Applying a new roof over an old roof is considered an acceptable practice, but we are firm believers in removing the original layer, especially if there is a possibility of old roof having issues.
There are 4 reasons why we advocate always removing the original roof:
1. If you know you need a new roof that usually means it’s at the end of its usable life. Signs to look for are blistering, brittleness, breaking, cracking and curling of the shingles and, of course, leaks. There’s no benefit to covering up old problems, and they will most likely affect the durability and warranty of the new roof.
2. Adding multiple layers increase the amount of radiant heat of the roof which in turn reduces the effectiveness of your attic ventilation, which is often under vented in the first place.
3. Can your roof carry the extra weight? Since the 1970s the use of trusses are more common. Trusses are often spaced at 2 ft. intervals with a layer of 3/8” – 1/2” plywood or OSB. Ever wonder why you see dips in new roofs? Well now you know! Then add another layer of shingles to the weight and see what happens.
4. Warranties are only as good as the installation. Manufactures and claim adjusters like to point out such things when a homeowner decides to file a claim for a new roof due to hail and wind damage. They have learned from experience that multiple layers do not do as well during storms.
The biggest benefit of tearing off an old roof is that it is an opportunity to inspect and repair any water damage or rot with the roof decking, soffits and eaves, and flashing. It’s also the time to inspect attic ventilation (which has direct impact on the life of your roof shingles), and lay down a high-performance underlayment over the decking that acts as a secondary barrier against water leaks.
Bottom Line: Even though 2 roof layers are legal outside city limits, we do not like covering over any old roofing materials with another layer because we don’t believe in covering up problems. We urge you to remove both layers of roofing so that the new roof investment will last as long as its intended to, and that you have peace of mind that your roof is sound from top to bottom, inside and out.