it is also the angle that you set your miter saw to. Like Paul said.

[marson]

What most carpenters would do is hold their speed squares on the 3/12 mark, and look down and see what angle the plumb cut corresponds to. Mathematically orientated carpenters can use a scientific calculator and enter 3 / 12 INV TAN. Which I guess is technically the angle of the rafter to the ceiling joist, but it is also the angle that you set your miter saw to. Like Pat said.

[Harry K]

Or use a bevel gauge to set the saw to. That's what I do and it avoids the problem of odd angles. Been a long time since I actually read degrees of angle on anything.

Harry K

[Pat]

I'll try again. Put the board on the miter saw. The angle is 76 degrees measured from the fence. The saw is at 90 degrees. You have to move the saw from 90 degrees to 76 degrees. So you move the saw 90 - 76 = 14 degrees.

[Harry K]

Because the angle at the ridge is 14 degrees, not 76. Eyeball the cut that will be needed. It is clearly way less than 90. The only 76 degree cut on your roof will be the the birdsmouth cut to sit flat on top of the plate.

Be sure to cut two short trial pieces and try the fit. With a 1x8 just a minor error on saw set or when they made their pattern rafter will result in a noticeable gap.

Harry K

Harry K