Cathedral ceilings covered with asphalt shingles require adequate ventilation like any other roof to prevent damage to the shingles. There should be a space of at least 1 inch (25 mm) between the roof sheathing and the insulation to allow for the unobstructed air movement. For most residential roofs, 1 in. (25 mm) air space is sufficient, but for lower sloped roofs (less than 5:12) or long roof runs (>30 feet from eave to ridge) a minimum of 2 in. (50 mm) is recommended.
When a vapour barrier is used, cathedral ceilings require a minimum total net area for inlet and outlet vents equivalent to 1/300 of the total ceiling area. Cross ventilation should be ensured by locating half the required vent area at the eave and the other half at the ridge.
If a vapour barrier is not used, then the vent area should be doubled to 1/150 of the total ceiling area.
A typical installation is shown in the following diagram.
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.
CANADIAN ASPHALT SHINGLE MANUFACTURERS' ASSOCIATION (CASMA) is a corporation which was created on July 24, 1989 pursuant to the Canadian Corporation Act and continued under section 211 of the Canadian "not-for-profit" Corporation Act on October 12, 2012.
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