I am going to replace my roof. I have one layer of shingles down and was going to reshingle on top of them. Someone told me that when you put new shingles over old ones, it shortens the life of the new shingles. Is this true? Also, is the cost between shingles that last 25 years and those that last 50 years (asphalt-fiberglass) worth the cost difference in roof replacement?
When replacing your roof, it's usually a good idea to remove the original layer of roof shingles, but only if you intend to live in the home for most of the life of the new roof. You are correct that second layers typically don't last as long as single layers. Here's why: Asphalt roof shingles remain waterproof until they dry out. Having an old layer of roof shingles under your new layer accelerates that process as the original layer holds a lot of heat. As a result, you'll typically lose one-quarter to one-third of the roof life.
On the flip side, removing the original roof shingle layer drives up the replacement cost, so the decision is an economic one. If you intend to live in the home for as long as the roof will last, then remove the first layer as you will be the one that benefits from the extended life of the new roof. But if you think you might be moving in the next ten years or so, don't bother. A new buyer won't pay you a dime more just because you have one roof layer instead of two.
As for whether or not shingles last as long as a manufacturers' warranty promises, the answer is probably yes, but only if the installation and maintenance are absolutely perfect.
Roof warranties only cover materials, so the skill and training of your contractor can have an adverse affect that won't be revealed until it's way too late. A so-so installation can cut a 50-year roof down to 25 years of service, sinking the return on your coverage accordingly. Plus, a material-only warranty means that the labor isn't covered. Hence, you'd only be able to file a claim for the remaining part of the roof shingle life, granting you maybe half the cost of buying new shingles without a dime for removal of the old layer and installing the new roof. So beware of warranty coverage limits and loopholes when you're considering a roof replacement, and resist the temptation to select roofing material based on a possibly empty marketing claim.