I've written about how to prevent ice dams by fixing attic air leaks and insulation, as well as several hack methods showing how to remove ice dams, but I still get a lot of questions about ice dam prevention. For homeowners with a one-and-one-half story house or a house with a vaulted ceiling and no true attic space, correcting attic air leaks and insulation can be an extremely expensive project.
In these cases, it's not always cost effective to fix the problems that are causing the ice dams - the 'repairs' might outweigh the costs of controlling the ice dams, and even if the repairs make economic sense, it's not always in the homeowner's budget. In those cases, I recommend ice dam control from the exterior.
Remove The Snow
Pulling snow off the roof with a roof rake will keep ice dams to a minimum. This becomes a constant chore, but it's better than dealing with water leaking in to the house. Just raking the first several feet of snow from the eaves is typically enough to prevent the formation of ice dams, but in some cases, this will cause ice dams to form higher up on the roof. The trick is to get the shingles exposed to the sun; once that happens, the sun will warm the shingles enough to prevent ice from accumulating.
Raking snow off the roof with a roof rake is a safe way of removing snow, as long as you don't get too close to your overhead power lines. In theory, a roof rake could cause some premature wearing of shingles by removing the aggregate, but I've never seen any real life evidence of this. Some roof rakes have little wheels at the bottom that prevent the rake head from actually rubbing on the shingles.
While the roof rake pictured above is the most common type, there are many other variations of this designed to make the work easier - one such version is the MinnSnowta Roof Razor®.
Removing snow from the eaves is an effective way to prevent ice dams, but it won't work 100% of the time. Two years ago, I inspected several houses with ice dams forming right where the snow stopped being removed. This is not typical, but it can happen during especially cold, snowy winters. When this happens, people start to get depressed and wonder why they live in Minnesota.
The fix for this is to have all of the snow removed.
For two-story homes where using a roof rake from the ground isn't practical or possible, the options are to risk your life getting up on an icy roof to shovel the snow off, hire someone else to risk their life, or install roof de-icing cables as a preventative measure.
Roof de-icing cables, also known as heat cables or heat tape, should be considered a last resort when it comes to preventing roof leaks from ice dams. De-icing cables themselves aren't cheap, it costs money to have them professionally installed, and they'll cost money to operate - between five and eight watts per foot. I've also heard that they can damage shingles, but I've never seen any evidence of this.
On the flip side, de-icing cables are very effective. When de-icing cables are properly installed and operational, ice dams won't cause leakage. De-icing cables won't prevent the formation of ice at the eaves, but they'll keep enough ice melted to create drainage channels for water. If you choose to install roof de-icing cables yourself, here are a few tips:
- Measure the areas where you need to install your de-icing cables first, and buy appropriately sized cables. For a simple 15' section of roof with no overhang, a gutter, and one downspout with an extension, you will need a 60' heating cable.
- The cables should extend 6" up the roof past the exterior wall line, through the gutters and downspouts, and 2/3 of the way up the valleys.
- Don't bother removing the snow from your roof; you could damage your cables, and you could potentially create another ice dam higher up on the roof, defeating the purpose of the heating cables.
- Don't expect the snow and ice to melt the way it does in the promotional photo above. It's like comparing a photo of a fast-food burger to the burger that actually comes in the fast-food bag.
If fixing the causes of ice dams isn't a possibility and safe removal of snow isn't possible, de-icing cables or de-icing panels may be a good choice. Sometimes this is the most cost-effective way to prevent roof leakage from ice dams.
Reuben Saltzman, Structure Tech Home Inspections