Venting a Shed Roof – To Peak or not to Peak?
Hello,
Casey here. Long time Reader, first time poster.
So I’m detailing a vented shed roof in climate zone 4c and am looking for some advice on ventilating the upper end of the roof.
There are plenty of articles on here with regards to roof venting, I’ve read most of them, but none (that I can find anyhow) that address shed roofs (that don’t terminate into a wall) peak/wall ventilation output.
I’ve attached a quick sketch to help illustrate my question, but essentially, which path is the most ideal ventilation path for a vented shed roof? And why so?
1) topside – furthest from wall
2) bottomside – furthest from wall
3) bottomside – closest to wall
Thank you in advance of response and I look forward to the exchange.
~Casey
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Replies
The vents need to 1) achieve air movement while 2) keeping out the weather and critters.
1) is possible through a variety of means
2) is most practical by venting out the bottom of the rafters, at both the high and low sides of the roof.
Standard detail is to have ventilated soffits at both ends of the rafters. If the vent channel is sized properly then this system will work. As long as the air has an unobstructed path in and out of the vent channel you will be in good shape.
Thank you for the thoughtful response plumb_bob
Sure, that all makes sense. My current strategy - see attached sketch - is a continuous soffit vent at the lower eave which would then direct air movement to a continuous ridge vent @ the peak, both being as close to the fascia / furthest from the wall as possible. I've done this same detail on another similar type house and it works fine from my understanding. But as you pointed out, practicality will work just fine as long as the vent channels are sized properly and unobstructed.
Also, I've seen conflicting arguments for situating the soffit vents (both low & high side) either closer to the wall or to the fascia. If I remember right, the arguments were mostly centralized around pressure differences at those locations. Do you have an opinion on this?
Thanks again, have a great rest of your day!
~Casey
Casey,
Looking at it solely from the perspective of efficient air-movement, and not leaving any dead-ends that might accumulate moist air, the two options closest to the peak are the best - and venting up into the ridge flashing wins out. However situating a vent-opening on the roof surface opens you up to potential bulk water intrusion in windy conditions, so I would opt for venting through the soffit close to the fascia.
Honestly though, I'm not sure it matters all that much. I've seen all three done successfully.
Great, thank you for the thoughtful response Malcolm.
Sure, that completely makes sense - there is certainly a good chance that wind driven rain could enter that topside opening, especially here in oregon! And if there isn't much of a difference between the three ventilation strategies but we can feel confident about keeping the water out of the cavity, then I'll elect to place a continuous soffit vent near the fascia, as you've suggested. Thank you much for the recommendation!
Also, I've seen conflicting arguments for situating the soffit vents (both low & high side) either closer to the wall or to the fascia. If I remember right, the arguments were mostly centralized around pressure differences at those locations. Do you have an opinion on this?
Thanks again, have a great rest of your day!
~Casey
Casey,
I agree with plumb_bob.
I would GUESS that under windy conditions there will be pressure differentials created that will interrupt the normal flow of air through the rafter vent channels, but this is just intermittent, and the regular flow from bottom to top will be the prevailing flow.
The detailing of the ceiling air barrier will be a much bigger factor in the success of this roof than the vent location.
Sure that makes plenty of sense to me, thank you for the input! Much appreciated!