Roof Replacement - One Layer or Two?

I am going to reshingle my roof. I have one layer of shingles down and was going to reshingle over top of them. Someone told me that when you put new shingles over old ones that it lessons the life of the new shingles. Is this true? Also is the cost between shingles that last 25 yrs and those that say 50yrs (asphalt-fiberglass) worth the cost difference or even last that much longer.
woolley_mark 8-12-07 7:50AM

When replacing your roof, it is usually a good idea to remove the original layer, 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 shingles remain waterproof until they dry out.  Having an old layer of shingles under your new layer accelerates that process as the original layer holds a lot of heat.  Typically, you'll lose one-quarter to one-third of the roof life.

On the flip side, removing the original 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 years.  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 manufacturer's 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 its 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 with a not a dime for removal of the old layer and installing the new roof.  So beware of warranty coverage limits and loopholes, and resist the temptation to select roofing material based on what may be an empty marketing claim.

For more info, see my column Roof Repair 101.

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