We put a second layer of 30 yr. shingles over the existing shingles about 17 yrs. ago. We notice a few pieces of shingles on the ground and was wondering if this meant it was time for a whole new roof. Someone told us that the second layer only had a half life of the 30 yr. rating. Is this true ?
I don't know if a precise formula like that exists but the second layer does have less firm foundation than the first and it would be logical to expect shorter life span. Still, you've got no less than 17 years from it already. And shingles have started to separate. I think you've got a good life out of it. It's sounds like a great time to strip it all down to the decking, remove old flashing, check for signs of rot, repair what needs to be repaired, put new flashing, felt and redo the shingles anew.
If it showed no signs of damage, I might have left it alone for a few years longer but since it's already started going, you should be looking at a possibility of a tear-off sometime soon.
I agree that there is no precise formula.Â Many times, anything associated with our homes is ultimately unpredictable.Â I think it's probably time to strip all old roofing off and install a new roof. This will be the best piece of mind and ensure the proper protection of your home. Â
If you want to be sure, I think you should probably replace your shingles since they have already shown signs of damage. I advise you not to wait for it to get worse. Summer is a perfect time to replace your roof shingles so I suggest you do it while the weather is fine. That way, when the rainy season comes, your house will be ready and protected.