roof rafter formula

[Peg]

Hello, I'm a carpentry student deperately trying to understand how to figure out the basic formlua for gable roof rafters. I have read several books and am actually working on a shed, but I can't seem to find a clear formula, step by step for getting the legnth of the rafter, and the placement of the bird's mouth! Can someone give me some information?
Example: Shed 10' x 10', 6/12, double ridgeboard 2x 2"x6", 6"overhang.
thanks

[mlkmj]

A Squared + B squared = C squared.

[Rich]

What mlkmj suggests is absolutely correct. I have a little bit different (sometimes more intuitive) way. First you'll want to get the exact horizontal run from outside face of birdsmouth (outside of wall) to face of ridgeboard In your case I'm assuming it would be 9'-10 1/2". The roof factor for a 6/12 pitch is The square root of the pitch squared plus 1. SQRT(((6/12)*(6/12))+1)
.5*.5 = .25 add 1 for 1.25 and take square root of it
equals 1.11.
Take 1.11 times the horizontal run found earlier and get 10'-11 1/2" (approximate with my current calculator). That is only from birdsmouth to ridge.. add your overhang to horizontal run to get overall length.

[Joe Bartok]

Peg: This Hip-Valley Framing Calculator (http://ca.geocities.com/xpf51/HVFRAMING/FRAMING_WP.html) has links to sketches and formulas that may help you. The dimensions returned by the calculator are to the theoretical framing points through centerlines and require further adjustment to find the allowance for the ridge board, seat cut, HAP, overhang at the soffit, etc. as per the formulas.
Joseph Fusco also has a well written and easy to understand Basic Roof Framing (http://www.josephfusco.org/Articles/Roof_Cutting/raftercutting.htm) article.

[Peg]

Thanks folks,
I will play with the numbers now that I have a basic understanding how to approach this task.
And thanks for the web site, never can have enough information!