Gutters and Ice Dams

(c) 2009 Jim Morrison

Many people I come across (incorrectly) believe that having rain gutters on your house causes ice dams. Since many houses without gutters still experience ice dams, we know this isn’t true, and the literature very clearly backs this up. However, myths die hard and there are people who still swear that it is (hey, there are still folks who believe the earth is flat, too), despite the vast body of building science that has exonerated rain gutters in this instance. We only get bad ice dams every five to seven years or so in this neck of the woods. Some houses never get them. If we get some significant snowfall followed by prolonged sub-freezing temperatures for a few days, the ice dam cometh.

Here’s how it works:

You are trying to keep the interior of your home heated to 70°F or so, but heat is constantly escaping the conditioned portion of your home to the underside of your roof no matter how much insulation you have. If your attic isn’t properly vented, this heat continually warms the underside of your roof until the flakes on the bottom of the “snow blanket” covering your roof begin to melt. This snowmelt runs down the roof until it gets to the low edge. This is where your roof overhangs the house, so the snowmelt is no longer being warmed by the house, and it freezes. After a while, the ice builds up (sometimes more than a foot high), damming water behind it and creating those enormous icicles that look great on postcards, but are scary to walk beneath. If the dam gets high enough, it will back water up under the roof shingles and stain the interior and exterior of your walls. Because rain gutters and ice dams both occur in the same place, the gutters usually get the blame.

So what can you do?

First of all, make sure your attic is well insulated and well vented. There should be at least one square foot of net free ventilation for every 300 square feet of attic floor area (you’ll need twice as much if your insulation has no vapor retarder). The ventilation area should be balanced with ½ of the area at the ridge, and the other ½ at the soffit. This should keep the house cooler in the summer months and will reduce the formation of the ice-dam in the winter months. It can usually be achieved with a ridge and soffit vent system, but each home is unique. Make sure clothes dryers and bathroom vent fans aren’t exhausting into your attic either. They must be vented directly to the exterior.

Insulating is a little trickier, but essentially there should be insulation between every heated and unheated portion of your home. The basement ceiling, all exterior walls and the attic floor should all be insulated. When installing fiberglass batts, remember the vapor retarder should face the heated area (translation: paper to the warm side). If there is insulation against the roof sheathing in your attic, it is working against you and should be removed. It may seem counterintuitive, but your attic should be uncomfortably cold in the winter, preferably within 10°F of the outside air temp. Insulating the roof sheathing will keep heat in the attic and make your home susceptible to future ice dams.

With your well-vented, well insulated attic comes other benefits! The additional ventilation prevents the buildup of intense summer heat. Attics can get to about 150°F in the summer. When the attic is cooler, the roofing material will last longer. And since you have a nice thick blanket of insulation between you and the attic, any heat that does build up in the attic, will stay there, leaving the house more comfortable.

If you are unlucky enough to experience an ice dam, please do not climb up on a ladder and hack away at it. If you do, you are very likely to fall off the ladder and at the very least, you will damage the roof shingles. The safest method of destroying an ice dam is to attach a garden hose to the drain cock at the bottom of your water heater. Then standing in a safe spot (not on your snow and ice covered roof), spray hot water on the outside edge of the dam, melting a 12 inch wide hole in it, but be warned, when the dam melts, the icy water it once held back will come cascading off the roof, so stand to one side. Better yet, hire someone to do it.

Installing warming cables on the roof’s low edges will also melt holes in ice dams, but that is a last resort in my opinion. It is treating the symptom (ice dams), rather than the problem (lack of sufficient insulation and ventilation), but they are effective. Some houses are going to get ice dams no matter how well insulated and vented they are. In these cases, it makes sense to also install ice and water shield beneath the low roof edges and valleys. This is the black rubbery membrane installed beneath the low roof edges and valleys in new roofs. When ice dams form, water backs up beneath the shingles, hits the membrane, and runs right into the rain gutter. It is a great product and one of the cheapest insurance policies going.

There are people who will maintain that ice dams are made worse by clogged gutters. To which I respond: Then keep them clear! I don’t believe that rain gutters effect ice dams in any way, but there are many good reasons to keep them clear. Rain gutters are designed to collect roof drainage and carry it away from the house. This prevents rain water from washing down the exterior walls causing paint to peel, hastening wood decay, and causing leaks. Missing, clogged, or poorly installed rain gutters are THE number one cause of damp and wet basements in this part of the country.

Properly installed and well maintained rain gutters and downspouts are an essential component in your home. Keep them clear and well maintained at all times for maximum effectiveness.

Whatever you do, don’t remove the gutters from your house in response to an ice dam. You’ll still get ice dams, and you’ll have created a host of other problems for yourself!

Jim Morrison
978.851.6315
Morrison Home Inspections
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