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

On the side of a 2ft framing square are all the numbers you need.
I have a Johnson brand square, but all are similar. On the top row it gives you the" length common rafters per foot run". Your roof slope is 6/12, so look under the 6 on this line. The number is 13 42, this represents inches per ft run. So if your building is 8ft wide, have of that is 4ft, the center of the building. So take 4, and multiply by 13. 42 inches. =53.68 inches. This is 4ft 5 11/16. Cut a 6/12 on the end of your rafter. Tape down from the point 53 11/16 and make a mark. At this point, mark a 6/12 angle down the side of the rafter the same direction as the top cut. This will be the backside of your birdsmouth. I forgot to mention about your ridge board. Most people use a 2x stock. You need to deduct have this thickness (3/4in) in your line length of 53 11/16.The length would then be 52 15/16 inches.

If you have 50bucks go to Lowe's and buy the construction calculator, its shows how to figure this very quickly,

To his measurements you'll need to add your over hang if you have any

It's very hard to explain on paper without showing how in person. The numbers I gave were without the overhang. They sell books at Home Centers that can illustrate better than I can explain. I didn't really explain how to cut a birdsmouth. It's too hard to explain without showing in person. I know a lot of carpenters that can't even layout rafters!

Hope I helped a little.

I Thought That Was As Good As One Could Do. I Tried To Explain Stair Stringers Lol. And How To Cut Crown Is Always Lengthy

So To Sum It Up Take Jeffs Numbers And Go Buy A Book With Pics To Guide You. There 20bucks And You'll Spend That On Rafter Material Miscuts

How much overhang?