If you’re thinking about replacing an asphalt shingle roof, you may be wondering whether or not it is necessary to remove the old shingles before installing the new ones. The answer is...maybe. It depends on local codes, how your roof is built (notably how many layers of roofing it already has), and how pristine you want the new roof to look.

The main advantage of leaving the old roof on is obvious—you can minimize the mess, removal costs, and disposal expense if you can leave the old shingles in place. Other advantages are that you end up with an extra barrier against leakage and a little extra insulation.

But shingles are heavy. When roofs are built, they’re designed to carry a certain load. Every layer of shingles on a roof adds to the load the structure must carry. With this in mind, some states allow only one layer of asphalt roofing, some will permit two roofs, some allow three. Check with your local building department.

If any areas of the roof have become severely damaged, it’s usually better to remove the old shingles and start from scratch. This also gives the roofer the opportunity to examine the sheathing for water damage. Even if you’re going to add a roof on top of an old one, it’s a good idea to remove any loose shingles and apply a new layer of 30-pound roofing felt before laying the new shingles.