While putting a new shingle roof over an older one seems to make sense because it might provide an extra of protection from the elements, a multiple-layer shingle roof actually creates more problems than benefits. The bottom layer (or layers) continue to deteriorate, becoming more brittle, buckling, and cracking underneath over time. It also creates additional dead weight on the roof and makes securely nailing the new roof shingles through the old ones more difficult. 

    Plus, to top it all off, leaving the old roof in place under a new one typically shortens the life of the top layer of shingles. And, because the old roof is not considered an acceptable substrate for a new roof by shingle manufacturers, any manufacturer’s warranty is voided. The only real benefit is a savings in labor and disposal fees for removal of the old roof shingles.

    Also, 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:

• What are common problems of asphalt shingle roofs?

• 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?

• 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? 

• Why is a popped nail in a shingle roof a problem? How do I fix it?

• What are the most common problems with wood roof trusses?

• What causes a lump or dip in the roof? 

• If my roof is not leaking, why does it need to be replaced?

• How can I be sure my roofing contractor got a permit?

• How many layers of roofing are allowed on a home? 

• What are those metal boxes on the roof?

• What does "lack of tab adhesion" in an asphalt shingle roof mean?

• Why do roof edges start leaking?

• Why do my dormer windows leak? 

• Do home inspectors go on the roof? Do they get in the attic?

• Should I put gutters on the house? 

• How much of a roof truss can I cut out to make a storage platform in the attic? 

• What's the difference between an "architectural" and a regular shingle roof? 

• What does a home inspector look for when examining a roof? 

• Do stains on the ceiling mean the roof is leaking?

• How can I tell if the house needs a new roof?  

• Why does my homeowner's insurance want a roof inspection?

• What are the hazards to avoid when going into an attic? 

     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.