Ridge Vent for Single-Pitch Roof
potton
| Posted in General Questions on
What type of ridge vent could I install on a 2:12 SINGLE-slope, raised-heel truss cathedral roof, covered with an elastomeric membrane? (See attached drawing)
(The ridge is 60′ long and facing prevailing wind. Roof overhang is 24” on low wall, and 36” on high wall. That house to be built in zone 6).
I can find SINGLE slope ridge vent for steel roof, but none for membrane roof…
Do I need a ridge vent anyway, or a vented soffit on the high wall be adequate?
Thanks!
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Replies
Potton,
Vent the soffit , or if it is open vent in between the rafters.
Because of the low pitch it's a good idea to increase both he size of each vent and the depth of the vent channel.
While I would indeed prefer to be ok with vented soffit, could you explain why you do not prefer ridge-vent...? (I kept reading it is better...)
My raised-heel being 10 inch 'thick' (see my sketch), relying on soffit to exhaust air, would kind of allow 10'' (or so) to stay trapped below the roof, wouldn't not...?
Potton,
The trade-off between on a shed-roof between a ridge-vent and a soffit-vent is that the ridge vent may be more efficient, but is situated in a position that makes it more vulnerable to bulk-water intrusion from wind or ice-dams - especially on a low slope roof. How is the flashing at the edges of the roof (gables, peak) being detailed? Is there a curb?
If you are worried about moisture accumulation at the peak - and I don't know if this is a real or just a theoretical concern - why not block out your ridge-flashing with something like Cor-a-Vent and hold your fascia board down several inches so that the air can exit that way?
Just out of curiosity: Why is the framing for your overhangs so thick? Usually that 's only the case if they are going to be exposed with no soffits.
Well, no specific details (waterproofing membrane, drip edge, tape, finishing membrane, straight steel fascia under drip edge), no curbs.
The roof is 62ft long with a few changes of 'width'. (attached drawing)
The intent is to have the ceiling really vapor thight, so I don't know how much I need to worry with moisture, but just wish to build the best way, and keep it simple if possible..
The builder told me he needed a minimum of 10-1/2'' height for the overhangs, but I don't know why...
Thanks
From your comment, can I conclude that with my 34 t0 38 ft long slope,
- providing nearly 5ft of elevation difference between inlet soffit and outlet soffit (see attachment),
- having 10 to 12 inches of air channel above the cellulose,
- giving great attention to ceiling's vapor thightness.
there is no worry to have, and no need for ridge vent?
Thanks
Potton,
Yes, as l0ng as the soffits at both the eaves and peak are perforated or have a strip-vent the roof will be fine. The more net area you can provide for venting at the intake and outlet the better.
Thanks Malcolm,
So you are saying I should aim for much more than 1 sq. ft. of
ventilation for every 150 sq ft (split between intake soffit and outlet soffit)
Is there a point it becomes too much...?
What appears to make theses low-slope ventilated roofs work is a larger vent channel and increasing the area of the intakes and outlets. Make sure the ratio is at least 1/150 . There is no downside to going above that.
Malcolm,
A roofer keeps telling me that relying on my peak overhang (3ft) open soffits as a continuous outlet, might not work because it is facing prevailing wind...
Is he wrong on that?
Dual slope roofs face the same concern - the soffit area on the downwind side is at a low pressure compared to the ridge - meaning that wind induced flow (downward) is fighting convective flow (upward). Yet these roofs work well - because these forces are rarely equal.
Maybe someday people will realize that powered ventilation, successfully used in crawlspaces, sealed attics, bathrooms, heating systems, HRVs, etc , can be designed to vent roofs better than highly variable passive venting. It can be designed to reduce infiltration/exfiltration. It can be smart, responding to conditions such as moisture levels and ice dam potential. Very low CFM are needed to remove moisture, so operating cost isn't significant.
Jon,
So, if I understand you well, you agree that my long 2:12 39ft single slope (with 12'' air channel) - with ridge facing wind - will ventilate well with only eave and ridge open soffits, and no needs for roof outlets or ridge vent?
(The roof will be a light-grey elastomeric membrane)
Yes. 10-12" (plus scaled up soffit intakes/outlets) produces very little resistance, so drying flow will be more than a shorter span 4:12 with 2" air channels (a standard, well proven design). 10-12" is also much more than what European codes consider safe for a flat roof.
This calculator refuses to do a 2:12 slope and doesn't account for wind (which causes much more flow than stack effect), but you can try some sample roofs to see that bigger vents do make up for lower slope and longer span.
Air seal the interior side well (to keep moisture out of the roof) and test it with a blower door. I'd use a smart-retarder.
Jon, the temperature charts produced by the calculator are really interesting. I do not understand however, why if I only double the air gap height, the Required Inches² of Net Free Area doubles; if I quadruple, the required NFA quadruples, etc... Why is the NFA not fixed to an area ratio (1:150 or 1:300) ??
It's about lowering total resistance to compensate for less pressure. With a large vent channel and small intakes/outlets, the latter would excessively restrict the flow, which you want to increase. So increase your inlet/outlet NFA, say to 50% of the vent channel size.
Thanks Jon
Jon, ugh.. how do you correlate in2/li.ft. with depth of air channel?
Please give me an example, say for a vent channel 12'' deep.
12" deep with 50% is 12 inches deep x .50 x 12 inches/ft = 72 in2/li.ft (of roof edge). Since your case is well above what is needed, going down to say 50 in2/li.ft should also be fine. Numbers typical for higher slope roofs (eg 18 in2/li.ft) would be too restrictive.
Jon,
I suppose you mean 50 in2/li.ft (of roof edge) as a TOTAL sum of eaves and peak vent area? (Or do you mean 50 in2/li.ft of eave length + 50 in2/li.ft of peak length ? )
50 in²/li.ft of edge on the low side and again on the peak. Ie, net free area on each end of the vent channel. 50 total wouldn't be much more than a standard slope, 1:150 roof.
Low side and peak are 58' long, so 50 x 58 x 2 = 5800 in² total for my 1600 ft² would equate to a ratio of 1600 x 144/5800 = 1:40 ... almost three times more than 1:150...
Well well, I was not expecting that much... ;~)