We'll assume the 30' are horizontal measures. 30 x 30 gives you 900 sq. ft., which you have to multiply by a slope factor to convert the area to surface area. For instance, if you have a 6:12 slope, the slope factor is 1.12, and the surface area is 900 x 1.12 or 1008 sq. ft. Then you have to adjust for the waste, which depending on the material may go from 5 to 18%. Assume you have a waste allowance of 10%, and you get 1008 x 1.1, which gives you 1,109 sq. ft. There is 100 sq. ft. in a square, so in this example, you've got just over 11 squares. If you're figuring in bundles, then most bundles cover about 33 sq. ft., or 3 bundles to the square. You'd need 33 bundles. Did I miss anything?

Thank you Pgriz! That was very easily explained. You are a lifesaver. I am assuming that the slope factors are driven by the pitch and I would substitue a pitch# where you have indicated the six in our example?

Thanks again!

That's right. The relation between pitch and slope factor is as follows: 2:12 - 1.01, 3:12 - 1.03, 4:12 - 1.05, 5:12 - 1.08, 6:12 - 1.12, 8:12 - 1.20, 10:12 - 1.30, 12:12 - 1.41.

In geometric terms, the pitch describes a right-angle triangle, where the rise (say, 2) is the vertical side of the triangle, the run (12) is the horizontal side of the triangle, and the slope itself is the hypothenuse. The slope factor is the ratio of the hypothenuse to the horizontal part.