BCAB #1402jag - Roof Space Ventilation, Subsection 9.19.1

July 18, 1995

BCAB #1402

Re: Roof Space Ventilation, Subsection 9.19.1

Project Description

The project in question is a new ski lodge. The 9 in 12 pitched roof, with no attic space, is constructed with 16" wood I-joists. The webs of the joists are drilled with 13/4" holes at 24" o.c. with the centre of the holes being 23/4" from the top of the joist. These holes are intended to provide roof space ventilation.

Reason for Appeal

Subsection 9.19.1. requires roof spaces above insulated ceilings to be ventilated. Article 9.19.1.1. requires vents totaling at least 1/300 of the insulated ceiling area with at least 25% of the vent area at the top of the slope and 25% at the bottom. Article 9.19.1.3. deals with ventilation of roofs with no attic space. Cross purlins are required unless the roof slope is 1 in 6 or greater, the framing members run in the same direction as the roof slope and there is a 75 mm clearance between the underside of the roof sheathing and the top of the insulation. Venting in such a roof must be distributed approximately 50% at the top and 50% at the bottom.

Appellant's Position

The appellant contends the roof system meets the Code requirements for ventilation of roof spaces. The area of the roof vents exceeds 1/300 and is distributed approximately 50% at the ridge and 50% at the soffit. The framing members run in the same direction as the roof slope, the slope exceeds 1 in 6 and there is 100 mm clearance between the roof sheathing and the insulation. The appellant contends that cross purlins are not required but the ventilation holes in the I-joist web provide cross ventilation between joist spaces and this, in fact, exceeds the requirements of the Code.

Building Official's Position

The building official maintains that the laminated valley rafter runs at 45 to the roof slope and with the deletion of the cross purlins the valley areas are inadequately ventilated. In the interpretation of the building official the ventilation holes in the joists would have to be larger at the full length joists and get smaller as they approach the valley rafter.

Appeal Board Decision #1402

It is the determination of the Board that Sentences 9.19.1.3.(2) & (3) are not applicable because the valley joists, which the Board considered to be "roof framing members", do not run in the same direction as the roof slope. The roof slope was considered to be perpendicular to the ridge. Therefore, cross purlins conforming to Sentence 9.19.1.2.(1) are required unless it can be shown, to the satisfaction of the building official in accordance with Section 2.5, Equivalents, that the cross vented I-joists provide an equivalent level of roof space ventilation.

George R. Humphrey, Chair