Prevent Ice Dams


Old 12-12-16, 12:46 PM
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Prevent Ice Dams

I was told the following about preventing ice dams:If you have proper ventilation and insulation as well as proper installation of a roof (not just the shingles but all the other components you can't see) is the big stopper to ice dams. After all that it's best to just roof rake".
My question is who can determine if I have "proper installation of my roof"- a roofer?
Old 12-12-16, 01:16 PM
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It's not proper insulation of the roof. It's proper insulation below the roof..... usually right above the second floor..... that needs to be addressed.

The idea is to keep all the heat in the house so that the attic space and area right under the roof are the same temperature as outside. It's when the roof is warmer than outside when the melting and ice damming occur.
Old 12-12-16, 01:20 PM
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Hi music,
Ice dams are created by heat escaping into the attic and contributing to melting the snow. That melt seeps down under a blanket of insulation (the snow) and as it exits at the edge of the roof it encounters freezing temps, thus the start of an ice dam.

The one item missing from your list is air sealing of the attic floor. Air leakage is a major contributor of heat and moisture.

As for determining if your roof has the ice and water shield, at least near the edge, during warmer weather someone can lift the edge shingles and look. That's assuming the shingles are not old and brittle.

The ice and water shield does not reduce the risk of ice dams, just the risk of water leaking in once a dam forms.

More if needed.

Your too fast for me PJ

Last edited by Bud9051; 12-12-16 at 01:21 PM. Reason: comment
Old 12-12-16, 01:21 PM
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Proper installation of the roof has more to do with preventing leaks caused by ice dams than it does with preventing them. The key to this is usually installation of ice and water shield on the roof deck under the roofing at least 3 feet up from the eave edges, and properly installed flashing in valleys and where the roof meets vertical surfaces (if your roof has any). These measure help prevent water that backs up under the shingles from leaking through the roof deck.

Proper ventilation and insulation *and* air sealing are the best ways to prevent ice dams from forming. However, under the right (wrong) conditions, ice dams can form on any roof, hence the need for the protective measures above.

The basic mechanism of ice dam formation is snow melting on the upper parts of the roof, running down to the cooler areas of the roof near the eaves, and refreezing. As the ice builds up on the lower sections, it prevents more and more water from reaching the gutters, and the dam builds faster. The ice and water backs up under the shingles, especially during a thaw, and can leak through the roof decking if there is no secondary barrier like ice and water shield.

The upper roof is warmer than the lower roof because warm air in the attic rises to the upper portion causing it to be warmer than the eave sections. Proper insulation and air sealing helps minimize the amount of warmer air in the attic, and proper ventilation helps the warm air escape through the ridge vents, to be replaced with cooler air from the eave vents. This helps keep the roof deck cooler and minimizes snow melt.

But as I mentioned, if you get sunny days when the temperature is around freezing, followed by colder nights, ice dams can form even if everything else is done right.

That's where roof raking can help (or to compensate for not having everything else right). The key to roof raking is that the whole roof has to be cleared, especially the upper parts. Just clearing the lower area can actually make ice dams form faster because the lack of snow allows the lower roof to cool off much faster when the temperature drops below freezing.

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