Overlaying new shingles over old ones is commonly touted as an inexpensive way of repairing a damaged roof instead of replacing it. It is true that overlay is inexpensive compared to complete replacement, but it does have its disadvantages. Here are some of the complications you may experience with a shingle overlay:

The New Shingles May Not Lie Flat

Any distortion in the old shingles will prevent the new ones from lying flat on the roof. For example, shingles have a tendency to curl on their edges when they age; this is mainly due to weathering. Whatever the cause of the curls, they will prevent the new shingles from lying flat, which means it will be easy for the new layer to get blown off the roof due to wind forces.

Moisture May Get Between the Shingle Layers

It's difficult for the space between the two layers to be watertight. Melting snow or rainfall may get between the shingle layers, where it may take time before evaporating since it won't be exposed to direct sunlight or air. The longer the roof remains wet, the higher the chances that the wood sections will rot, or it will encourage mold growth; both of those things weakens the roof.

The New Weight May Be Too Much For the Roof

There is also a chance that the additional layer of shingles may make the roof too heavy for itself. Excessive weight may damage other parts of the roof such as the framing and joists. This is especially likely if there were hidden damages in the roof's structures that were not unearthed before the installation of the overlay. Don't forget that snowfall may also add more weight to the already burdened roof. The combined effect of all those things may cause the roof to collapse.

It Complicates Your Next Repair Job

Lastly, no roof lasts forever, and even your current one will have to be replaced at some point. Unfortunately, having multiple layers of shingles makes the replacement task more difficult and expensive. The labor and disposal costs will be relatively high; you have to tear off all the layers and cart the relevant materials to the nearest landfill.

Therefore, think twice before deciding to lay new shingles over existing ones. An overlay is only advisable if other parts of the roof are in excellent conditions and are likely to last for many more years still. Talk to a roofing repair contractor to inspect your roof and tell you exactly what's in store for you if you install new shingles over it.