I just got off the phone with a home inspector friend of mine and we were discussing the upside versus the downside of multiple layers of asphalt shingles, so I thought this would make a great blog that would help educate our readers.
First of all, multiple layers of shingles simply means layering a new asphalt shingle roof on top of the old one rather than stripping the roof completely to expose the sheathing and replacing it with new shingles. Most building codes allow this, but is it a good idea? Here are the pros and cons:
Pros to Overlaying a Shingle Roof
● It’s cheaper. The most common pro for overlaying a roof is saving money. The average re-roofing job will cost about $600 less than a tear-off job, and you’ll save a little on cleanup, disposal costs and dumpster rentals, too.
● It’s not as messy. Because you’re not ripping off an entire shingle roof, obviously the job won’t be as messy.
● It’s faster. Again, when you’re eliminating the entire stage of tearing off (and cleaning up) a roof, it will save time.
Cons to Overlaying a Shingle Roof
● Can’t detect leaks or other damage. When you’re just layering over the old roof, you won’t get the same opportunity to fully inspect the roof’s sheathing. There may be leaks, bad flashing, damaged or rotted wood, algae growth, etc., and you can’t fix or prevent what you can’t see.
● Added weight. You’re essentially adding a second roof on your house, and that added weight puts stress on the roof decking, which is especially worrisome in our climate that gets heavy rain and snowfall. Note: the average 30-square roof uses 90 bundles of shingles, and a bundle of asphalt roofing shingles weighs about 100 pounds, which means you’re putting roughly 9,000 additional pounds of weight on your roof.
● Gets hotter. Having multiple layers of shingles is going to make your roof hotter because the heat gets trapped between the layers, which could mean premature aging of the shingles. This is especially true if you’ve overlayed your new shingles on top of warped shingles.
● Affects inspection reports. If you have multiple layers of shingles and are trying to sell your house, a home inspector will likely view layered shingles as a negative since it could indicate potential problems for the new homeowner. It could also shorten or even void your shingle warranty in some cases.
As you can see, there are a lot more cons than pros when it comes to overlaying a new asphalt shingle roof rather than tearing off the old one and replacing it. The only real upside is cost, and the savings just aren’t worth it, especially when you realize that at some point you will need to replace your roof, and at that time you will spend any money you saved this time round.