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Low slope residential roof vent question

Climate zone 7, lots of snow (high rockies)

Is it worth venting over the top of the sheathing to prevent ice damming on a 60' long metal roof with a pitch of .75"/12" ?
I am not talking about the "attic venting".
The assembly, as it currently stands, inside to out:
1_Gypboard ceiling
2_11 7/8" TJI perpendicular to long low slope w/ R-55 closed cell spray foam.
3_1/2" Foam thermal break between sheathing and TJI upper flange.
4_roof sheathing.
5_roof underlayment
6_VENT SPACE???
7_Floating metal roof (for thermal expansion)

An alternate roof material I am considering is a ballasted torch down roof, but the owners are somewhat averse to this. SO, with the metal roof, can anyone weigh in on the effectiveness of venting? I am considering a product called "Delta Trela" which is a dimpleboard vent product that goes under the metal roof. Questions? Comments? Suggestions?

Asked by Erik Lobeck
Posted Mon, 02/17/2014 - 14:37
Edited Mon, 02/17/2014 - 15:34

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6 Answers

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1.
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Erik,
First, I assume that you have checked with the manufacturer of the metal roofing that you intend to use, to be sure that the manufacturer approves the product's use for low-slope roofs.

Assuming that the manufacturer approves, I don't think there is much advantage to including a vent channel under the roofing.

In general, I'm not a fan of low-slope roofs in cold, snowy climates. But it's probably too late to convince you to design a roof with a steeper slope.

Answered by Martin Holladay, GBA Advisor
Posted Mon, 02/17/2014 - 15:54

2.
Helpful? 0

Martin,
Thank you for your input. Yes, manufacturer says Ok, and it is warranteed by installer. So, regarding the low pitch roof, it is largely a result of local zoning. IMHO, low slope roofs in snowy climates can sometimes be of benefit in cases where snow and drip may be undesirable in certain locations below at grade (the usual alternative being multiple intersecting gables). In this case, it is a monopitch and all snow drip is being directed to an area without foot traffic below (if zoning allowed I would increase the pitch somewhat). What I am left with is the roof design I described above. That said, continued question:
1_Venting above sheathing, if I place the metal directly on underlayment (no vent space), will I not have created a "vapor sandwich?" with the closed cell SF below?
2_Will venting not work in your opinion due to a lack of pressure / temp differential ? Or is there another reason?

Answered by Erik Lobeck
Posted Mon, 02/17/2014 - 16:21

3.
Helpful? 0

Erik,
Venting above the sheathing will work (assuming, of course, that the roofer specifies asphalt felt instead of one of the new, vapor-impermeable synthetic roofing underlayments). You are quite correct that the venting will help the roof sheathing dry out if it every gets wet. That is a real benefit.

Your original question concerned ice damming. Because you have spray foam insulation with a high R-value, and because air leakage through your roof assembly is unlikely, I don't think that ice damming will be a serious concern. My answer was made in that light -- in an attempt to save you money.

The entire question of whether we need to allow roof sheathing to be able to dry in one direction on the kind of roof you describe remains unsettled. Including a vent channel above the roof sheathing would be a conservative approach, with no downsides other than the increased cost.

Answered by Martin Holladay, GBA Advisor
Posted Mon, 02/17/2014 - 16:37
Edited Mon, 02/17/2014 - 16:38.

4.
Helpful? 0

Martin,
Got it. That helps. Agreed. (Cost/benefit/conservative/etc) I suppose an additional benefit of not venting would be an increased assembly R value when we need it most-in the winter. (R-1 per inch for snow?)
May I delve somewhat deeper and ask why the issue remains, as you say, "unsettled" ?

Regards,
Erik

Answered by Erik Lobeck
Posted Mon, 02/17/2014 - 16:46

5.
Helpful? 0

Erik,
Eventually, all roofs leak. If a building has a vented, unconditioned attic, a dripping roof leak is easier to notice, easier to locate, and more likely to get fixed quickly than if the roof has a cathedral ceiling.

If your roof leaks, it may well be necessary to replace rotten roof sheathing.

The unanswered questions are:

1. If you build a roof with closed-cell foam installed on the underside of the roof sheathing, will it take longer to detect a roof leak?

2. Will more sheathing rot before roofing repairs begin?

3. If the sheathing is dry on the day that the closed-cell foam is installed, is there any reason to think that the roof sheathing will gain moisture via any mechanisms other than a roof leak?

Lots of people have opinions on these questions, but we don't really have a lot of data yet. So these types of roofs might last just as long as other types... or they might not.

Answered by Martin Holladay, GBA Advisor
Posted Mon, 02/17/2014 - 16:54

6.
Helpful? 0

Ok, if I wanted a reasonable level of safety without significant add cost in this scenario, what would your opinion be on utilizing pressure treated ply for the roof sheathing? Maybe in the hopefully unlikely event of a roof leak and membrane defect at least maybe there would be minimal chance of a structural roof rot condition?

Answered by Erik Lobeck
Posted Tue, 02/18/2014 - 13:07

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