What is an ice dam on a roof?
An ice dam is a ridge of ice that forms at the edge of a roof and prevents melting snow from draining. As water backs up behind the dam, it can leak through the roof and cause damage to walls, ceilings, insulation and other areas.
How do ice dams form?
Ice dams are formed by an interaction between snow cover, outside temperatures, and heat lost through the roof. Specifically, there must be snow on the roof, warm portions of the upper roof (warmer than 32°F-0°C), and cold portions of the lower roof (at freezing or below). Melted snow from the warmer areas will refreeze when it flows down to the colder portions, forming an ice dam.
Although the primary contributor to snow melting is heat loss from the building's interior, solar radiation can also provide sufficient heat to melt snow on a roof. Gutters do not cause ice dams to form, contrary to popular belief. Gutters do, however, help concentrate ice from the dam in a vulnerable area, where parts of the house can peel away under the weight of the ice and come crashing to the ground.
Thermal bridging through walls: many older homes have insufficient insulation, particularly at junctions where the roof and walls meet. This problem is most notable with low-sloped roofs and short overhangs. As heat is transferred through framing members it warms the roof deck and melts the snow; that water then runs down under the snow to the overhang where it is no longer heated by interior factors, and it freezes again.
Air leakage through walls: through ceilings is another common problem as a lot of heat is transported by air. This warms the attic and causes that same melting to occur.
Ventilation equipment such as furnaces, duct work and air conditioning is rarely seen in attics anymore, but if you have such equipment up there it is likely contributing to ice damming. In addition to that, you are probably adding 25% to your operational costs by having air handling equipment in what is seasonally the coldest and hottest part of the house.
Pot lights: they are not so much a problem due to the heat they generate, but they are very hard to seal. And worse than other air leaks, as it passes by the bulbs, this heated air magnifies the problem.
Snow: this may be hard to wrap your head around, but snow has an R-value of between 1 and 2 depending on density. When there is a significant amount of snow on a roof it insulates the attic and warms the air inside the attic, causing snow to melt. Ice doesn't form directly on the roof surface; the water is absorbed into the layer of snow, turning it into ice. So water can easily run down underneath to the cooler point of the roof where an ice dam will form.
Sun and wind: wind will blow snow off a roof, most noticeably at the ridge, and when roofing materials are exposed to the sun they warm up and melt the surrounding snow.
Ice dams are problematic because they force water to leak from the roof into the building envelope. This may lead to:
rotted roof decking, exterior and interior walls, and framing;
respiratory illnesses (allergies, asthma, etc.) caused by mold growth;
reduced effectiveness of insulation. Wet insulation doesn’t work well, and chronically wet insulation will not decompress even when it dries. Without working insulation, even more, heat will escape to the roof where more snow will melt, causing more ice dams which, in turn, will lead to leaks; and
peeling paint. Water from the leak will infiltrate wall cavities and cause paint to peel and blister. This may happen long after the ice dam has melted and thus not appear directly related to the ice dam.
Keep the entire roof cold. This can be accomplished by implementing the following measures:
Install a metal roof. Ice formations may occur on metal roofs, but the design of the roof will not allow the melting water to penetrate the roof's surface. Also, snow and ice are more likely to slide off of a smooth, metal surface than asphalt shingles.
Seal all air leaks in the attic floor, such as those surrounding wire and plumbing penetrations, attic hatches, and ceiling light fixtures leading to the attic from the living space below.
Increase the thickness of insulation on the attic floor, ductwork, and chimneys that pass through the attic.
Move or elevate exhaust systems that terminate just above the roof, where they are likely to melt snow.
A minimum of 3" air space is recommended between the top of insulation and roof sheathing in sloped ceilings.
Remove snow from the roof. This can be accomplished safely using a roof rake from the ground. Be careful not to harm roofing materials or to dislodge dangerous icicles.
Prevention and Removal Methods to Avoid
electric heat cables. These rarely work, they require effort to install, they use electricity, and they can make shingles brittle.
manual removal of the ice dam using shovels, hammers, ice picks, rakes, or whatever destructive items can be found in the shed. The roof can be easily damaged by these efforts, as can the homeowner when they slip off of the icy roof.