Preventing Ice Dams

Robert Robillard

10 years ago

Heat Loss Seen on Condo On Left. Condo on Right Has Spray Foam

How To Prevent Ice Dams

What is an ice dam?

An ice dam is a ridge of ice that forms at the edge of a roof and prevents melting snow (water) from draining off the roof. The water that backs up behind the dam can leak into a home and cause damage to walls, ceilings, insulation, and other areas. A roof leak is often the first indication that you may have ice dams. Here are some tips on preventing ice dams in your home.

Figure 1 shows a cross section of a home with an ice dam.

Image: extension.umn.edu

Where Do Ice Dams Form?

Ice dams form when there is snow on the roof and freezing temperatures.

Ice dams usually occur after a heavy snowfall and several days of freezing temperatures. Warm air inside your home leaks into the attic and will warm the underside of the roof causing snow and ice on the roof to melt. The melted water will drain along the roof, under the snow, until it reaches the cold overhang. The overhang tends to be at the same temperature [below 32 degrees] as the outdoors and the melted water will refreeze and form an ice dam and icicles. The ice dam can cause damage to the roof, which will result in water leaks to the inside. The water back ups and finds cracks and openings in the roofing and then flows into the house. The result is water spots or dripping water on the ceiling under the roof damage.

Ice Dam Causes

The most likely causes of ice dams are due to heat loss, air leakage and the general warming of the house structure due to conduction from the heated building. So preventing ice dams will focus on improving heat loss and air leakage.

IceDam

Preventing Ice Dams

Ice dams are not a rood shingle issue, there an air leak sealing problem and usually a combination of lack of insulation and ventilation. There are four sure-fire steps you can take to prevent ice dams. These steps are best done in order of priority listed below:

Seal Air Leaks
Increase Insulation
Improve Ventilation
Install Waterproofing Membrane Under Shingles

Seal Air Leaks

Warm air flow from the house leaks into the attic space and for this reason, insulation alone will not stop air leaks or ice dams. It is extremely important to get your attic air leaks sealed if you want your insulation to really work well.

We use a combination of expanding foam and ridged insulation to seal all gaps, cracks and joints where air may escape.

Common Air Leak Areas

Recessed can lights [Ridged insulation boxes between the joist and on top of the lights work well]
Ceiling mounted electrical boxes
Bath exhaust fans
Plumbing vent pipes
Wall partition top plate seams
Plumbing and electrical penetrations
Chimney chase-ways. [Cut 24 gauge or thinner sheet metal to fit tightly to the chimney and the framing. (IMPORTANT: This is a draft stop only. Do not put insulation on top of it. Insulation must still be kept at least two inches away from the chimney.)
Plumbing chases

Example of massive air leak

Add Insulation

Cellulose or fiberglass insulation can be added on top of existing insulation, undersized or poorly installed batt insulation. If additional insulation depth is required install over and on top of joist bays. Make sure to install ridged foam or Styrofoam baffles between the roof sheathing and the insulation below, to create a 1-2″ air gap and to prevent insulation from blocking the soffit vents.

In many instances it may make sense to install closed-cell spray foam insulation to the underside of the roof. Achieving R40 in an attic will cut down on heat loss by conduction. Many state codes require an R-value of 30-49 above the ceiling for new homes. Building Science Corp recommends an R-60 roof, however, we’ve seen great success with R-values as low as R-40 (when properly installed.)

Contractor blowing foam insulation

Improve Ventilation

Increasing ventilation will help keep the roof cool but only after you’ve air leak sealed and added insulation. If you fail to do the first two steps, adding ventilation will actually work against you by pulling more heat out of the house and against the rood surface.

Install A Waterproof Membrane

There’s a reason when rubber roofs don’t worry about ice dams! While not a solution, adding a minimum of 6-feet [or more] of self-adhering ice and water shield membrane under your shingles, and along your roof edges, eves and valleys can help prevent pooled water from ice dams from getting into your house.

Sawtooth Pattern

How to Deal With An Active Leak

Rake Roof of Snow:

Remove snow from the roof. This eliminates one of the ingredients necessary for the formation of an ice dam. Using a roof rake to clear the snow from the first three to four feet along the roof edge is a great way to prevent ice dams. This should be done immediately after it snows. Roof raking eliminates one of the ingredients necessary in an ice dam recipe and is BEST done before the ice dam forms.

Cutting a channel in ice

Removing Ice Dams With Steam

Steam is by far the safest method and poses the least risk of further damage. A steamer uses a home’s water supply, heating it to 300 degrees. The steam is forced through a hose and wand, where it is delivered in a low-pressure stream used to cut through ice. The most efficient way to steam ice off a roof is to cut it into chunks and throw the chunks from the roof. A true emergency sometimes means you cannot wait for a contractor to use a steamer.

In this case, I have cut channels through the ice dam to allow the water behind it to drain off the roof.

Cutting channels in the roof to drain melted water

In these situations, I have used a hammer claw, a large chisel ad even a hammer drill with a chisel attachment to carve the channels. Your goal is to allow the roof to drain, and you need to be super careful not to damage the roofing material underneath.

Tip: Stop chipping when you get close to the roofs surface. I suggest cutting channels every 2 to 3 feet. Removing the snow above this ice dam is also another way of eliminating the one ingredient of the ice dam – snow that melts water.

Crimping clips to hold wire

Deicing System:

If air leak sealing, adding insulation and ventilation are not an option, then I suggest installing and using a ice melt system as a last resort. These systems are helpful with complicated roof designs, high and hard-to-rake areas. Roof and gutter deicing cables, or “heating cables,” provide an effective and economical method for draining the roof of snow melt.

I also advise my clients against using cheap hardware-store versions. These cables are not efficient ice melting systems and turn on when temperatures reach a certain point, snow or not. Instead, I use a commercial-grade, self-regulating heating cable that has sensors to detect snow and ice dam conditions. The system is “smart”; it turns on when it snows and off a few hours after it stops.

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