Canadian Asphalt Shingle Manufacturers’ Association
TECHNICAL BULLETIN #2
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.
Typical/usually, depending upon the depth of the vault, there will be a form of soffit or roof edge venting and a ridge vent, but not always. The vaulted space may run the width/length of the roof.
No pics at the moment. I am not in the office.
At times, Infrared can detect cooler temperature if this space above and below the assembly. Chuck, or one of the other astute InterNACHI colleagues may port some insight.
Absolutely Robert, roof edge and ridge venting together are key indicators. However, many times I see Cathedrals with soffits and Roof Rake venting, which of course, if it’s properly venting the rake, leaves the last couple of feet without anything.
Never seen (in Canada) horizontal venting, mainly because it would be hidden. It was usual in the U.K. where we used lats to hold the tiles and perforated fascia at the gables.
Most tile and slate roofing I see here in Canada puts the tiles and slates directly (although profiled tiles would need some form of battening I guess) onto the roof deck which means the ventilation needs to go below the deck and therefore vertical.