Venting a 3/12 hip roof

I have a single-story addition on the back of my two-story house in Riverside, IL (Zone 5). We had a hip roof with a 3/12 pitch built to replace a flat roof on the addition. The roof will have 18-inch vented soffits on all three sides. Our contractor hasn’t had much experience with low pitch roofs. What type of attic vents should be used on the roof and how many of them? The attic floor space is about 360 ft.².



How much ridge is there? Is this attic space separate or does it adjoin the main attic?

Thanks for your reply.

The ridge is at the the exterior wall of the main part of the house. It is not connected to the main part of attic at any point. The ridge is about 3 feet above the room’s ceiling, including the depth of the 2 x 8s. It extends about 10 feet horizontally. I’ve attached a photo (if this forum allows attachments).

Id install a kitchen vent on both sides.
Thats plenty for that small area.

Thanks for the reply, Roof_lover.

What’s a kitchen vent, like the kind used for venting range hood exhaust fans?

ye,just google it ,black tiny squareish vent with glossy finish

Thanks, Asus_Versuta. I see what they look like. Something that low to the roof won’t be a risk for ice damming on a 3/12 pitch?

Run a clerestory vent at the main wall intersection

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If you have 18 inch vented soffits on a hip roof, you are going to be very heavy on the intake Net Free Vent Area (NFVA). Meanwhile, when the roof is hipped, as opposed to gabled, you give up 2/3rds of the ridge line so that your capacity for ridge vent is greatly reduced.

International Building Codes suggest that you follow the 1/300 rule for static ventilation if you have a perfectly balanced ventilation system (50% intake, 50% exhaust, in terms of NFVA… or, you can drop as low as 60% intake to 40% exhaust… but you cannot go beyond this ratio of intake to exhaust and continue to operate with the 1/300 rule for ventilation.

So, you have 360 sf attic floor. Divide attic floor by 300 to get your total NFVA requirement. multiply that by 144 to convert to square inches. So you need 173 square inches total NFVA. (Hint: you are going to have way more than 173 inches NFVA in your soffit panels!). If you divide 173 by 2, you will have your NFVA exhaust requirement of 86 square inches. You can achieve this with 5 feet of a shingle over ridge vent, like the GAF Cobra III which specifies 18 inches NFVA per foot. But this means that you also need to limit your intake to 86 square inches, and just taking the attic floor of 360, I’m guessing that 3 sides of the roof will give you a perimeter of about 55 feet. If you are using perforated aluminum soffit panels, you are probably going to have over 1000 square inches intake NFVA. So rather than a 50:50 ratio intake to exhaust, or even a 60:40 ratio, you are going to have like an 90:10 ratio, or worse, intake to exhaust.

So your ventilation system is out of balance, and you cannot use the 1/300 rule to be within International building code compliance. You can use the 1/150 rule which effectively doubles the requirement for total NFVA, so we go from 173 to 346 square inches total NFVA, and your intake alone covers this requirement. IN this case, you should simply add as much ridge vent as you can to get your ratio of intake to exhaust as closely balanced as possible. But still, you will be far from balanced. But you will be in compliance with the 1/150 rule.

Now, all of the above is for “static ventilation.” You could think about using dynamic ventilation which does not use the 1/300 or 1/150 rule. CertainTeed has the best specifications for dynamic ventilation requirements. Take your attic floor, multiply by 0.7 (or 0.9 if the roof pitch is above 7/12), and this gives the CFM requirement. 360 * 0.7 = 252 cfm. A Lomanco 14" BIB wind turbine gives about 740 CFM in 8 mph wind speed (which is the annual average wind speed for Florida, and I would imagine is also similar elsewhere). So a single Lomanco 14" turbine would possibly be your best bet - as I’m not a big believer in unbalanced static ventilation systems.

Did you have fun?
I know i did.

A traditional turbine would be very unsightly on this. I know i dont want some 2 foot tall ugly vent on my porch messing up my view of the house.
What ever it is , needs to be low profile.
And he talked about snow intrusion.
Snow can enter that vent.
Especially on this section.

I agree that the flashing of the vent be round.
Thats why i suggested the kitchen vent.
Its walls are square but its flashing is circular.
Snow can fall and clog up the vent
But still not get above the vents flashing.

A solar turbine also wouldnt be unsightly.
Almost half the height of a traditional turbine.

Thanks for the detail answer, Jonnie18.

You can stifle your exhaust by using smaller sq inches of vent.

This is dumb homeowner question, what happens when air intake exceeds exhaust? The Google search results were too technical for me to understand.

We’re actually using HardieSoffit panels so only a small portion of the panel has holes.

I appreciate all of the advice. The architect, carpenter and roofer are pointing at each other and telling me ventilation is somebody else’s problem!

stop overthinking just tuck in couple vents and call it a day,holy moly ::

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That is by intent, " if you can’t blind them with brilliance, baffle them with bullshit". Ventilation is a fairly simple process, no different than a flue damper on a woodstove. It parallels the covid scam… with all the drama.

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In reality, the 1/150 rule doesn’t seem to require any exhaust ventilation. I’ve asked multiple experts (i.e. CertainTeed & Lomanco technical departments) about the limit to the unbalance of the 1/150 rule. As the 1/300 rule is limited to a 60:40 ratio of intake to exhaust, I also have asked, does the 1/150 rule limit to 90:10 or 95:5 or 99:1 or…?

As far as I have found out, there is no limit - certainly there is no limit spelled out in any building code that I have found. So, to be code compliant, and to go with the 1/150 rule, as long as there are at least 376 square inches NFVA installed in the soffit vent (which seems unavoidable with 18 inch soffit panels) then the roof really doesn’t need any additional ventilation. At least, in order to be code compliant.

I agree that a turbine can be a bit unsightly… but most people over estimate the aesthetic quality (or negativity) that a turbine vent will have on a roof. I’ve found that most roofs with a pitch greater than 6/12, you cannot even see the turbine vents as long as they are installed on the back side of the roof. But, yes, we have a 3/12 here, and a small roof at that.

If it were me, I’d go with a shingle-over ridge vent, or no vent at all - with the one caveat, I am a Florida trained roofer, and thus, I have zero experience with issues concerning snow and ice.

Convection, heat rises. In cold climates, we have condensation problems in the winter