Ice Dams on Flat Roof

mcgeebcDecember 8, 2010

I have a flat roof with rolled roofing over an unheated and uninsulated sun room. Enough heat leaks into the sun room from the house that the snow melts and forms really bad ice dams. Those dams back up water all the way to the gable roof and caused some leaking last year.

I've considered a few options to help with the issue.

1) Roof cables - my problem here is that they normally attach to shingles. I don't know of any way to attach them with a rolled room. Does anyone have any ideas?

2) Add insulation to the ceiling. There is no attic, but there is about a 6-9" cavity in the ceiling. I guess I could cut holes and blow insulation.

3) Shoveling snow off the roof. This is a pain, but may be my only option.

Does anyone have any other ideas?

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It's not clear to me how ice dams form on a flat roof. It must have some sort of pitch so a "dam" forms on the downslope side and prevents the water from runing off. Otherwise your problem is that a puddle forms in the low point in the center and leaks through (although I'd expect this to happen during/after significant rains also). Lastly your roof slopes back towards the main roof and leaks at the intersection.

If you have "dams" that are actually blocking runoff, one suggestion is to fill a nylon stocking with ice melt pellets and rest it across the dam. The ice melt will slowly eat its way through the dam to the base roof and let the runoff out.

If the sunroom below (and the cavity in the ceiling) is truly unheated then I can't see the insulation doing much for you. yopu actually want to vent that space so then underside is as cold as the top.

Shoveling the roof sounds like the best short term corrective action you could take. Sheet roofing on flat roofs is supposed to be tight enough to withstand some ponding, but there still needs to be some slope (using crickets) and a drain mechanism.

Pictures might add some clarification.

Hope that helps.

    Bookmark   December 8, 2010 at 1:31PM
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Sorry. There is a slight pitch.

    Bookmark   December 9, 2010 at 9:30AM
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"It's not clear to me how ice dams form on a flat roof. "

Seen many times.

The snow does not melt over the entire roof all at the same time.

It melted snow then saturates the remaining snow and the whole mess can freeze.

When it warms up more, the frozen slush thaws but dams of ice remain and prevent drainage of the melted ice.

Increasing the pitch would probably help, but if you want/allow the room to be warmed (even by the sun) you should probably check how much insulation you have.

    Bookmark   December 9, 2010 at 3:33PM
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