How do I figure out the length and seat cut of rafters if my building is 8ft. Wide with a 6/12 pitch

To figure out the rafter length from ridge to the seat cut, you take the run and times it by the number (6 pitch) on the framing square, minus 3/4" (which is 1/2 the thickness of the ridge).

Or I can tell you the rafters should be 4'-5 5/8" long from ridge to seat cut.

If you plan on doing rafters and/or stairs go pick up Swanson's Little "blue book" for rafters and stairs. This book is usually included when you purchase a Swansons speed square. It is actually a little blue book.

Get the little blue book! Use the Swansons speed square to make your cuts. Use the "Common" marks on the speed square. If you use a ridgeboard, (2x) deduct 3/4 of an inch from overall length just as skiberger said. To cut your birds mouth, or angle cut that sits on wall, mark the plumb cut, at 4/12 or 5/12 or whatever, measure "up" however deep you want it, then square off the plumb line to mark the part that sits on top plate.

There is software on the internet that does this for you and let's you decide the truss style for most efficient use of wood (based on roof load). Of coarse, your roof load would be negilgable. Search on the WEB you should find some free-ware.

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Originally Posted by skiberger

To figure out the rafter length from ridge to the seat cut, you take the run and times it by the number (6 pitch) on the framing square, minus 3/4" (which is 1/2 the thickness of the ridge).

Or I can tell you the rafters should be 4'-5 5/8" long from ridge to seat cut.

If you plan on doing rafters and/or stairs go pick up Swanson's Little "blue book" for rafters and stairs. This book is usually included when you purchase a Swansons speed square. It is actually a little blue book.

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The pitch is a relationship between rise to run
In this case the rise is 6.5 inches per foot of run,which is half the building span.. ( 4 ft in this case )
Mulitply the run by the pitch... 4 x 6.5=26
26 inches is the total rise of the rafter
To get the length of the rafter simply apply A squared + B squared = C squared

48 inches is the rafter run & 26 inches is the rafter rise

48 x48 = 2304
26 x 26 = 676
2304 + 676 = 2980
square root of 2980 =54.58

rafter length to seat cut = 4'- 6 5/8 inches minus 3/4 " for half the thicknes of the ridge assuming using 1-1/2" stock

Rafter length = 4' 5 7/8 "

Or you can buy a little blue book and save all the math.