When a roof needs to be replaced, it is sometimes alright to just add a layer on top of the current roof. It depends on two factors, what condition the current roof is in and the amount of layers that are already there. Layering shingles over the existing shingles is a quicker fix but can lead to damage in the long run if the current layer is already heavily damaged.
Condition of the Roof matters most.
If your existing roof has damaged shingles, this can lead to further damage to the new layer of roof. This damage can result in extensive interior damage to the house. A roof that has shingles in a relatively good condition can withstand another layer of shingles. This is an attractive option because a homeowner has to pay less hours of labor. It takes time to strip the roof off a house. The reduced number of hours means that the homeowner can save a bit of money.
Keep in mind, however, that adding layers of shingles also adds weight to the roof. This added weight adds stress to the support structure of the house. Some building codes only allow for two layers of shingles. It is important that the first layer of shingles aren't cupped, warped or bent because this can lead to damage of the second layer of shingles. The second layer will take on the flaws of the first layer. Some manufacturers will void their warranty of a roof if there is a second layer applied. It is important to check with the manufacturer before adding a second layer to the roof.
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