We frequently hear from homeowners seeking answers to repair and maintenance problems. Many of these questions and answers are compiled in our Home Improvement Library. Here is a recent question.

Question

I’m having a roof replaced, and parts of the existing roof has mold. Does the old roof have to come off or can a new roof be applied over the existing roof?

Answer

Applying a new roof over an old roof is considered acceptable practice, but we are firm believers in removing the original layer, especially if the old roof has issues like you describe.  There are 4 reasons why we advocate always removing the original roof:

1. If you know you need a new roof that usually means it’s at the end of its usable life. Signs to look for are blistering, brittleness, breaking, cracking and curling of the shingles and, of course, leaks. There’s no benefit to covering up old problems, and they will most likely affect the durability of the new roof.

2. Adding multiple layers increase the amount of radiant heat of the roof which in turn reduces the effectiveness of your attic ventilation, which is often under vented in the first place.

3. Can your roof carry the extra weight? Since the 1970s the use of trusses are more common. Trusses are often spaced at 2 ft. intervals with a layer of 3/8” – 1/2” plywood or OSB. Ever wonder why you see dips in new roofs? Well now you know! Then add another layer of shingles to the weight and see what happens. Anytime trusses are used we advise clients to check with the manufacture to insure the truss is designed to carry the additional loads.

4. Warranties are only as good as the installation. Manufactures and claim adjusters like to point out such things when a homeowner decides to file a claim for a new roof due to hail and wind damage. They have learned from experience that multiple layers do not do as well during storms.

The biggest benefit of tearing off an old roof is that it is an opportunity to inspect and repair any water damage or rot with the roof decking, soffits and eaves, and flashing. It’s also the time to inspect attic ventilation (which has direct impact on the life of your roof shingles), and lay down a high-performance underlayment over the decking that acts as a secondary barrier against water leaks.

Bottom Line: Even though 2 roof layers are legal, we do not like covering over any old roofing materials with another layer because we don’t believe in covering up problems. We urge you to remove both layers of roofing so that the new roof investment will last as long as its intended to, and that you have peace of mind that your roof is sound from top to bottom, inside and out.

Learn more about Mosby Building Art’s residential roofing service.  And if you’d like to have your roof inspected and a new roof installed right the first time, please call the Mosby office at 314.909.1800 or contact us here.

Related posts:

1. When Do You Need a New Roof?

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Tags: remove old roof, roof inspection, roof installation, second layer of roofing, two layers of roof

This entry was posted on Thursday, February 16th, 2012 at 12:35 pm and is filed under roofing. You can follow any responses to this entry through the RSS 2.0 feed. You can leave a response, or trackback from your own site.