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Porch roof is 3/12. House roof is 6/12. Porch roof meets house roof over the interior of the house. Porch will have a ceiling but the overhang will be vented. Space above porch ceiling is open to attic space. Main roof will have a ridge vent. Attic insulation will be R19 or more over interior of house.
Would anyone like to venture a guess as to where an ice dam is most likely to occur on this roof?
Attached is a 69K sketch of the roof framing.
Replies
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I don't know, but I'd put Ice/Water Shield from the eave to the pitch change if I was roofing it.
Rich Beckman
*Ice dams occur when warm air gets through the living space into the attic and makes contact with the roof. The warm air melts snow, The water drips down to the gutter area away from the warm air, and then freezes again The ice dam in your picture will form where the porch meets the main roof, if the area is uninsulated and/or invented. It will settle in either on the overhang or by the gutters. Of course, if you live in Phoenix, the ice dam will form in your freezer. We have debated venting and attic insulation ad naseum. Check out some the prior posts. Ditto on ice and water shield well into the roof area and on all valley's and perforations.
*Ted, Where do you live? I assume that being concerned about ice dams it gets pretty dam cold there. May want to bump that R-19 figure to above 30. With adequate insulation and adequate ventilation at the eaves and ridge you can drastically reduce ice dams.Also looking at your diagram. Is there a reason for framing the gable end like that? I would suggest a gable end truss instead. It will save you plenty of time and materials.Another vote for the ice and water shield up past the slope change. A 3/12 pitch is getting too low for a shingled roof.Scott R.
*Thanks to all for the replies. Ice dam conditions occur in this area occasionally. It hit -30F for a couple of nights a few years ago, and many people had leaks during that period.I understand the theory behind ice dams forming. In this roof, I think temperatures in the attic would gradually decrease from the house area to the space out over the porch. Somewhere there would be a point where above-freezing changed to below-freezing. I guess that point could vary, so maybe the idea of ice and water shield over the whole area is best. That area is 16'x44' times two (since there are two porches, one on each side of the house) so it won't be cheap. Scott, thanks for the suggestion on the gable...but I'm not a professional builder. I'm a solo (or soslow?) builder. Those roof trusses span 32 feet, and I built them by hand using 2x6's all around with hand-nailed & glued plywood gussets. My two daughters are pretty good at driving nails now. Since "labor" is "fun" for me (in a masochistic kind of way), it was cheaper to build 2x6 trusses than buy standard 2x4 trusses. This is a farm house...no neighbors in sight. I can drive nails until 2am if I want (and sometimes I do).
*And working with the kids makes for great memories.What CAD are you having fun with?
*Ted I live in the mountains of Colorado. I have a 5/12 pitch roof with a 6 foot wide porch, on the north side of my house. My attic is insulated to R-30. I have continuous soffet vent and continuous coravent ridge vent. Sounds similar to your situation. The first winter we had snow on the roof for 2 months solid. We also developed 4 inches of ice. I had used Ice & watershield as advised here and I had no water problems, but I know ice isn't good for anything but skating and whiskey. So since then I've installed heat tape every fall. I don't see any options.It's good talking to you again.Terry
*piffin,I use AutoCAD r14 for the accurate drawings, and 3d-Home Architect just to visualize. They don't save me any time, but they do allow me to lay things out when it's raining/dark so as to save some dry/daylight time. A large part of my motivation was to learn to use the CAD software. It's also kind of fun to be able to have a termite's view of the underside of the house...Terry,I'm sure we don't normally get as much snow as you do, but occasionally we get hit pretty hard. Those are the occasions I'm concerned about. Do you install the heat tape only in the gutter (or at the eaves)? Or try to snake it over the whole porch roof? In other words, do you try to prevent dams at the eaves, or over the whole roof?
*PiffinThe ice dam starts at the eave on my house and continues to grow up the roof, if undeterred. I lay the heat tape in the gutter and weave it 3 feet up the roof. This stops the ice from growing up the roof. It's worked for 5 years.Terry
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Porch roof is 3/12. House roof is 6/12. Porch roof meets house roof over the interior of the house. Porch will have a ceiling but the overhang will be vented. Space above porch ceiling is open to attic space. Main roof will have a ridge vent. Attic insulation will be R19 or more over interior of house.
Would anyone like to venture a guess as to where an ice dam is most likely to occur on this roof?
Attached is a 69K sketch of the roof framing.