Is it ok to install new shingles on top of old ones?

Here at Rydel Roofing in Ottawa, we are often asked if it is ok to install new shingles on top of old shingles. There is contradicting information on the internet regarding this. We believe the answer is a hard no and while there are a plethora of reasons why it is a bad idea, we have put together 5 of the most important ones.

  1. The existing shingle on your home is more than likely what is referred to as a three-tab shingle or organic shingle. These paper-based shingles coated with granules have done a great job of protecting your home from water, wind, hail, snow and everything mother nature could throw at them but they have also retained a certain degree of moisture. Additionally, there is a very high chance that moisture has accumulated beneath and between the shingles. If one were to install new shingles on top of these existing shingles, they would essentially be trapping this moisture in between the new shingles and the old ones creating a perfect environment for mould and bacterial growth. Over time, this will not only reduce the lifespan of the new shingles but also reduce the lifespan of the plywood supporting them.
  2. The second reason why it is a terrible idea to place new shingles on top of old shingles is that even if the old shingles look fine and flat, as time passes they will inevitably curl and warp in an upward direction. This warping will put upward pressure on the new shingles and create small gaps. As strong winds hit your shingles, these gaps will become susceptible to uplift and they will inevitably be pulled right off your roof.
  3. We believe in doing the job right the first time and that means completely removing all of the old roofing materials off your roof before installing new materials so we can inspect the plywood. If the plywood on your roof is mouldy, rotten or sagging, we will replace it and start with a fresh sheet.  Here at Rydel, we will even replace up to 3 sheets of plywood free of charge!
  4. Every shingle manufacturer offers great warranties for their products. It has become the industry standard to offer at least a 25-year warranty against manufacturer defects and if a homeowner installs new shingles on top of old shingles, these warranties become null and void.
  5. The single most important reason why you should never install new roofing shingle on top of old shingles is that it is illegal and for good reason. In the event that your home catches fire, the fire department will need to be able to cut holes in your roof both to prevent backdraft and to give access to water. If a roof is covered with multiple layers of shingles, it becomes nearly impossible to cut these holes using an axe.