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I have a building with 3 roofs on it, the top layer is starting to curl, etc, due to its age. My question is if having 3 layers of shingles will extend the useful life of the roof? I know this is vague but I hope you see where I'm going with this.

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    Why not tell us where you are going? I guess you want to try and just remove the top layer leaving the two older layers? – JPhi1618 May 20 '16 at 19:22
  • Where I am going with this is: at what point should I worry about roof failure, because there are 2 "backup" roofs under the top layer. Can I reasonably expect to get thirty + years out of this roof, or should I start budgeting now for a tear off in the immediate future. – davidaug May 20 '16 at 19:47
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    People don't put extra layers on a roof for added protection. They put extra layers on after the initial one fails instead of removing it and starting over. So your roof has already been replaced twice without removing the faulty layer first. – kinar May 20 '16 at 20:23
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    Additional layers work best when you put them on when the existing layer is still in good shape. If an additional layer is put on after the existing layer is already starting to curl, the lifespan will likely be greatly reduced. The lifespan of your roof will depend on what shape the lower layers were in when the additional layers were laid. – Michael Richardson May 21 '16 at 14:49
  • Multi layers can have issues with wind damage, especially if the roofing nails in the upper two tiers don't penetrate well into the sheeting. As to the "Backup Roofs", the two lower layers have been pierced and only the topmost provides the sealed membrane to shed water. – Fiasco Labs May 22 '16 at 4:21
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No. Layers of shingles on top of each other shorten the life of the roof due to the newest layer sitting on an uneven lower surfaces.

The best life will come from removing all the old layers down to roof boards or sheathing and then laying down the new roof on a well repaired flat surface.

Multiple layers of shingles happen because folks want to avoid extra removal work or to save the cost of removal labor and disposal costs. There is basically no truth to claims of benefits of stacking shingle layers.

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    Since the top layer is currently curling, it doesn't seem to have helped them at all. – JPhi1618 May 20 '16 at 19:36
  • @JPhi1618 - If they were cupping, I'd agree. They're curling, which means they're nearing the end of their life due to exposure. – Mazura May 21 '16 at 3:24
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    The cupping and curling is also brought on in part by the shingles settling into the low spots of the layer underneath when they get really hot in the summer time. A prime part of the shortening of their life span. – Michael Karas May 21 '16 at 4:40
  • The curling is likely due to having bought a batch of bad shingles. In the Midwest Certainteed was putting out bogus shingles for about 10 years (I think they've fixed it now), and several other manufacturers have had similar problems. – Hot Licks May 21 '16 at 21:41
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As Michael Karas said, this happened because the previous owner was possibly lazy or too cheap to afford labor and disposal... or they may have been physically impared and living on a budget.

Adding an extra layer of shingles when the first layer has met it's life expectancy can buy a few more years... maybe 5 - 10 years but the life expectancy of second layer is less than it could be (10-30) years. The third layer is a waste- only lasts a few years (maybe).

Most importantly, the extra layers adds a lot of weight (pushing the load limit for the roof). This is kind of dangerous as it might cause the roof to collapse, (especially if you get much snow).

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  • Are shingles really that heavy? I can't imagine them weighing more than an inch or two of snow. – Mark May 20 '16 at 21:07
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    @Mark Shingles weigh about 2lbs per square foot... or in the case of 3 layers, 6 lbs. For a small house (1000 square ft roof), that's 3 tons. – Ben Welborn May 20 '16 at 21:17
  • Or about four inches of fresh snow. More than I thought, but still not much. – Mark May 20 '16 at 21:21
  • Snow typically weighs 7-20lbs/cubic foot (depending on compaction), so @Mark 's estimate wasn't that far off. – Dan Is Fiddling By Firelight May 20 '16 at 21:22
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    3 layers of shingles is like 4.8 inches of snow... all year. Time is ticking... what if it snows another 6 inches? – Ben Welborn May 20 '16 at 21:25
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The rule of thumb is three layers maximum. After that you pull them all off and start over.

Half a century is about time to get a good look at the sheathing, and if any of it is bad, a good time to inspect the joists as well. I'm not saying you need a structural engineer, but you'll want someone on the squad who knows when to say, "This has all got to go." Be it just the roofing, the sheathing, or on to a few new joists.

Where you're going with this, is completely new roof(ing), assuming it leaks. The old shingles aren't helping; they all have nails through them from the latest install.

Beyond three layers, weight becomes a pressing concern.

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in my time as a project manager for a construction company in TX and CA I learned that 3 is the maximum number of layers (legally) allowed anyway. If you're near a coast with hurricanes or in tornado areas, 2 layers is the max due to length of fasteners required to hold into the wood. Time to take them down, inspect the framing / sheathing and start fresh.

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