Death by Uberschall
Active member
Bad idea to put new shingles on top of old shingles and here's why:Digital Jams":8aqmqnw9 said:Only advise I have is that if you are staying in that house for more than 15 years then have the roof ripped of because then in 15-20 years you can throw new shingles over the old ones.
1. Weight. Shingles are heavy and most decking won't support the added weight of a second layer without sagging between the rafters. Then when it's time to do the next roof, you'll be doing the decking along with it. Drive around and look at roofs, you'll see what I mean. Some decks are thin enough that one layer will cause the deck to sag.
2. Nail penetration. Adding another layer of shingles reduces the depth at which the nails penetrate the deck. The old shingles will not add any grip to the nails. Also, the second layer acts as a cushion when the nail goes in and gives a false level of clamping force. So your new shingles are more liking to come off in high winds.
3. Heat absorption. Most people don't realize that the decking and attic is what cools the singles down. Go in your attic mid summer and you'll know exactly what I mean. You have to get the hot temps out of the shingles, the only way is through the deck and out the attic. So you want good attic ventilation for the same reason. By adding another layer of shingles in between, you cause your new shingles to overheat and deteriorate rapidly. This will void any lifespan warranty you get with the shingles. Trust me, you make a claim 5 yrs in on a 20 yr shingle and the shingle manufacturer will come inspect the roof. If they find a second layer of shingles or poor attic ventilation, warranty is voided and you're on your own. If anybody ever says "X" brand of shingles were shit because they didn't last long, take a look and see how well they were cooled by the attic. Most homes don't have enough attic ventilation. This added heat will also soften your decking causing it to sag between the rafters.
4. Moisture barrier. You need to replace the asphalt felt that's under the shingles. The shingles are not water tight, they only rely on the basic principles of gravity for water run off. But they do not stop moisture. The asphalt felt does that. It is the vapor/moisture barrier for your decking and it deteriorates just like your shingles do.