Cathedral ceilings covered with asphalt shingles require adequate ventilation like any other roof to prevent damage to the shingles. There should be a minimum 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 roof designs a 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.