Another rule of thumb that can give you a ballpark estimate of the roof area on newly built roofs is to look at the number of sheets of plywood used for the roof deck.
How many roofing sheets are in a bundle.
It takes three bundles to make a square which in roofing is 100 square feet.
There are 21 shingles to a bundle and 3 bundles covers a 10 by 10 ft area.
With other bundle counts per square divide the sheet number by three and come up with the number of squares you need to cover your roof.
Each plywood sheet is 4 x 8 feet or 32 square feet which coincidentally is almost exactly equivalent to the coverage of one bundle of traditional three tab shingles.
Therefore order one bundle for every sheet of roof sheathing you have.
Of your roof area which is close to the sheet coverage area of 32 sq.
If it s a re roof project and you have access to the original paperwork from the last shingle installation that may be useful in showing the roof area.
Each of your bundles can cover 33 3 sq.