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How to Prevent Ice Dams on a Roof

By Edited May 6, 2015 2 0
How to Prevent Ice Dams on Roof
Credit: Dmcroof via Wikimedia Commons

Living in an area that has harsh winters requires regular home maintenance to ensure you will not have problems during the snowy winter months when everything is much more difficult to repair.

One of the most common problems in areas with temperatures consistently below freezing is the buildup of snow and ice on the edge of roof lines referred to as ice dams. Preparing for ice dams is just one of the items you should be concerned with when winterizing your home.

What is an Ice Dam?

An ice dam is a buildup of melting snow that refreezes at the bottom of the roof eaves preventing the melting snow and ice from draining properly into your gutters causing a lake under the snow on your roof. As you can imagine, this can lead to a whole host of issues not the least of which is water leaking through your shingles and causing damage in your attic or any rooms near the roof line.  

So what causes this buildup of snow and ice in the first place? 

More often than not, ice damming is caused by the difference in temperature in a very warm attic and very cold eaves. The heat from the attic rises and melts the snow on the upper part of the roof line which flows down to the bottom toward your gutters, but then refreezes as it reaches the colder areas above your eaves. Eaves are the portion of the roof line that hangs out over the house, therefore it is exposed to the cold.

The buildup feeds upon itself getting bigger and bigger. Soon the ice dam itself is enough to refreeze the water that approaches. The ice will form above and hang off the sides causing a big mess. If left alone, only sustained temperatures well above freezing are enough to dissipate the ice, but by then there has likely been some water damage to your roof.

How Does an Ice Dam Cause Damage?

If you think about a normal dam holding back a river, a lake eventually forms behind it. The water seeks to fill every low level or ground around it.

The same principle applies to what is happening behind an ice dam on your roof. As the water is prevented from draining off a roof into the gutter system, it starts to pool behind the dam.

Roofing shingles are laid from the bottom of the roofline toward the peak of the roof so that each successive row covers half of the last row. This engineering works great with falling rain because the slope of the roof carries the water downward. Gravity does not allow water to seep back upward under that last shingle.

However, as the water begins to pool, it back up allowing it to get under the previous row of shingles and starts soaking the wood underneath.

One of the first signs that you have a problem during a really snowy period of weather is when you see water leaking around windows or water stains on your walls near the ceiling.

Occasional water issues will cause ugly stains on wood trim and drywall. Persistent water issues will rot the wood under your shingles requiring an entire new roof.

What Causes Ice Dams
Credit: Photopro20 via Wikimedia Commons

How to Prevent Ice Dams

You first line of defense against persistently cold winters is to insulate and ventilate your attic properly. You do not want a really warm attic space in the winter. Ideally you need to have it at one cold temperature from peak to eave.

Assuming you have an unfinished attic, the first place you need to start is with good insulation in the attic floor space in between ceiling joists. The idea is to keep the heat in the main home from rising and heating the attic also.

The following chart shows the recommened insulation levels for each zone in the continental United States.

How Much Insulation do I Need in my Attic?

How to Prevent Ice Dams on a Roof
Credit: Department of Energy

Start by measuring the amount of insulation you have in your attic. You need at least an R-49 which equates to about 20 inches of fiberglass insulation of either bats or the blown-in kind. Simply place a 24 inch ruler into the insulation and measure it.

Check for any gaps in the attic floor from plumbing, venting stacks or electrical wiring and seal

Insualtion Baffles for Airflow
those with caulk or insulation. The idea is to eliminate any path that warmer air from the home might have to the attic.

However, do not place insulation around recessed lighting pods as it will create too much heat and be a fire hazard.

You may have noticed that 20 inches of insulation in your attic floor would take it above the joists in the floor, so how are you going to get that much coverage?

Well, most ceilings in homes are attached directly to the joists above them allowing for no room to go deeper than the ceiling joist. So if your joist are 10 to 12 inch boards, then you have to build up the insulation above the attic joist. Obviously this is impossible with blown-in insulation, but you can use that type up to the top of the joists, then overlap the joists perpendicular with insulation bats to reach the R-49 rating.

However, do not use a vapor barrier at this point.

Addressing the Eaves

How to Prevent Ice Dams on a Roof
Credit: Noles1984 via WikiCommons

You may have noticed that the eaves around your home have either vents spaces every few feet, or are simply made with vented soffit all the way around the home.

As you insulation the attic to bring it up to an R-49 rating, be sure that you do not cover any of the vents over the eaves. The attic needs to breath and it draws in colder air from the soffit vents, then breaths out through a ridge vent at the peak of the roof.

One way to prevent insulation from covering these vents is to use Styrofoam panels that fit between the rafters to keep a narrow channel open between the panel and the roofing boards.

An attic that is ventilated properly will pull more cold air inside during the winter months keeping it at a constant cold temperature along the inside of the attic. That translates into less melting snow on your roof and less potential for damming. Ideally you want the temperature of the attic at the eave to be the same as the temperature of the attic at the peak.

If you have recurring ice dam issues, it is recommended that you have a continuous soffit vent all around your eave as well as a ridge vent at the peak of your roof.

Install an Ice Belt

An ice belt is simply metal flashing that is installed along the eaves which extends above the attic in the area where ice is likely to form from an ice dam.

Ice belts will not allow melting water to flow under to reach the roofing boards since it is all one continuous piece as opposed to individual shingles.

This type of metal belt is places several feet up the roof line from the eave, under the shingles. Water may seep under the shingles during damming periods, but it will simply slide off the ice belt as opposed to being absorbed by the roofing deck.

If you do not want to install a metal belt, you can opt for special membranes that serve the same purpose.

Install Heat Tracing Cables

How to Prevent Ice Dams on a Roof
If you have heard about in-floor heating under tile or even concrete floors, the same principle applies to roof heating.

Heat cables are effective at melting ice and snow build-up at the eave, however, they do not prevent ice dams.

Your first line of defense should be prevention, however, trace heat lines are relatively easy to install along the bottom of the roof line on the exposed part of the shingles.

Easy Heat ADKS-150 30-Foot Roof Snow De-Icing Kit
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If you want to go really low tech with the cheapest solution for immediate issues, use a roof rake. Simply pulling the snow and ice from the eave will eliminate the dam.

However, for higher roof lines, this is not possible and I do not recommend getting on a ladder during really cold periods.

If you are experiencing recurring issues with ice damming, your best bet is to prevent it from happening in the first place by following the advice listed in this article. You may spend a little money upfront, but it will be a much smaller amount than what you will pay to replace a rotted roof and repair water damage inside your home.

Trimline 58784 Trimline Shingle-Over Ridge Vent
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