"Troughing" of
Glass Shingles


Technical Bulletin #22

Current as of March 2016



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Glass fibre asphalt shingles provide a very satisfactory water-shedding function on sloped roof assemblies. Glass shingles, due in part to their lighter-weight reinforcement, are much more flexible in warm weather. However, in certain situations, with certain products, small ‘troughs’ can develop within the shingle layers, as a natural outcome of the way shingles overlap, and their geometry/shape. Consequently, when applied to the roof, they often take the shape of (i.e. "telegraph") whatever lies beneath them

Diagram - Troughing of Glass Shingles

As the glass shingles are applied, there is a natural build-up of "layers" of shingles, which is essentially how the finished roof sheds water. In the first few years, as the summer sun heats up the roof, the shingles soften and slowly conform to the shape of the layers underneath. Just above the lower exposed edge of each shingle course, lies the concealed top edge of the shingle two rows lower on the roof. This concealed top edge may form a small "trough" in the unexposed part of the shingle (see diagram). On lower roof slopes (those below 8:12), water that enters this trough (through the shingle cut-outs or joints in the overlying shingles) can travel laterally along the trough, and penetrate the shingle layer through misplaced fasteners or shingle joints.

This potential for leaks due to troughing is one of the reasons CASMA strongly supports the standard accepted industry requirement to use approved roof underlayment under glass fibre shingles. The underlayment provides secondary water shedding protection, and also helps ensure long-term roof system performance.

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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.

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