Asphalt shingles provide a very satisfactory water-shedding function on sloped roof assemblies. However, due to their characteristic flexibility in warm weather conditions, shingles can take the shape of (i.e. "telegraph") whatever lies beneath them.
On normal slope roofs this does not affect the water shedding performance of the shingles. However, on lower slope roofs (those between 2:12 and 4:12), water that enters this trough, by the shingle cut-outs or joints in the overlying shingles could 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 requirement accepted in the industry, to install first an approved underlayment under the laminated shingles. The underlayment provides secondary water shedding protection and helps ensure a long-term roof system performance.
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.