Hello beverlygaydos. Welcome to the community.
How many shingle?
A 3/12 pitch means for every 12” of run you will have 3” of rise. If the foot print of your building is 33’x60’ you would divide 33 in half, which equals 16.5’ a run. Multiply 16.5’x3”= 49.5”. Divide 49.5” by 12 and you get 4.125’ gable height. Now to calculate the surface of your roof, first you would add the square of the gable height (4.125x4.125=17.016) plus the square of your run (16.5’x16.5=272.25) which equal 289.27’. Find the square root of 289.27’ which equals 17’ for your rafter length or rake. Now multiply your rake times your eave to get one side, then multiply by 2 for the total roof size. (17’x60’=1020x2=2040sq ft. of roof) One bundle of 3 tab shingles equals 33.3 square feet, so divide 2040 by 33.3= 62.2 bundles.
You will need extra shingles to cover your ridge and if you are using for your starter row. 8 shingles cover 10 ft. of ridge. You have 60 ft. of ridge, so you will need 8x6=48 shingles. There are 21 shingles in a bundle so that is an additional 2.28 bundles.
For starter of eave and 2 rakes you have 60’+34’=94’ to cover. Each single is 3’ long so 94/3=32 more shingles or 1.52 bundles.
If you add those together you need about 66 bundles. The rule of thumb thought is you should add 10% for cutting which brings your total to about 73 bundles and what you don’t use you can return.
Don’t forget your ice and water shield, felt and nails.
For a 3-12 roof you may want to consider a roll roofing product, check your local Home Depot for these products as well.
If you have more questions please ask.
Charlotte.
I am a Home Depot Associate trained and authorized to help people on the internet.