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How to vent (or should I vent) this addition over-framed roof?

evan_edstrom | Posted in Energy Efficiency and Durability on

I am doing an addition and remodel as an owner-builder. One of the details is an over-framed roof structure to alter the roofline. This sits on top of the existing roof. The old overhangs were cut off and the top of the wall extended to support the new overhang.

My first question is about venting the new roof. The roofers will install a ridge-vent (like cor-a-vent’s roof-2-wall), however the intake is going to be tricky. Because these aren’t full rafters, there is no soffit from which to draw in air. I have drilled vent holes on the new overhang blocking, but the amount of area at the bottom corner is small. I could add a supplementary wall vent like would be traditionally used at a gable end. Maybe a better option exists?

The second and perhaps more important question is about venting the original roof rafters. Since the original overhangs were cut-off and walled over, there is no way for air to reach the bottom end of the original rafters. I could cut holes in the original roof decking between these two areas, but am unconvinced this will provide sufficient airflow. Providing a wall vent that low seems tricky to detail properly, especially with a rainscreen and may not look great.

I had considered an unvented or hot-roof assembly for this portion of the house, but I’ve never done one before. It seems like it could solve the issue of trying to vent the original rafters and sheathing. I live in the Seattle area, so ice-dams are a minor but possible concern with a hot-roof. If I went this route, would I be better off with open or closed cell? Closed cell would keep interior humidity from reaching the sheathing, but surrounds the sheathing with vapor barriers which seems risky.

What would you do in this situation? I can make a drawing or take more photos if that helps.


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  1. Expert Member
    Michael Maines | | #1

    Evan, building codes usually require a vent area 1/150 of the area being vented. That can be reduced by half, to 1/300, if the vents are equally balanced high and low on the roof. In a situation like yours, especially with no moisture coming through the shingles from the interior, I would be perfectly comfortable with only a ridge vent.

    If you cut a hole in the existing roof, the existing rafters can vent into the new space.

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