Green Home Articles | Ice Dams

An ice dam is a build up of ice at the edge of the roof. It is called a dam, because the build up is large enough to dam up some water behind it.

It is caused by the melting of snow on the roof, which flows down the roof to the edge, where it freezes and builds up. Sometimes even the best roof designs can get them. Generally it is caused by a warm attic and cold eaves. That allows the snow to melt up on the roof, over the attic, and then refreeze when the water reaches the eaves. The eaves are cold since they are not over the house, but are hanging out there in the cold. If the ice doesn't fall off the roof, and hangs on, then the water melting above, will build up behind the ice.

What harm can they do??

The water that is building up behind the dam can back up high enough to get under the shingles and leak into the attic. This often shows up as water leaking from the window trim or wet spots in the ceiling close to the walls. If it happens once it can stain your woodwork and make those ugly water marks on your drywall. If it happens often it will rot the roof decking and other framing members.

INSULATE & VENTILATE

You want a cold roof that will keep the snow from melting on the roof. At least when it does melt on the roof, it will not freeze again when it reaches the eaves. Ideally what you want is a roof that is a constant temperature from top to bottom, from eaves to peak.

Adding insulation to your attic floor will help. You should have enough insulation in your attic floor to give you R-49 (or you don't live in an area where ice-dams are a problem). That is about 20 inches of fiberglass. This means giving up the attic floor as a place to store things, since to get that much insulation you will have to go deeper than the ceiling joists, and preferably lay it perpendicular to the joists. You should NOT use insulation with a vapor barrier (kraft faced) since you should not add a vapor barrier part way through your insulation. Before laying the insulation down, or before blowing it in, seal up any openings in the floor. Electrical wiring comes through holes, caulk them closed. The plumbing vent lines run through the attic, caulk around those pipe openings. If you have light fixtures which require the insulation to be left away from them to dissipate the heat, then make sure you do that, you don't need to create a fire hazard here.

At the eaves, make sure the insulation does NOT close off the openings between the rafters that allow the air to flow up from the eave (soffit) vents. If you are blowing in insulation or packing the batts in tight there, buy prop vents. These are styrofoam panels that fit between the rafters to keep a channel open. This will help stop the heat from escaping into the attic. That will help save on your heating bill as well as help to eliminate ice dams.

VENTILATION

Ventilation is important for many reasons. Key among them is the elimination of moisture that finds its way up to the attic. (This is the reason for sealing the holes) Moisture that makes its way to your attic will condense in the cold air. It will condense on your plywood roof decking and cause it to rot, and it will pass up through the shingles causing them to fail years before they should. It also keeps the attic cool in summer, which is also good for the roof, and good for your comfort in summer.

A well ventilated attic will be colder which means less snow melting on the roof. The closer the temperature of the roof is to the temperature of the part over the eaves, the less melt you will get to freeze again on the eaves.

How much ventilation should you have? If you are prone to have ice dams, then the recommended MINIMUM is 1 sq ft for every 150 sq. feet of attic floor area. Adding more will only help. Ideally you want it split 50-50 between the soffits and the roof vents. Best is a continuous soffit vent and a ridge vent.
Again, make sure the paths from the soffit vents up between the rafters is free of insulation so the cool air can rise up to the ridge of gable end vents.

By the way, mechanical ventilation is NOT recommended. If a fan pulls a negative pressure on the attic and the negative pulls moisture into the attic from the house, it can create a new problem of condensing moisture in the attic.

FLASHING

If you have a lot of insulation and ventilation and you still have ice dams, or, you have cathedral ceilings or a roof attic design that makes adding those impossible, there is an alternative. In these cases you can have an ice belt installed. An Ice Belt is metal flashing installed all the way along the eaves and extends to the area of the roof above the attic. It covers the area where the ice will form. It serves two purposes, first it provides a surface which permits the snow and ice to just slide off preventing most dams. Second it provides a surface that water won't flow up under as it as it does with shingles.
There are other membranes that serve the same purpose to keep the water out, that can be installed under the shingles at the eaves. Again, extending up the roof, it will keep the rain that gets under the shingles from wetting the roof deck and leaking into the house. Removing the rain gutters may be necessary since they can become a blockage holding the snow and ice at the edge of the roof causing a dam.

You can always install a metal roof. These, while they are expensive, will eliminate ice dams completely. The snow and ice will slide right off, and they have no horizontal seam that water can get up under.

HEAT TRACING

Electrical Heat tracing can be run along the eaves that you can turn on should an an ice dam begin to form which will melt the snow and ice on the eaves, eliminating the ice dam. This is relatively easy to install and relatively inexpensive to purchase. It only needs to be used when a dam is forming, so it is not a large energy user.

FINALLY,

You can buy a roof snow rake and rake the snow and ice off the edges of the roof as soon as it forms. This method may be the cheapest but for very high roofs this is not for the timid! However, if you have an ice dam now and water is coming into the house, then this may be your best bet right now. Getting the snow and ice off the roof will stop the source of water leaking into your house. Continue to keep the snow off the roof until you solve the problem with a long term solution. If you have to climb up there to do it . . . BE CAREFUL!