How do I figure Ridge Board Height?

[pappy77]

Hello,
I am building a screened porch on my house. The house has a 5/12 pitch on the roof. The wall span of the porch is 179 1/2" making the total run 89 3/4". I figured 89 3/4" to 7.479 Ft. I figured 5" to .416 Ft. I multiplied 7.479 X .416 and got 3.111 or 3 Ft and 1/8" for a total rise.

Seems like that would be right but when I set a practice board/post up on the gable wall and pulled a string with a line level to the point on the exhisting roof where the ridge board would contact, it was too low. I raised the total rise measurement the height of my ridge board (2x8=7.5")and looks like that is right. Problem is it's not right on paper and in my head.

My house is a ranch with a hip roof. The porch will have a gable roof but must tie in to the exhisting hip roof on one side at the end of the house. So this has to be right or I'll have a speed bump in my shingles.

Any help would be appreciated. Thanks.

[Rich]

The distance of 3.11' is to the bottom of the rafter. Typically rafter bottoms will sit on the very outside edge of the wall which is the bottom of the rafter. If you want to birdsmouth the rafter you just need to figure out what the drop will be and drop the ridge beam height by that much.
I hope that makes sense. I can CAD something up for you if you want.

[pappy77]

OK, that makes sense. If I take into account the distance from the bottom of the rafter where it sits on the outside of the wall, to the top of the rafter on a plum line and add that to the total rise, I will get total rise the the top of the rafter which will be where the top of the ridge board should be. Actually, if I want to get real precise, I could take into account the rise lost by the thickness of the ridge board since it is flat on top. Probably 3/8" or so on a 5/12 pitch.

Thanks very much for clearing that up for me!

[Rich]

You bet. I think I'll change my rafter calculator to include total rise also.