Technical Bulletin #6

Current as of June 2019

When the time has come to re-shingle your roof, one decision you will be faced with is whether to strip off the old shingles, or apply the new ones right over the existing layer. There is no easy answer. Although each roof must be evaluated individually, there are some general guidelines you can follow to help you make an informed decision.

You will probably have to tear off the existing shingles:

  • If an inspection of the deck reveals rotted or warped wood, or large gaps between the deck boards. Any rotten or damaged boards should be replaced before applying the new shingles. (Note: for best roof performance, you may want to consider re-covering "board" roof decks with a layer of minimum 3/8" plywood, before applying the shingles.)
  • If there are more than two layers of existing shingles on the roof. Most roofs are designed to carry the weight of three layers of shingles (plus snow loads). All roofs should easily handle the weight of two layers of shingles.
  • If the roof structure shows signs of sag across the ridge or truss lines. If the roof does not look straight or feel solid, you may want to have the structure professionally inspected to check for structural defects.
  • If the condition of the existing shingles is so rough and distorted that it would not be practical to flatten all raised areas enough for the next layer to lay flat

In some cases, it is not necessary to tear off the old shingles before applying the new ones; if your roof has only one layer of shingles, laying flat, and the deck is in good condition, a tear-off is not needed. Not only will the existing layer (although old and weathered) provide a secondary back-up roof for the new shingles, but it will also save you the cost and inconvenience of removing and disposing of the old shingles. However, it is suggested that before making a final decision, a verification be made with local building officials to ensure that you do not infringe on any building code. As well, consideration should be given to the shape/dimension of both the old and new shingles, as some combinations may telegraph the old layer through the new layer.

Even though shingles are not environmentally hazardous, unnecessarily dumping into landfills is a disservice to the community. Currently many companies are researching ways to recycle roofing tear-off material into other products. The next time your roof needs replacing, it is possible that technology will exist to more readily recycle those shingles, rather than throw them away.

The information contained in this bulletin is for general education and is not intended to replace advice from a qualified contractor or direction on usage/installation from the manufacturer. Consumers should be aware of the safety hazards associated with work on roofs and, before doing so themselves, should consider following CASMA s advice of using qualified contractors. This bulletin may be reproduced with permission on condition that it be reproduced in whole, unedited, with attribution of copyright to CASMA.