Adding a new second layer of shingles over an existing shingle roof is called a “roof-over” in the construction industry, and it does have a couple of advantages:

1. The cost of removal and disposal of the old roof is eliminated.
2. It has been required in Florida for about the past decade that any new roof installation must have the roof sheathing re-nailed with 8d (eight penny) nails at a closer spacing than previously required. This is not necessary for a roof-over, and saves a homeowner more money. 

    But the downside is that a roof-over typically does not last as long as a roof replacement and, when you do replace it, the tear-off and disposal of two layers of shingles costs extra. A maximum of two layers of shingles is allowed by the building codes. Plus, you have to do the re-nailing that was put off by the previous roof-over.

    One clue that a home has had a roof-over which is easily observable from the ground is that it looks a little lumpy overall, but especially at the roof penetrations, like plumbing vents and gas appliance flues. The photo above of a plumbing vent at a roof-over is a good example.

And here’s another example at furnace flue. Notice the waviness at the ridge also.

The way to be sure you are looking at a double layer of shingles is to get up on a ladder (if you are comfortable doing that) and examine the shingles at the edge. A normal roof has a “starter” row of shingles and then a single layer above it, for a total of two layers of shingles. A roof-over will have four layers, like in the photo below.

    Also, see our blog post Are two layers of shingles better than one?

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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?

• 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? 

• What is the life expectancy of an asbestos cement shingle roof? 

• Are roof trusses better than roof rafters (stick framing)? 

• 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 the dark lines running parallel to shingles on my roof?

• Can metal roofing be used on a low slope/pitch roof? 

• How can I make my roof last longer?  

• What are the warning signs of a dangerous attic pull-down ladder?

• How can I find out the age of a roof? 

• Should I buy a house that needs a new roof?

• Should I buy a house with an old roof? 

• 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.