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House roof vent area required 1/300 or 1/150 of the insulated ceiling area

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sunyaer

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Apr 21, 2022
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338
Location
Toronto
As per Ontario Building Code, house roof vent area shall be 1/300 or 1/150 of the insulated ceiling area. This house seems to be about 3,000 square feet, which would have about 1,500 square feet insulated ceiling area or minimum 1,000 square feet insulated ceiling area , while it doesn't seem to have enough roof vents. The first picture is taken from the front of the house, the second is from the back.

I looked around in the area I am living, many new built houses / row houses don't seem to have roof vent area meeting the 1/300 requirement.

My house is metal roofing, which was done by the previous owner. It looks to me there is no roof vent except three roof vent for the toilets.

Am I missing something?
 

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I'm assuming none of the roof is "hot roof" which doesn't require ventilation. I'm guessing there are soffit vents too, which get added in to the 5 to 10 sf of net free vent area required? See if soffit isn't perforated.

Each of the mushrooms (assuming roof vents) is .4 or so sf of nfa. Add that to the soffit - typically 6 sq in per sq ft or 1/24. So just a 96 sf of soffit is 4 sf of nfa, and I'd guess you have much more.

I think many codes say balance or equal low and high but Joe Lstiburek (https://buildingscience.com/) suggests less high. It has a tendency to wash insulation and create negative pressure pulling conditioned air into attic.

The question should be is what you have enough? Do you get roof ice - ice at eaves or icicles hanging from eave? Have you ever looked in attic to see if there is any condensation on underside of roof deck? If they did a good job on vapor barrier in ceiling, you're probably fine.
 
Vented soffits account for a significant amount of roof ventilation. Coupled with linear peak vents, minimum roof ventilation is not usually challenging to meet.

It is fairly rare to see other styles or roof vents as they tend to be the source of more leaks.
 
ICE said:
And people live there...amazing.
Click to expand...
Not only do we live here, most of us prefer it to where you live. There is nothing quite like going snowshoeing on a sunny winter day.

I have spent a fair amount of time in your neck of the woods and while it is beautiful, it is not for me.
 
sunyaer said:
As per Ontario Building Code, house roof vent area shall be 1/300 or 1/150 of the insulated ceiling area. This house seems to be about 3,000 square feet, which would have about 1,500 square feet insulated ceiling area or minimum 1,000 square feet insulated ceiling area , while it doesn't seem to have enough roof vents. The first picture is taken from the front of the house, the second is from the back.

I looked around in the area I am living, many new built houses / row houses don't seem to have roof vent area meeting the 1/300 requirement.

My house is metal roofing, which was done by the previous owner. It looks to me there is no roof vent except three roof vent for the toilets.

Am I missing something?
Click to expand...

When you have a moment on a construction site, esp. one that has through vents and a ridge-vent system, take the time to calculate the opening sizes and you may very well be surprised.

Most homes that I have seen exceed the ventilation requirements by a hefty margin.
For example, a 1,200 square foot (111 m2) home with a 4:12 pitch only requires 1/300 venting. That's 4 square feet. 576 square inches.
Assume a 30x40 footprint, trusses on the 40-foot length.

The average through-vent features an opening about 10 wide and 1" thick.
Assuming a 24" truss span, 20 trusses = 40 cavities. One vent on every cavity (standard) = 40x10=400 square inches.
A ridge cap, with a 1/4" wide gap (which is way less than normal) , running 40 feet, both sides, gives 240 square inches.
 
I think a lot of people don't know that the soffit and ridge are part of the venting systems. They are expecting to see something obvious on the roof that accommodates venting. If you don't know what to look for, it could be very easy to miss.
 
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