It would seem that a double layer of shingles should be twice as good as one but, unfortunately, that’s not the way it works. Applying your new roof over the old one does have a couple of short-term advantages though. You save the cost of removal and disposal of the old roof, and that also means the new roof is completed sooner.

    Those positives are outweighed by these problems:

•• Shorter lifespan - The old roof continues to deteriorate under the new roof. A couple of reasons are that two layers of shingles traps more heat and adhesion of the new roof is not as good as directly on sheathing.

•• Warranty voided - Most shingle manufacturers void their warranty when shingles are applied over a existing roof.

•• Costs much more next time - Three layers of shingles is not allowed by the building code, so the tear-off labor you saved on the double-layer roof will cost you much more on the next roof.

•• Ancient flashings - More roof leaks happen at the flashings at roof valleys and penetrations than in the field. Old flashings, often now corroded with age, are left in place for a double-layer roof.

•• More difficult fixing leaks - An additinal layer of shingles makes it more complicated to find and fix leaks.

•• Extra weight - Many roof decks are not engineered to accept two layers of shingles. Mobile homes, in particular, are not even allowed to have a double-layer shingle roof for this reason.

    If you are buying a house and want to determine if it has a double roof, also called a “roof-over,” see our blog post How can I tell if a roof has more than one layer of shingles?

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  To learn more about roofs and attics, see these other blog posts:

• Why is my roof sheathing sagging between the trusses?

• Why is granule loss a problem for an asphalt shingle roof? 

• What are the mistakes to avoid when doing attic improvements?

• What causes roof shingles to curl up at corners?  

• What causes shingles to buckle along a line on the roof? 

• Why is my roof leaking?

• What causes leaks at a fake roof dormer? 

• What causes a sagging roof ridge line?

• What causes bubble-like blisters in a built-up and gravel roof?  

• Why does it cost so much more to replace a steep roof than a low slope roof? 

• What is "ponding" on a flat roof?

• Is an attic required to have a light by the building code? 

• How can I inspect my roof for hurricane damage?

• Why is premature curl of roof shingles a problem?

• How can I tell if a roof has more than one layer of shingles? 

• What are the common problems with attic insulation? 

     Visit our ROOF AND ATTIC page for other related blog posts on this subject, or go to the INDEX for a complete listing of all our articles.