I have a metal shed that is about 10 years old and the roof has developed some small holes, my guess is from rust. As a temporary fix before building a new shed, I want to put a tarp over the roof. The shed is 10'x8'. The peak of the roof is about 12" above the wall. Obviously with the pitch of the roof it will add square footage to the area of the roof. How would you calculate the new square footage of the entire roof? I can't get on the roof to measure, so I guess I'm trying to find the area based on the rise and run of the roof. Anybody have a formula they can share?

Thanks
Bryan