It used to be very common to pile on the layers of roofing one over another. This, however, has changed. Building codes in most jurisdictions limit the amounts, because roofing - especially asphalt - can be very heavy. Most jurisdictions allow two layers of asphalt shingles on shallow roofs and three on steeper slopes (above a 4/12 pitch). (The IRC, or International Residential Code, permits three layers.)

Many homeowners are now wondering if they should have their new roof installed over the existing one or if they should just rip it all off and start new. There are a number of things to consider, such as whether or not the existing roofing is over damaged underlayment or roof structure.

Some areas are even banning the use of a second layer of roofing. In those locales where multiple layers are still allowed, it is important that you think about whether or not this is something that you want to do.

Here are some considerations to keep in mind before you decide if you should rip off the bottom layer or if you should pile on another.

The Weight: One of the biggest points to consider is the weight of the roofing materials to be added to structure. Additional layers or roofing may be too heavy for the home’s frame to hold up, causing problems with the home’s structural integrity. If the home is older, this is definitely a problem. 

Generally, there isn't an issue with adding roofing over only one layer. And we find that because our metal roofing is so lightweight, at times it can even be installed over two existing roofs. But local building codes need to be checked to verify.

Waste, Time, and Cost Reduction: Leaving the extra layer under the new one is going to reduce the waste from this particular roof installation.  (But eventually, it will most likely have to be removed.)  If leaving your old roof on is a viable choice for you, it will most likely save you quite a bit on the project. There will be no labor for removal of the old roof, and there will be less cost for waste disposal since that old roofing isn't going to have to be hauled away.  And since you're eliminating a major step, your new roof installation will take less time.

Appearance: Problems happen when you start to layer the roofing materials one over another.  Often installing a layer of asphalt shingles over old ones results in a lumpy, bumpy roof. Those old shingles are most likely curled or buckling, and the new roofing won't smooth them out.  However, we have found that when we install our metal roofing over asphalt, we are able to get a more even result.

If you want a smoother, better looking roof, then we usually suggest that you skip the second layer and go for a fresh one. We would be happy to come out and take a look at your existing roof, however, and let you know whether or not a tear-off would be our recommendation. 

Having a new roof is an important thing to think about, but when the time comes to consider if you should add a second layer, always adhere to the local regulations and the recommendations of the roofing company that you hire. 

Quarve Contracting, Inc. is a licensed Minnesota residential building contractor, and we specialize in exterior home improvements, including roof installation.  We serve the greater Twin Cities metro area.