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Roof and attic ventilation

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Every roof must be ventilated - there are no exceptions: the only question is how much ventilation is required.

The section of Code governing ventilation of roofing areas (attics) is – unlike most of Code - pretty simple.

Code (section for those of you who want to check our work) requires that ventilation areas of either 1/300 of the ceiling area (or 1/500 for shallow slope roofs) be ventilated, and that at least 25 per cent of that figure be at the top of the ceiling space and 25 per cent at the bottom.  Now, most folks out there will figure that 25 per cent plus 25 per cent is only 50 per cent, but the intent here is to allow some flexibility in design. In other words, a builder can have 75 per cent openings at the soffits, and 25 per cent at ridge vent, for example.

The other critical requirement is that except for gable roofs near walls, there must be at least 63 mm – that’s 2 ½” – of space must be provided between the top of the insulation and the underside of the roof deck. [].

And just in case the Code didn’t make it obvious enough that airflow must be provided in a roofing system, clause hammers the point home:

“Ceiling insulation shall be installed in a manner that will not restrict the free flow of air through roof vents or through any portion of the attic or roof space.”

The intent of attic ventilation is to ensure that any moisture that exists, or may exist, in the roofing system, particularly in the sheathing, doesn’t remain in place: it can turn to vapour and move through the ventilation system to the exterior, hence all of the above requirements.

Unfortunately, there are some who believe it's acceptable to use site-applied spray foam applied directly to the underside of the roof deck in a cathedral ceiling. Clearly, Code shows that's quite false - and doing so could cause serious degradation  of the roofing system.

At this point, readers have probably grasped the key point, but are asking a reasonable question: why?

One of the problems with unventilated roofing systems, or “hot roofs” as they are called, is that in our Canadian climate there are invariably going to be days where the combination of temperature and humidity lead to a dew point in or under the roof sheathing. If that moisture can’t be vented to the exterior over time, it will create moisture degradation issues. In fact, the incredibly waterproof nature of spray foam can actually trap moisture under an unventilated roof deck.

Simply put, our office will not accept this construction method without approval after submission of an Alternative Solution crafted by a qualified engineer licensed to practice in the province.


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