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My wife (licensed Architect) and I (builder with degree in architecture) are designing a small house in Mazama, WA where the design snow load is 100 pounds/s.f. We have spent much time there recently in a just completed house and have been studying how the roof works (or doesn’t work) Most of the house is under a 10/12 gable. The snow seems to slide off pretty regularly. No icicles at the eave except below the valleys (which we will not have) The roofing is painted metal with exposed fasteners. We watched about an inch and a half of new snow melt off in the 30 degree sun yesterday. The rafters are 2×12, insulation is bat R-30 with a 2 inch air-space, vents at the eave and continuously at the ridge. There is a dormer with what looks to be a 4/12 pitch. Snow doesn’t slide off, but it does creep like a glacier and curl inward toward the windows. The snow/ice on the dormer roof is now almost making contact with the exterior wall – overhang is about 18″. I read the article in issue #124 and want to hear more thoughts on what we can do to really make our roof work. We will have a large dormer and are considering a slope of 4/12 for its roof, and 10/12 for the main pitch.. How about using composition shingles so that the snow is less likely to creep and curl? What about using only shed roofs, 4/12 or less, and allowing the snow to sit there and not slide? Would a bigger airspace (like 4″) be more effective in keeping the roof “cold”. Also – what can we do for roofs above exterior decks? Having a 10/12 pitch change to a shallower one seems to only catch and hold snow on the low roof. Having snow slide and drop from an second-story roof to a lower one also seems like a bad idea.
Suggestions???
Replies
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Brian: Many have posted on other threads about this topic. The archives are a little bothersome to use, but can be worth it. There was a recent thread about 100 psf requirements in the WA Cascades. Here's are my thoughts:
Snow stays put much longer on composition shingles than on metal. Even longer on "architectural" comp shingles. My snow doesn't move, slide or creep on a 12:12 roof with arch comp shingles. Even this year when we've had lots of wheather in the 30's (plus, not minus for a change). 2" sounds like a great vent for a cold roof. Do you have 24 square inches of vents per bay in the eave and ridge? Focus on the pinch points. Valleys are especially trick to get good air flow to and through. I wouldn't go to 4" air gap in the bays. Rather use that depth for more insulation. 2 inches of sprayed urethane will really tighten up the house, add more R-value and leave you lots of bay depth to fill with cheap FG batting. Does require backing to maintain the air gap but that can be foam baffles, blue foam (for more R-value) or cardboard.
In addition to a cold roof, I put 2 courses of Water & Ice above the eaves on my own house. Belt and Suspenders. Avoid ice dams in the first place and be ready for them if they happen.
Plantings or rock placement can keep people out from under the dripline and away from falling snow. Every other year a kid dies up here from sliding snow. Gutters tend to get ripped off each year if your snow creeps. I like 4 feet of 1x1" or 1.5x1.5" "L" angle of sheet metal over each doorway to divert rainwater to the side. Elsewhere, let the plantings and rocks take the water runoff. And use the ground slope to take the water away from the house. When snow drops a story and collects, it can have impressive densities and depths. So I would agree that dropping snow from one roof to another is not the best. -David
*Hi Brian,I just completed a house directly south of you in Leavenworth. My house has a 12/12 pitch - metal roof with concealed fasteners, snow never gets more than 2 inches deep before it goes flying.Whenever 2 roof planes meet i.e. valleys/dormers it seems to form a snow wedge that as David said, can have impressive density and depth. I guess I don't have any real answers for you, observe other houses in snow country.Oh yeah, make sure your plumbing vent stack exits just below the roof peak and you don't need gutters in Mazama (beautiful area)jim
*You could try snow gaurds and stick with your plan. Snow gaurds are installed in a pattern at the bottom edge. There only function is to break up the snow as it slides down the roof. It also gives a nice look to the roof.
*Well maybe a good place to start is with the idea that the snow has to go somewhere sometime. It can either stay on the roof and go away as melt water. Or, it can slide off the roof as snow. While it is on the roof either a natural or designed dam can stop the melt water so it refreezes and causes ice on the roof.If you think in these terms and deliberately design the roof to do what you want it seems to me you can't help but have a good roof. What design elements to use? There has been so much written in Canada and Alaska by the governments and universities that you could spend a month reading it all. You don't need to spend that long but you get the idea why it is foolish for anyone to give a quick answer here.So, follow the fundamental of deciding where you want the snow to go and in what form and deliberately building for that and you won't have any trouble living with the pesky stuff.
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My wife (licensed Architect) and I (builder with degree in architecture) are designing a small house in Mazama, WA where the design snow load is 100 pounds/s.f. We have spent much time there recently in a just completed house and have been studying how the roof works (or doesn't work) Most of the house is under a 10/12 gable. The snow seems to slide off pretty regularly. No icicles at the eave except below the valleys (which we will not have) The roofing is painted metal with exposed fasteners. We watched about an inch and a half of new snow melt off in the 30 degree sun yesterday. The rafters are 2x12, insulation is bat R-30 with a 2 inch air-space, vents at the eave and continuously at the ridge. There is a dormer with what looks to be a 4/12 pitch. Snow doesn't slide off, but it does creep like a glacier and curl inward toward the windows. The snow/ice on the dormer roof is now almost making contact with the exterior wall - overhang is about 18". I read the article in issue #124 and want to hear more thoughts on what we can do to really make our roof work. We will have a large dormer and are considering a slope of 4/12 for its roof, and 10/12 for the main pitch.. How about using composition shingles so that the snow is less likely to creep and curl? What about using only shed roofs, 4/12 or less, and allowing the snow to sit there and not slide? Would a bigger airspace (like 4") be more effective in keeping the roof "cold". Also - what can we do for roofs above exterior decks? Having a 10/12 pitch change to a shallower one seems to only catch and hold snow on the low roof. Having snow slide and drop from an second-story roof to a lower one also seems like a bad idea.
Suggestions???