New roofs are often added without removing the old roofing material. This reduces the cost of re-roofing.
Multiple layers of roofing may mean faster wear and a shorter life expectancy for the new roofing materials, because of the uneven surface that is created with the second layer lying over the first. 1. Multiple roof layers increase the dead load on the roof and make the structure work harder. In some cases, this causes roofing deflection. 2. Multiple roof layers often have nails that are too short to penetrate the roof sheathing. Roofing materials are more inclined to be blown off in heavy winds when multiple layers are used. 3. Adding new roofing over old often means that old flashings are not replaced. These critical components then become the weak link in the new roof. 4. Over-roofing prevents a proper inspection of the roof sheathing. Roofs are typically only replaced after leakage has occurred. In many cases, you don't know how long the leak has been going on and how much concealed damage may have been done. Putting new shingles over old ones sheds no light on this issue. 5. Multiple roof layers inhibit the ability of the roof covering materials to dry after a rain. This is particularly important with wood shingles or shakes. This can accelerate deterioration of the roofing.
With many roofing materials, the recommended maximum number of layers is one. Asphalt shingles are often installed with two layers. Most consider this acceptable. Three layers are not recommended, although this system, unfortunately, is very common.
Detecting Multiple Layers On Your Roof
You should try to determine the number of layers of roofing materials. You have to know how the roofing is applied to verify the number of layers. In most cases, there is a starter of some sort and then the shingles or panels are laid. You can determine the number of layers either from the lower edge of the roof or from the rakes, if there are gables. Understand that, in some cases, roofers will cut off the old roofing material around the edges so that it will look like only one layer when there are really two.
In some cases, you can get a look at the number of roofing layers at flashing details, because flashings are often not replaced when new roofing is applied over old. This, incidentally, is one of the weaknesses of re-roofing. The flashing materials are often durable enough to outlast the roofing covering material. Many flashings, however, are not durable enough to last through two roofing lives, and flashing problems are common when there are multiple layers of roofing.
In some cases, you can determine that there have been two layers of roofing by the nail pattern protruding through the sheathing from below. You have to be careful, however, because the shingles may have been removed, but the old nails were driven down into the sheathing. Nails should not be driven through the sheathing where there are open eaves (there are no soffits). This is unsightly from below and makes painting difficult. Sheathing boards often splinter where the nail heads protrude through the underside.
Asphalt shingles are often installed over wood shingles and, in some cases, over slate.
Re-roofing is much more expensive if you have to strip the old shingles first. The stripping cost is also increased if there are three layers rather than two, for example.