Is It Not Good to Lay a New Roof on an Old Roof?

Real Cost

• Cost is obviously a major concern for most homeowners. But consider how much you actually save by installing new shingles over old roofing. Removing old shingles is not usually a major part of the roofing expense, and shingles installed over existing roofing may not last as long as a new roof, so you may be looking at another re-roofing in a few years.

Building Codes

• Most building codes limit the number of layers of roofing which can be applied. Generally the limit is two, although some older codes will permit three layers. Neighborhood associations and residential development restrictions also may apply; many will not permit re-shingling. Adding a second layer of shingles also can affect cooling costs, since the second layer will add heat to the roof and make cooling harder and more expensive.

Weight

• Shingles are heavy, even modern lightweight versions, and adding a layer can greatly increase the load on the rafters, trusses and other framing. In areas of heavy snow and ice, this can affect the structural integrity of the roof. It also will be impossible to inspect the roof decking and underlayment if old shingles are not removed and will make it harder to replace metal flashing installed at roof junctures.

Insurance

• Many insurance companies will no longer insure houses with multiple roof layers. Virtually all refuse coverage on three-layer roofs, but some still allow two layers. One consideration is fire danger. A fire can smolder undetected between shingle layers, then erupt into a major disaster. Also most shingles are made of asphalt or other combustible materials, so adding layers just adds fuel for a fire. A homeowner's policy or insurance agent should clearly explain these options.

Warranties

• Warranties of shingle manufacturers may not apply if they are installed over existing shingles because of the possibility of failure not due exclusively to the new shingle. Some roofing companies also will refuse or limit guarantees when they must install shingles over an existing roof, because of the greater likelihood that nails will not hold properly or roofs will develop unsightly bumps and create openings for moisture to creep in.