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Community and Q&A

How low of a roof slope is too low for venting

omc2011 | Posted in General Questions on

We will have two roofs with a clerestory in between on our new house.

The upper roof is a straight run shed style cathedral roof with a 2.5/12 pitch and 24 inch trusses.

The lower roof is a straight run attic with 2/12 pitch and standard trusses.  The total height of the attic at the highest point will be about 6 feet and it should be possible to put gable vents in on each end of the attic.  There will be black steel roofing on both roofs.

There is plenty of room for both roofs to be vented with at least a 2 inch venting channel and still be well insulated but it is not clear to me if there is sufficient slope on these roofs for venting to be effective.  It is also not clear if the slope matters on the side with the attic?

If ventilation will not work then we will need to consider an unvented roof assembly.  Foam above the sheathing is unlikely to work aesthetically given the additional required thickness so I expect at that point it would be ccSPF and dense packed cellulose under the sheathing.  However, I have also read about using a smart vapor retarder in conjunction with cellulose or fiberglass in an unvented roof assembly.  We are planning to have a chase between the bottom of the trusses and the ceiling to allow access for running electrical so it should be possible to get a proper airtight barrier at the bottom of the trusses.

We are in zone 6 in New Brunswick, Canada. 

I have read the many articles and Q&A posts on GBA on this topic but have not found anything about minimum slope requirements for proper venting.

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  1. Expert Member

    C. Robinson.

    Anything below 3/12 is considered a low-slope roof unsuitable for venting. There are factors that could mitigate for a slightly lower pitch. Roofs in windy areas that would help move air, and those with a southern orientation, but when you are close to the limits of any assembly you are incurring risk.

    The unvented roof assembly using cellulose and fiberglass is one suggested by (I think) 475 High Performance Building. Whether it works or not, I don't see anything in the NBC that permits it, unless NB has amended that portion of the code.

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