2:12 roof- Soffit vents sufficient?
Flatrooforbust
| Posted in General Questions on
Will soffit vents be sufficient for venting this 2:12 pitch roof?
Is it worth adding any powered soffit vents?
Or turbine-style vents on the top of the roof?
Thanks!
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No. It's an open question as to whether even using a combination of soffit and venting at the peak will be sufficient, but only soffit venting certainly won't be.
Thats unfortunate.
What are the options for something like this then?
1. Do as Akos suggests. Vent at the soffit and peak, increase the vent area and the depth of the ventilation channel, hope you live in a benign enough climate that a ventilated low-pitched roof works.
2. Build an unvented roof.
3. Increase your roof pitch.
I live in the interior of BC. We don't get 100% RH as Florida might, and we rarely get to -20C in the winter.
What sorts of climate are most prone to issues of moisture in the attic space?
Would increasing bottom chord slope help with ventilation?
Are there any high-NFA soffit vent products you might recommend?
The attic area is 1344 sq ft/150= 9 sq ft of NFA/2=4.5 sq ft NFA each of top and bottom soffit vents.
Thanks a lot for the information. I really appreciate it.
One of the climates most prone to issues of moisture is that just to your west in Coastal BC. The interior is dry enough I'd bet a 2/12 roof of the shape you show is probably just fine as long as you follow Akos' advice.
Increasing the pitch of the bottom chord will unfortunately make the ventilation less effective by decreasing the size 0f the open area above the insulation.
How you get the amount of vent area at the soffit is mostly dependant 0n how you want it to look. I think from your drawing you have exposed rafter-tails? If so I'd block in between them leaving an appropriate gap above to the sheathing and use a U shaped perforated flashing there as an intake.
Our code allows for a roof like that provided there is 2 1/4" gap between the insulation and roof deck and the vent area is increased to 1/150.
There are a lot of places here that have vented low slope roof and they seem to hold up well. I know certain areas with a lot of humidity and rain have issues with mold even with vented pitched roofs, if you are in one of those places, might be best to consider an un-vented assembly.
With any vented roof, the most important item is to air seal the ceiling. This means no ducts or air handlers in the attic, no pot lights in the ceiling and not bath fans dumping into the attic space. If those are taken care of and you provide sufficient vent area at both high and low sides, the roof should work well.
For the high side you could go with facia vents which will give even more height delta and help with air flow.
Power vents should always be avoided. These usually depressurize the attic and drawing in air from the house, creating more problems than they solve.
I live in the interior of BC. We don't get 100% RH as Florida might, and we rarely get to -20C in the winter.
I believe our code is similar for the air gap and the vent area.
With the high heel truss I should have a reasonably large air gap after insulation. (6"+)
Is there any way to figure out if there is sufficient venting besides code?
Is more always better with venting?
Look around the area and ask a couple of folks with flat roofs if they have had issues (besides the typical flat roofs leaking water).
People imagine roof venting as this large amount of air flow that washes across your deck to dry it up. This is not what happens.
In reality, what venting needs to do is keep the attic dewpoint about the same as outdoor air. If the dewpoint there is close, you can't get any condensation. Outside of air leaks your only source of moisture in your roof deck is condensation from night time radiative cooling of your roofing. This can easily dry from the sun during the day (dark roofs help with this).
Usually the biggest source of moisture in the attic is air leaks from the house, so if you are very diligent with air sealing, you need very little venting to avoid issues.
If you already have 6" of vent space, you are pretty good shape. Instead of trying to figure out how to add much more than code venting, I would focus your effort on detailing your air barrier. Getting that right will make much bigger difference in the robustness of your assembly and also helps with energy use.
You people are fantastic.
There are quite a few old houses in town with 2:12 or less peaked roofs. Some of them I don't see any sort of ridge vent at all, and maybe 2 gable end vents. How the hell does a roof like that even work?
Its an owner build house, so air barrier detailing is going to be one of my main goals.
Malcolm mentioned "peak venting" (I don't think Fascia vents meet the BCBC). Is that just soffit vents at the high end under the truss, or is that something different?
Yes. You can install much the same venting at the peak as at the soffit.
I'm not sure just asking how these roofs are doing around where you are is a particularly good way of knowing whether yours will work. A lot of older houses stay safe by leaking immense amounts of warm air into uninsulated roofs, which keeps them dry. That's not something you want to emulate in a new build. Here is Martin's (and BSC's ) take in the subject: https://www.greenbuildingadvisor.com/article/insulating-low-slope-residential-roofs
Darn, I can't seem to reply to your comment Malcolm.
When I mentioned soffit venting, I did mean that I would install the vented soffits at both the high and low end of the roof where the truss tails are.
Does soffit venting normally mean only at the bottom eave and peak venting would be under the high eave?
Are condensation issues normally only during colder months?
Ah - that explains it. Yes, the distinction comes from most roofs not having soffits at the high end, but y9u are right, with a shed roof you have soffits all around.
You do generally have more problems in the winter, but clear summer nights can cause condensation from the night-time radiant cooling Akos mentioned.
Ahhh okay. That makes things a LOT clearer!
I looked at ~10 houses built in the last 2 years in my area, with ~2:12 pitch roofs. The majority of them use a single 3" perforated channel at both the high end and low end of the roof as venting.
One in particular used the 3" perforated channel at the low end, and then two 14" square vents on the top of the roof. These two vents on the roof move enough air that it melted about 3' of snow for 6' around each of them.
Do you think it may be a non-issue as long as I add soffit vents at the high and low end?
Thanks again for all the help. Last night I was ready to have the whole roof redesigned which would have been a nightmare!