A roof overlay is a type of roof installation that uses the old shingles as the underlayment for the new roof. Although overlays can be done well, in some cases problems can arise. If you have a home that has had an overlay in the past, it's important to know when it is time to replace it. 

1. Layer Thickness

In most municipalities, a roof overlay is only allowed once, which ensures that there are no more than two layers of asphalt shingles on the roof. Any thicker and the chances of issues increases. First and foremost, a thick layer of shingling can lead to more weight on the roof support system than it can support, especially when additional weight is added such as in a snow storm. More layers also increase the chances of leaks occurring if one layer fails. If there are more than two layers of shingles, you need to replace the roof.

2. Ventilation Issues

A standard asphalt on asphalt roof overlay shouldn't result in ventilation issues as long as the upper shingles are installed flush on top of the lower level of shingles. Issues will only occur if there are voids between the layers where moisture can accumulate easily but vent out poorly. Metal panels overlaid on asphalt are more likely to experience ventilation issues. For this reason, a proper overlay uses wood batten strips between asphalt and metal to provide ventilation. 

3. Decking Damage

It's a general rule that the old shingles must be in decent condition if an overlay is to be done. There can be minor leaks and damages, but nothing major that could affect the new shingles. Unfortunately, the decking beneath the old shingles is harder to assess, since water damage to these plywood sheets is often hidden by the old shingles. If you notice water spots on the attic ceiling or other signs of leaks, a new roof is needed because the decking has failed regardless of the overlay.

4. Warped Under Layer

Another prerequisite of an overlay is that old shingles need to lay flat. If they are curling or wavy, then that will affect the upper layer of new shingles so that they too will eventually become warped. Shingles that don't lay flat can allow water beneath them, a major concern that requires a new roof.

5. Aging Issues

Although overlays are a suitable option that can save money, they typically don't have the same lifespan as a completely new roof. For this reason, you can expect a roof overlay to wear out more quickly. It's important to inspect the roof often so you catch aging issues early before major leaks occur.

Contact a roofing company if you need to replace an overlaid shingle roof.