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Question DetailsAsked on 10/3/2014

do i need to remove one layer of shingles to re-shingle my roof?

will it affect my insurance or the longevity of the shingles if they are on top of a layer that was installed properly and doesn't currently leak?

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1 Answer


Whether it will affect your insurance is hard to tell - and of course if you ask them they will say YES, do not roof-over, plus it will go in your file that you are roofing-over. Will normally affect longevity - I would guess by maybe 5-10 years, because they are not as uniformly supported, wind can get under them a bit easier to lift them, and the buried layer will run hotter than usual and start to deteriorate faster under the new layer as well as make the surface layer hotter on extended hot days due to the increased underlying insulating factor.

One significant factor is if you roof-over (unless you spot check the existing underlayment) you do not know what the condition of the existing underlayment is - it may be well deteriorated with splits all through it (very common with tarpaper) so by roofing over you are counting on just the two layers of shingles to positively stop ALL water, as the underlayment may be essentially shot. This is particularly important to you if your roof is subject to strongly blowing rain or ice damming, or even mid-winter freeze-thaw on snowpack, which forms invisible ice damming and water backup under the snow pack. In addition, putting new shingles over deteriorated underlayment means the nail holes have no water barrier, so you may get a lot of small leaks into the sheathing that will start to deteriorate it.

Also, you have no way of knowing what condition the flashing is in, so you may have leaky flashing material underlying your new work - especially in valley areas where you might end up overlaying new woven valley over a valley flashing that is rusted clear through.

Ditto to possible concealed sheathing damage or rot. All ofthe above make enforcing a warranty on either the manufacturter or installer a very iffy thing - basically you are trusting them to be fair, because you handing them a ready-made excuse for failure. And the worst thing is, you are counting on the underlying old materials the most in the case of a roofer who does not do a top notch job on the reroof, so while a roof-over might work fine when done by a very careful and expert roofer, if you get a shoddy or less competent one, then you are counting on old and at least somewhat partly deteriorated materials to stop the resultant leaks - not a bet you would get good odds on in Vegas, I am sure.

For my money, I would certainly consider it for a barn or tractor or outdoor equipment storage shed or such, and maybe for a house in normally very dry milder climate areas without snowpack, but for the $500-1000 extra (at least in my area) for a normal sized house tearoff, it is not worth putting the other $4-9,000 for the roof replacement at risk. Plus it may or may not void the new shingle warranty, but even if it does not say so in writing effectively it does void it because unless the warranty claim is because the new shingles disintegrated in just a few weeks due to an obvious manufacturing problem, the manufacturer (and roofer) can always blame any failure on the fact they are overlying other shingles that did not provide the proper nailing surface.

There are a few other prior questions with responses under this answer that might interest you too.

Answered 6 years ago by LCD

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