In the roofing world, the Building Code (around the St Louis area) allows for up to two layers of Asphalt Shingles on a residential roof.
I see this all the time – new roofs going over the top of old roofs.
And if you are thinking of doing this, I'm here to gently persuade you to PLEASE reconsider.
Two layers, also known as a "roof over" may seem like a less expensive alternative to a full roof replacement, but let's consider this:
1) If you do not do a complete tear off of the old shingles and inspect the roof decking, you don’t know what you will be nailing your new shingles into. There could be rotted wood, or worse, spaced decking (see pic below). Neither of these are a SOLID reliable nailing surface which means your shingles are not going to remain on your house for long.
2) Also, many, not all, shingle manufacturers will not honor the warranty on the shingle if it is installed on top of another shingle.
How awful to not have a warranty after spending thousands of dollars.
3) Roof overs are much more difficult to repair and maintain. They make small repairs somewhat complicated and re-flashing is a little more challenging.
4) Because of the multiple layers, there will be an EXCESSIVE build up of heat which can cause the top layer to prematurely FAIL.
5) Your next roof will be even more expensive because the money you saved on tear off you will now spend in additional labor to tear off 2 layers and dump fees for 2 roofs instead of one.
Roof Overs are more common than I’d like to admit, but there are a bucket full of crappy contractors out there that are willing to do it. Problem is they tend to not buy the longer roof nails and the shingles don't anchor to the decking. Roof Overs are prone to wind damage because of the unreliable nailing surface they have been installed over. Valleys are prone to leaking because they are not lined properly. Roof overs DO NOT have the life expectancy of your typical roofing system. They FAIL prematurely because of the heat build up between the layers. There is no way to ventilate that. They damage easier from hail events because the two layers create a softer surface for hail to impact. I could go on and on and on....
Love your home. Please don’t roof over your problems. Correct them and do it right and they won’t come back to haunt you later down the road. ;)
Owner/Founder, Gwen Co LLC
"Everything you love is under that roof."