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I’ve got a roof where during the winter the top half of the roof gets direct sunlight for about half the day. The bottom half of the roof sees no direct sunlight at all due to being in the shadow of a wall.
What seems to be happening is that the sun on the top half of the roof melts the snow which runs down the roof and freezes, eventually forming an ice dam. Last winter it backed up enough to leak, this winter the ice melted off the roof before it leaked. A valley intersects this roof, and the worst of the ice seems to build up in the valley.
There is good ventilation under the roof (vents in the eaves and a ridge vent on top. Insullation is R60 with at least 2″ between the insulation and the roof for air flow. Roof is a 9/12 slope, with a 14/12 slope on the other side of the valley. There is one row of the that sticky ashphalt paper under the shingles and the rest of roof has felt under the shingles.
Any suggestions on what I can do? I’m concerned about the leakage, but also that when the ice does fall off, it’s several hundered pounds of ice and could easily kill someone who happened to be in the wrong place at the wrong time.
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Sure reads as if correct practices were followed for a cold roof. With R60 it also sounds as if you are in a cold geographic zone - is that correct? We also get plenty of snow (near Canada border) - how about you? Does snow also melt on areas of the roof that are in the shadow? Do the ice dams form at the eaves or at the demarcation point near where sun meets shadow? Do you also notice the snow melt early in the season, before the ridge vent is covered with snow? Randy
*Caleb,I too had a similar problem where the roof area facing north below a dormer never sees sunlight until spring. The snow would build up and snow melt from the top of the roof would run down to this area and refreez. All I could do was install an electric snow melt system in the gutter and halfway up the valley. It is thermostat controlled and is switched so I can turn it off if the forcast isn't calling for precip.This has been in operation for 2 winters and works great.Rich
*Sounds like an application for heat tapes
*Dear Caleb,I had an idea. We call them idears in Vermont, but Mike told me to say that it is really just a theory. Ok so here's my theory about your roof ice:Maybe excessive heat from the house is causing the snow to melt too fast. I know you have R-60, and the vents, and the sticky paper. Hidden passages allowing warm air to blow into the attic, screwy ductwork causing pressure imbalances, excessive humidity carrying latent heat into the attic, and even attic mounted equipment can cause these troubles. Sometimes even a south/west facing gable wall can warm up and dump heat into the attic.These are just theories mind you, but I'm sticking to them. You can read more hypothetical rantings here.Regards, Fred
*With reply #1 I was trying to obtain additional information. Lacking that, I will say that I see many more ice dam problems that will respond to proper venting, insulation, and snow free ridge vents(a passive approach), than require heat tape. I've also met numerous homeowners who are adamant that they want heat tape (which will address the symptoms), and have no interest in a bit of detective work to determine if that is the only solution. Of course I also see severe reverse slope adjacent homes (that would respond to $200 worth of topsoil), but the owner opts instead to invest several thousand dollars in an interior "Beaver Board" system to drain away the severe basement water infiltration, caused by the ponding next to his foundation wall. To each his own.Randy.
*Here's the answers to your questions:Yes I'm in a cold area (near Ottawa, Canada). Yes the snow stays on the lower half of the roof all winter. In fact, there is a clear line where the snow stops, that corresponds to the shadow cast by the wall that blocks the sun. I haven't been up to the roof to examine it closely this year, but last year there was a sheet of ice under the entire snow covered portion of the roof with a large build up in the valley and near the eves.Fred, I've followed your thoughts on this forum and have read your pages over the past several years. I don't believe air leakage into this area is the problem, as great care was taken to plug every penetration of the vapour barrier. If I was to check the temperature under this roof, where exactly should I check it, what exactly should I be looking for.Heat tape is my last resort solution, I'm pretty sure it will fix the problem, but I'm wondering if there is a better way.Thanks for the responses so far.
*caleb... this is one of those cases where nothing is going to change the conditions so you have to change the protection..you can't stop the sun from melting the snow.. and you can't stop the shaded portion from refreezing.. so either heat tapes to melt the rest ( not my choice ) or... your ice & water have to extend up beyond the zone of refreezing so the dam cannot migrate high enough to remelt and leak in..
*What about a change in roofing materials? Perhaps a metal roof would shed the snow better.