Field guide

Roofer Ice Dam Pages: A Cold Roof Is the Fix

The roof is warm in the wrong place

An ice dam is a ridge of ice that forms at the edge of a roof, and it forms because of a temperature difference across the roof deck, not because of the weather. Snow on the roof acts as an insulating blanket. When heat escapes from the living spaces into an under-insulated attic, it warms the underside of the roof sheathing, and the upper section of the roof deck climbs above 32 degrees Fahrenheit, even while the outdoor air is well below freezing. The bottom layer of snow melts into liquid water, which trickles down the slope beneath the snowpack until it reaches the overhang, the eave. The eave extends beyond the exterior walls, so it gets no heat from the attic, and its temperature matches the sub-freezing outdoor air. The meltwater refreezes there, and over hours and days the cycle builds a continuous ridge of solid ice.

Once the ridge exists, the damage follows a fixed path. Water pools behind the dam, finds nail holes, overlapping shingle seams, and wood joints, and soaks attic insulation and leaks into interior ceilings. Ice dams develop most rapidly when outdoor temperatures sit between 20 and 30 degrees Fahrenheit, cold enough to freeze the runoff at the eave, warm enough to melt the snow on the upper deck. Colder temperatures freeze the runoff before it reaches the eave. Warmer temperatures melt the snow uniformly. The page that explains this chain is the page that can sell the fix, because the fix is not about the ice. It is about the heat that made the ice.

Insulation, air sealing, and ventilation, in that order

The most permanent way to prevent ice dams is to keep attic temperatures as close to the outdoor temperature as possible, and the work has an order. First, seal the air leaks, because stopping warm indoor air from leaking into the attic is the top priority, and insulation alone cannot do it. Then verify the insulation meets the Department of Energy recommendation for cold climates, climate zones 4 through 7: R-49 to R-60 in the attic. A poorly insulated attic can easily reach 40 degrees Fahrenheit or higher in mid-winter, which is warm enough to melt snow on the deck above it. Then balance the ventilation, because even with excellent insulation, some heat escapes, and ventilation flushes it out before it can warm the sheathing.

Balanced ventilation means a continuous flow of cold exterior air in through soffit vents at the eaves and out through a ridge vent at the peak, sweeping away incidental heat and moisture. The rule for the amount is the 1/300 rule: 1 square foot of net free vent area for every 300 square feet of attic floor space, split equally, 50 percent intake and 50 percent exhaust. Rafter vents, also called insulation baffles, have to be installed at the eaves to keep the insulation from blocking the soffit vents, because a blocked intake is the same as no intake. A cold roof is a safe roof, and the three steps in that order are what makes a roof cold.

What a roofer's page should not promise

Heat cables are the promise to avoid. They melt a channel through the dam, which looks like a fix and treats the symptom, while the warm deck keeps making ice above the cable. A page that sells heat cables as the solution is a page that will see the same customer again next winter, with the same leak and less patience. The honest line is that cables are a stopgap for a roof that cannot be fixed this winter, and the fix is the insulation, the air sealing, and the ventilation, done in that order, before the next snow. The roofer who says that on the page is the roofer who gets the insulation job, the ventilation job, and the customer who tells their neighbors.

Frequently asked questions

What causes an ice dam on a roof?

Heat escaping into an under-insulated attic warms the upper roof deck above 32 degrees Fahrenheit, melting snow that refreezes at the cold eave. The temperature difference across the deck, not the weather alone, makes the dam.

How much attic insulation prevents ice dams?

The Department of Energy recommends R-49 to R-60 in cold climates, zones 4 through 7. A poorly insulated attic can reach 40 degrees Fahrenheit or higher in mid-winter.

What is the 1/300 rule for attic ventilation?

One square foot of net free vent area for every 300 square feet of attic floor space, split equally between intake at the soffits and exhaust at the ridge.

Do heat cables fix ice dams?

No, they melt a channel through the dam and treat the symptom. The fix is insulation, air sealing, and balanced ventilation, in that order, so the roof deck stays cold.