Ice Dam + Flat Roof


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Old 12-17-09, 07:29 AM
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Ice Dam + Flat Roof

Hi everyone,

Winter is here and I've got an ice dam forming on the roof of a flat-top (low-incline) house. I just went to check the attic space and it seems cool, definitely not warm enough to melt snow. There is no insulation or ventilation. Could it be cool because the heat is running out the top? To add to the fun, the original shape of the home is a rectangle, but in the 1930s a triangle addition was put on the front (grey section), and the attic area of the addition is not accessible. Any help would be appreciated, I can't even seem to find information on whether or not ice dams are a real concern for flat, non-shingled roofs.

 
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Old 12-17-09, 09:57 AM
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It sounds like the building has been around for awhile, so it must be holding it's own. But a couple of issues. First, any changes you make may affect more than the ice build up. For example, insulate that attic space real well, and the snow may build up instead of melding off, creating an increase in the snow load, thus potential structural problems. Simply shoveling off accumulated snow can be an answer.

Second, ice formation occurs at some unique combination of temperatures based upon the insulation, ventilation, the sun, and outside temperatures. Yes, ice dams are a concern for your building and all others as the accumulated weight of snow, ice, and water can exceed the building's load capacity.

I would start by gaining access to that closed off section of attic. Then look at options for insulating and venting, or sealing everything up with what is called a hot roof. A good place to start is with a couple of contractors providing recommendations and estimates. Any structural issues will usually go beyond a DIY project, so talking to a professional at the start just makes sense.

I've seen several low slope roof get totally replaced with pitched roofs and traditional attics, but I'm not sure what your budget is.

stay warm,
Bud
 
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Old 12-17-09, 10:54 AM
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Hi Bud, thanks for the response.

Originally Posted by Bud9051 View Post
Simply shoveling off accumulated snow can be an answer.
Unfortunately it's a two-story home and the problem is on the second story. There's no way I'm going up there in the winter with a shovel.

A good place to start is with a couple of contractors providing recommendations and estimates. Any structural issues will usually go beyond a DIY project, so talking to a professional at the start just makes sense.
That's what I was hoping to get on this forum. But I know it's hard to say anything without seeing it in the flesh. The previous owners had set up a hot-wire across the length of the edge of the roof - perhaps a contractor recommended this. Unfortunately the only outdoor plug is ruined because it's right under an ice-cracked-leaky gutter. But that's another story.

Cheers,
Marc
 
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Old 12-17-09, 11:32 AM
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Glad to see you understand the view from here is limited. We always worry when giving advice, that the home owner will start cutting and building based upon our advice, which is often limited.

The general process of weatherizing takes into account attic heat, primarily because it carries moisture with it via the air leaks. So, your first stop is air sealing. Don't stop at what you thing are all of the air leaks, you will discover your home is a piece of swiss cheese, they all are. But sealing everything you can is the most cost effective of all improvements.

I hate hidden attics, you can't see problems early on to perhaps fix them before they come through the roof below.

I have a flat roof story for you. went to help a friend, almost flat roof with about 6" of ice at the eaves. It had been covered with rolled roofing, not shingles. Warm day so I was trying to dislodge the ice on the edge when a chunk let go. The flood of water behind it was astonishing. Had to be 50 plus gallons, but that rolled roof was holding up, no leaks. Tried to look into the attic, but previous owner had sheetrocked over the hatch. Wonder what he was hiding?

A picture might help. others will be by. http://forum.doityourself.com/electr...your-post.html

Bud
 
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Old 12-18-09, 05:05 AM
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A few more detailed follow up questions:

1) Does the fact that the attic space is currently cool tell me anything about the temperature of the roof?

2) If I turn on the hotwire now after the dam + icicles have already formed, does this pose any danger? If the icicles fall, they'll go onto the first floor overhang, not someone's head, so that seems fine. The gutters below are already all frozen so I don't see it doing any more harm.
 
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Old 12-18-09, 05:22 AM
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Originally Posted by MarcGuay View Post
A few more detailed follow up questions:

1) Does the fact that the attic space is currently cool tell me anything about the temperature of the roof?

2) If I turn on the hotwire now after the dam + icicles have already formed, does this pose any danger? If the icicles fall, they'll go onto the first floor overhang, not someone's head, so that seems fine. The gutters below are already all frozen so I don't see it doing any more harm.
1 With heat below and cold above, and little or no insulation, the attic will be somewhere in between, which is warm enough to melt the snow, resulting in ice. A vented attic needs good insulation, air sealing, and good ventilation. An un-vented attic, needs super insulation and super air sealing. What little heat escapes has to go up through that roof deck without appreciable melting any snow.

2. I've never used hotwires, but I would assume they are built to get wet and frozen into ice at times. You should be ok turning it on.

Bud
 
 

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