House roofs should never exceed three layers of shingles. Building and city codes require roofers to limit tile layers to two. At most, you can safely lay up to two layers of organic asphalt or fiberglass roofing shingles with an inclination of up to 18.5 degrees. More than two layers add too much weight to your roof.
Some local building codes don't allow overlaps; in those municipalities, you can only have one layer of shingles. It's always a good idea to review the applicable codes before starting a repair or renovation project. In some cases, 3 layers may be allowed, but this is rare. Most areas only allow a maximum of two layers.
This will also be limited by other factors. The contractor can walk the roof and also inspect from the attic for rot before installing a second layer of shingles. If there is rotting wood, it will need to be replaced before performing any other roof repair work. New layers of shingles can be added to a roof, but there are more factors that need to be considered.
Overlapping can have its benefits, it can also inadvertently cause damage to a new layer of shingles if the older layer was not inspected properly. As mentioned earlier, the small savings you can get from installing an additional roof might not outweigh the disadvantages of doing so. Single-layer roofs are better because you don't have to deal with warranty issues. Most roof warranties are considered void when installed over an existing roof.
One reason behind this is that two layers of roof accumulate more heat, which can cause unwanted damage to the new roof. This causes damage to the new roof and shortens its lifespan. Therefore, you may find yourself replacing the roof sooner than expected. This makes the project even more expensive than originally planned.
An additional roof layer may look pretty good on the surface, but single-layer roofs are much better. One thing you need to do is check that your home is capable of supporting the extra weight of the new roof layers. The roofing professional can check all other variables and then advise if your roof would be a good candidate for a second layer of shingles over the old one. But if you have a tight budget and the current roof has a layer of shingles, it is possible to make a new roof with nails if the situation is right.
If you're considering adding a second layer of shingles to your roof, know that it's not the best option for every situation. Just remember to consider the cost of stripping and disposing of two layers of shingles next time. In the long run, the additional layers can trap heat and moisture, causing the underlying structure to break down faster and tearing off two layers of shingles at a later stage will more than likely be much more costly than replacing the single layer of shingles twice. While roof replacement is sometimes required, adding a second layer of shingles over the damaged part (also called an overlay or scale) may be a viable option in some situations.
In most cases, with new advances in shingles and other roofing system products, there's really no need to have a second layer. A full roof batten and a new shingle allow for a complete roof inspection, a new layer of felt on the roof deck and evenly placed shingles. Before you add a second layer of shingles, you'll want to check your roof's warranty to make sure this type of repair doesn't diminish or negate it. As long as the cladding (material on which the shingles are laid) has not suffered significant damage, or the roof structure itself is able to support and support the weight of the additional layers, there should be no problems.
One of the biggest benefits associated with double coating, rather than stripping and placing a whole new set of shingles, is simply cost savings. It is recommended that homeowners do not attempt to place a second layer of shingles on the roof, unless they are qualified to do so. Since an overlap involves roofers simply laying a new layer of shingles over old ones, they don't have to remove existing shingles. If you're looking to do something reasonably priced as a short-term solution, then a second layer of shingles would be a good idea, as long as your roof can handle it and it's within local building codes.