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Here's a quick guide to show your the pros and cons of having asphalt roof shingles applied over your existing roof:
There are two options available for reroofing installations. One would be to tear off the old roof before applying the new one (tear off). The second would be to lay new shingles over the existing roof (lay over). While the second choice is the less expensive of the two options, it is not necessarily always the best choice.
There are advantages to tearing off the old roof before installing a new one. For example:
• If there are any defects in the roof deck, they will be revealed when the roof is torn off. These defects should be repaired before applying the new roof.
• If condensation problems exist in the attic, they too will be revealed when the roof is torn off.
Properly designed attic ventilation can then be installed in order to help eliminate such problems.
• When the old roof is torn off, waterproofing shingle underlayment can be installed before applying the new roof. This will help protect against leaks created by cyclical ice damage and wind-driven rain.
• Tearing off the old roof and starting with a clean deck before reroofing may result in a smoother finished roof system. Although there is added cost to these advantages, each lessens the likelihood that the validity of the manufacturer’s shingle warranty will be impaired. If the old roof is torn off, your contractor should be responsible for the cleanup and disposal of the old shingles, but make sure your contract states this clearly.
If you do plan to reroof over existing shingles, first check if your local building codes limit the number of roof layers that can be applied to a residence in your area. Most restrict maximum to 2 layers. Your contractor should know the pertinent code requirements.
Eric pretty much nailed it.
It is not usually worth it in my opinion and especially given the nature of most leaks and the condition of most sheathing.
Roofs aren't cheap so I think the math usually plays out that the full tear off is a better idea based on the fact that you get a better install and will likely have a roof that will last longer.
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