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Snow on odd shaped roofs (Vassar Gym) or deep snow can make roof venting difficult; Venting towers are a common solution (C) D FriedmanRoof Ventilation Design for Deep Snow Areas
Avoid Condensation, Ice Dam Leaks, Attic Mold, & Roof Structure Damage in areas of snow cover

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Roof ventilation designs for areas of deep snow cover: here we discuss the problem of increased attic moisture, condensation, ice formation, and ice dam leaks that can occur on roofs where deep snow cover interferes with exit venting at the ridge vent.

Our page top photos shows a roof venting tower on a difficult-to vent roof on the Vassar College Campus, Poughkeepsie, New York in the U.S.

This article series describes inspection methods and clues to detect roof venting deficiencies, insulation defects, and attic condensation problems in buildings. It describes proper roof ventilation placement, amounts, and other details.



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Roof Ventilation Design Options for Deep Snow Areas

Reader Question & Discission: how to handle roof venting when snow covers the ridge vent

Photo of heat tapes on a roof edgeI desperately need to know once in for all, if ridge venting is feasible in a climate with large amounts of snow fall.I have read many online discussions on this topic but can't get a definite answer. I live in Saskatoon Saskatchewan and this past winter lasted 5 months with above average snow fall.

Our photo left, not the reader's building, illustrates a traditional "solution" for ice dam leaks and one indicating that the roof probably lacks adequate ventilation: the reliance on heat tapes to melt drainage channels through ice formation at the roof eaves.

[Click to enlarge any image]

There was in access of 12" of snow on the ridge of my roof for 2 months. I currently have 2 gable end vents and 4- 6"x 12" soffit vents with recently upgraded attic insulation of R50. I never experienced any ice jams whatsoever, even with the heavy snow falls, but I did get leaks from the bathroom exhaust roof vent which I will be exhausting through the gable ends when I re single the roof.

I know that ridge venting is the most effective way of venting a roof, but I can't see how it can work in this climate. My asphalt singles are long over due and I absolutely have to change them this summer because they're  leaking. I would appreciate a prompt reply to this request and any added info to what would be the best venting system for my roof. - J.R. 6/22/2013

Reply:

Vent shaft atop ridge vent for deep snow areas - Mackinlay et alsI will research this question further to search for authoritative citations; Meanwhile, based on your comment below and offering an OPINION,

If the ridge vent is completely snow-covered it is not going to provide effective exit venting; I've seen roofs on which the snow melts away at the ridge because of high volumes of heated air escaping from the attic but that's its own problem, not a solution.

The photo at left (Mackinlay et als, discussed and cited below) is that author's illustration of an approach to successful roof exit venting in climates of deep snow cover.

You're not getting ice dam leaks and should have less risk of that problem with added insulation, making perfect winter venting less of a concern than otherwise.

Your action to vent the bath fan out through gable end is correct; be sure the vent line itself is insulated and pitched so as not to send condensate back into the bath ceiling, nor to freeze-up an accumulation of water that later risks leaks back into the ceilings below.

The ridge-vent over-vent shaft shown at left (Mackinlay (2000)) is discussed in detail later in these notes and may be what you need.

Gable-end venting is a traditional but less effective means of attic venting as I explain at InspectAPedia - it does not uniformly vent the under-roof surface; Worse, having both gable end vents and ridge vents will subvert the effectiveness of intake venting at the soffits or eaves when the ridge is not snow covered. Air exiting at the ridge will pull in air from the gable ends not from the eaves.

Luckily not every possible problem against which we warn is necessarily going to occur in every home. If your home's attic shows no signs of ice dam leaks, condensation under the roof sheathing, or moisture-related mold growth, then considering that your ridge vent is working only out of the snow-cover season, your idea to keep the gable vents in use makes sense.

Ideally you would enable the gable end vents in winter when the ridge is snow-covered, even increasing that exit venting using a booster fan if needed; Then when there is no snow covering the ridge vent - (for the rest of the year) you'd close off the gable end vents (a simple plastic cover in the attic is fine) so that the ridge and soffit exit and intake vents work together optimally.

The photo at below left illustrates ice dam leaks appearing as stains on the under-side of a soffit or roof eaves. At below right, deep snow on a Mansard style roof in Hyde Park, New York easily obstructs typical roof vent turbines or towers used to vent the low slope roof atop such buildings.

Soffit intake venting blocked (C) D Friedman Deep Snow on a masard style roof can easily obstruct roof exit venting (C) Daniel Friedman Hyde Park NY

Reader Comment:

Thank you so much Daniel four your prompt reply. I have done considerable research on your site and other installation sites. I agree with all the info you have given me, including the closing and opening of the gable end vents when the ridge vent is covered with snow.

But, because my attic has 16"- 18" of blown in insulation I would have to devise a way to open and close them from the exterior of the house. I may be able to achieve this with a shutter type vent,but if there is a better venting system or solution I would love to here it, so please continue your search for authoritative citations on this venting issue. Once again, Thank You so much for your time and may you have a great day.

Follow-up:

Gable end vent in a building wall - photoI agree you don't want to crawl through (and disturb) the blown-in insulation; and for some roof designs there simply is not enough space to access gable end vents from inside the structure.

A gable end close-off does not have to be perfectly air-tight to avoid un-wanted summer "short circuit" venting problem I've discussed in these articles.

Our photo, left, from a different building than the one discussed here, illustrates a traditional gable-end vent on a building located in New York.

You might devise covers that hang from brackets above the gable end vents outside, like traditional exterior storms/screens, improved if you like with a pair of magnets to help hold the closure end down; I'd give some thought to enough magnets to avoid having the covers blow around in windy conditions;

Such a system can often be installed from a ladder or perhaps even using a pole from ground level, but  if you are in an area of high wind you'll want actual mechanical latches.

A useful reference that provides an in-depth discussion of engineering roofs for deep snow areas, including an excellent study of under-roof moisture and temperature control through ventilatin is

Mackinlay, I., R. Flood, and A. Heidrich. "Roof Design in Regions of Snow and Cold." In Proceedings, Fourth International Conference on Snow Engineering, 2000, pp. 213-224

This article appears in Erik Hjorth-Hansen, Ed., Snow Engineering Advances and Developments: Recent Advances and Developments, Taylor and Francis, 2000.  The book has quite a few pertinent articles.

The authors Mackinlay et als, an architectural firm, focus on the importance of a good vapor retarder (not just on superinsulation) and on  keeping indoor relative humidity at or below 50% in winter (I think their number might be too high - see HUMIDITY LEVEL TARGET).

Higher internal moisture and lower outdoor temperatures both increase the importance of the vapor barrier in avoiding loss of insulation value and in avoiding damage to the building from moisture (and ice) accumulation inside the structure (presumably the attic). They also discuss my pet topic: the ice dam problem likely to occur on warm sloping roofs with little ventilation, and they advocate for a ventilated, cold roof, citing Tobiasson, Buska, and Greatorex (1998, "Attic Ventilation Guidelines to Minimize Icings at Eaves").

Those authors defined an "icing envelope" of combined conditions now widely agreed-on in concept: outdoor temperatures below 22F, attic temperature above 30F. Those authors tested the use of mechanical ventilation to keep the attic adequately cool. That is what lies behind my earlier suggestion to consider adding a gable-end fan at one end (or both ends if soffit intake is adequate).

Macinlay et als illustrate an interesting solution to the problem of snow-covered ridge vents: the addition of a vent shaft that adds height above the snow cover to permit air intake at eaves and air exit through the vent shaft.

Watch out: when using mechanical ventilation in winter, depending on roof slope, shape, free air space, a single central shaft may not move enough air over the entire under-roof surface, leaving some areas exposed to condensation.

In fact Macinlay demonstrated by instrumentation that with adequate inlet and outlet vent areas natural ventilation produced results as good as mechanical ventilation. But you'll see in figure three from their paper (photograph excerpted above) that makes clear that the exit vent shaft is quite large, and would also need to be well-secured against winds.

Watch out: using fans without adequate outdoor air intake will depressurize the attic and increase both heat loss and possible moisture movement through the attic insulation. Air bypass leaks (AIR BYPASS LEAKS) exacerbate this problem.

Macinlay's paper discusses other important variations among varying climates comparing seasonal temperature variations in winter that differ, for example between New York and northern California or perhaps Colorado, and also the need for consideration in some climates of the effects of wet snow - or rain on snow (see the paper's citation of Colbeck (1977).

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ROOF VENTILATION for DEEP SNOW at InspectApedia.com - online encyclopedia of building & environmental inspection, testing, diagnosis, repair, & problem prevention advice.

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