Garage roof

I have a detached garage that needs new asphalt shingles. The roof already has two layers. Would it be bad to add a third layer of shingles?? If so can a drip edge be used over existing shingles.

a third layer with 3 inch roofing nails are a thing of the past.
but if you insist then a garage is better to do it on than a house.


i do belive 2 layers is the max by national building codes.

It is a terrible idea and against any codes that I know of and will not last very long and the longer nails may toggle loose.


I think i better strip off the old roofs. When purchasing shingles is there a rule to estimate waste, starter course, caps. I need about 450 sq feet of coverage. It is a hip roof.

As Gweedo said, not a great idea. Some building codes do allow up to 3 layers of roofing material, call your local building dept. and ask if it’s even an option. If you have the money do a full tear IMO.

order 5 sq of mat and tear it off.


Thanks all for your help. I will tear it all off and start new. I was thinking 5 square, but wasn’t sure it was enough, because i have to special order the color to match the house. i wanted to be sure.

Thanks again,

If you provide exact dimensions, you can have the right quantity of materials ordered.

Gweedo gave you an approximate good guess, but that did not take into consideration any starter strip shingles and special hip and ridge cap shingles, unless you are installing a standard 3-tab shingle on your garage.

If it is an architectural shingle, you will not be able to correctly use the left over shingles as your starters and H & R cap.

Length x Width of the actual eave edges?

Rafter Run length, from eave to the peak?

Length of each hip rafter?

Length of the ridge board?


Ya, it’s a standard 3 tab shingle, it’s just that no one has the color in stock.