Do gutters cause ice dams?

What causes an ice dam?

University of Minnesota Extension names three conditions that have to hold at the same time: snow on the roof, an upper roof surface that averages above 32°F, and a lower roof surface that averages below 32°F.¹ Snow melts on the warm part, runs down under the snowpack, and freezes when it reaches the cold part. The ridge of ice grows for as long as meltwater keeps arriving. Water backs up behind it and works under shingles, which are made to shed water and cannot hold a pond.

1. 1Snow on the roofThere has to be something to melt.
2. 2Upper roof above 32°FHeat from the house melts the snow from underneath.
3. 3Roof edge below 32°FThe meltwater reaches the cold overhang and freezes.

The warm part of the roof is warm because of the house. The Minnesota Department of Commerce says so in one line of its Home Energy Guide sheet on ice dams: "The primary cause of ice dams is attic air leaks."² Warm, moist air rises through gaps around plumbing vents, wiring, chimneys and light fixtures and heats the roof deck from below. Extension describes the same path and writes that in many homes it is "the major mode of heat transfer that leads to the formation of ice dams."¹

The U.S. Army's Cold Regions Research and Engineering Laboratory offers the plainest demonstration. Its researchers photographed two identically built buildings near Watertown, New York, within minutes of each other. One roof carried large ice dams and icicles and the other was ice-free. The ice-free building was the one with the heat turned off.³

Why does the ice end up in the gutter?

The overhang is the one part of the roof with no heated room under it. It stays at outdoor temperature while the roof over the living space warms, so meltwater freezes there first.⁵ The gutter hangs at the outer edge of that cold zone. Whatever water gets past the dam lands in it and freezes too.

Paul Fisette investigated ice dams as a builder and as a researcher at the University of Massachusetts Amherst. His summary is direct:

Contrary to popular belief, gutters do not cause ice dams. However, gutters do help concentrate ice and water at a very vulnerable roof-eaves area. As gutters fill with ice, they often bend and rip away from the house bringing fascia, fasteners and downspouts in tow.

Paul Fisette, University of Massachusetts Amherst⁵

The gutter is where the damage shows, which is why it gets the blame. Take it away and the three conditions Extension lists are unchanged. The dam still forms on the roof deck above the overhang, and the ice hangs from the shingle edge instead of filling a trough.

How cold does it have to be for an ice dam to form?

It has to be below freezing outside, with snow on the roof. The Army researchers instrumented several buildings in northern New York and reported that problem icing "developed very slowly, if at all" when the outside temperature was above −5.5°C, which is about 22°F.⁴ Near the freezing point the eave is barely cold enough to refreeze anything, and the dam does not grow much.

Snow depth matters as much as cold, because snow is insulation. Fisette puts its value at roughly R-0.5 to R-1 per inch, so ten inches of snow adds R-5 to R-10 on top of the roof and holds the house's heat against the roof deck. In his words, "The worst ice dams occur when deep snow accompanies cold weather."⁵

How much does attic insulation change the roof temperature?

Fisette works one example by hand: an R-20 roof under ten inches of snow, 70°F indoors and 20°F outdoors, ends up with roof sheathing between 30°F and 37°F. That is warm enough to melt snow while the overhang sits at 20°F.⁵ The chart applies the same arithmetic to a second attic insulated to R-50, the minimum the Minnesota Department of Commerce recommends where space allows.²

R-20 atticR-50 attic

View as a table

Estimated roof sheathing temperature over the living space, 70°F indoors and 20°F outdoors

Snow on roof	R-20 attic	R-50 attic
5 inches	26 to 30°F	22 to 25°F
10 inches	30 to 37°F	25 to 28°F
20 inches	37 to 45°F	28 to 34°F

This arithmetic counts only heat conducted through the insulation. It leaves out air leaks, which the Department of Commerce calls the primary cause, and it leaves out attic ventilation, which carries heat away. A leaky ceiling runs warmer than the chart shows. The comparison still holds. By this arithmetic the R-20 roof can reach 32°F under about 6 inches of snow, and the R-50 roof not until about 16 inches.

What stops ice dams?

The Minnesota sources give the same order of work. Extension writes, "First, make the ceiling air tight so no warm, moist air can flow from the house into the attic space," and then consider more ceiling or roof insulation.¹ The Department of Commerce says sealing the leaks with caulk or expanding spray foam prevents nearly all ice dams, and that insulation alone is insufficient.² Extension suggests hiring a weatherization contractor who runs a blower door test and uses an infrared camera to find where the ceiling leaks.¹ Minnesota's Weatherization Assistance Program provides free home energy upgrades to income-eligible homeowners and renters.

The sources differ on ventilation. The Army laboratory found insulation combined with roof ventilation the most reliable cure: 57 buildings at Fort Drum were modified to its guidelines, which "essentially eliminated the chronic icing problems of the past."³ Extension says only a small amount of natural ventilation is needed once the sealing and insulation are done well, and that mechanical attic ventilation is not recommended in Minnesota.¹ The Department of Commerce warns that adding roof vents "will not eliminate ice dams, and often make the problems worse."² All three agree on sealing and insulating first.

What should you avoid doing to the ice?

• Heating cables. The Department of Commerce says they shorten the life of the roof and cost money to operate.² Fisette adds that the cable tends to create a new ice dam just above it.⁵
• Chipping, chemicals or heat. These can damage shingles, gutters and other parts of the building.²
• Working on the roof in winter. Extension warns that anyone on a winter roof, or working on it from below, risks injury and may damage the roof, and it advises calling professionals.¹

Extension does describe two stopgaps. Pulling snow off with a roof rake removes one of the three ingredients, though the rake can scuff the roofing. If water is already coming into the house, a channel melted through the dam with tap water on a warm day, working upward from the lower edge, lets the trapped water drain. The channel stops working within days.¹

Is an ice dam a gutter problem or an attic problem?

It is an attic problem. The people who cure it are energy assessors and insulation crews, and a new gutter will not change what the roof does next winter. Extension makes the same point about interior repairs: fix the heat loss at the same time, "or the damage will occur again."¹

The gutter work comes afterward. Fisette's list of what ice takes with it is a useful spring checklist from the ground: the gutter run itself, the fascia board behind it, the fasteners and the downspouts.⁵

• Gutter runSagging or bent sections
• Fascia boardA gap where the gutter pulled away
• FastenersHangers or spikes backed out
• DownspoutsLoose, split or hanging free