Roof pitch formula

Thanks for the reply old school. it is a 12/12. And I do know the the roof area is bigger that the footprint. I have never used a formula before. So I'm a little unsure if this really works. I really want and need the work so I don't want to be over or under squares ya know? Thanks again old school iv seen you posts many times and respect your opinion. I don't usually post things on here cause some guys make you feel stupid if you don't know how to figure things. But anyway thanks again.

How close do you want to get? Is it 12/12? You know that the roof area is greater than the foot print. You know that you have to add for the waste! 17% should do for that. If it is 12/12 and the foot print of the building is 20 squares, the rafter length is 1.41 of the run. That would give you 28.2 squares. If you multiply that by your 17% waste factor that would be 33 squares. Is that close enough?