I have been receiving a number of calls lately related to leaking roofs and ice dams. This is a common phenomenon that often occurs as we begin our transition to spring. There is a fair amount to cover on this topic, but I will do my best to be concise. Notwithstanding, this will be a longer than post average as this seems to be an important topic… Especially if yours is the leaking roof!
Ice dams most often form when accumulated snow on a roof melts from the bottom, due to warm air in an attic. As the water travels down and reaches the uninsulated part of the roof near the eaves/over the soffit, it freezes because the surface temperature of the roof in this spot is below zero. When temperatures are extremely low during the middle of winter, the cold air keeps the roof deck frozen even though there is a warm air source below. As temperatures rise, the warm air from below now has the upperhand and it melts the snow from the bottom. If temperatures are still below freezing (as is usually the case at night even into spring) this water will refreeze at the eave, which is usually a near match to the air temperature. A freeze thaw cycle begins and soon you have an ice dam forming.
There are a number of things that can allow warm air to escape the conditioned space. Here are the most common ones:
Inadequate Insulation: Many older homes are only insulated to R-24 or less in the attics. This can allow the heat from inside to reach the underside of the roof decking. Insulating to R-60 will dramatically reduce heat transfer.
Air Leakage: Air leakage is when air from the conditioned space is able to escape through breaks in walls, around fixtures or through gaps in vapour barrier (there are others as well). Drywall, if it is taped and plastered, can act as an air barrier. But, for example most tongue and groove wood ceilings would be a situation where vapour barrier would be even more critical than normal circumstances as there are often voids in the wood. There is actually a fair amount of building science research out there that shows that air leakage is as big of a problem as lack of insulation in home energy efficiency.
Ventilation: Poor ventilation in an attic can be a big contributor to ice damming, especially if it combined with the points above, as it often is. If air is escaping the conditioned space below and has nowhere to go, it will sit under the decking and melt the accumulated snow on top. If there is good air circulation and warm air will be filtered out before it has a chance to wreak havoc and the chances of an ice dam will be reduced. In an ideal world, a home will have soffit vents at the eaves, blocked soffits on the rakes or gable edges and a ridge vent or Maxi-vent at the peak. You want cool air to be drawn in at the lower edges and for warm air to be forced out at the peaks.
Fixing the problems above will greatly improve your chances of remaining ice dam free. In some instances it may not be feasible to do all of the above, and in those cases, making as many changes as possible and adding a deicing cable may be the best path. Heating cables aren't terribly effective, but they can create a path for pooling water to get out if no other option is available.
This also highlights how important a properly installed roof system is. If water does get behind the primary roofing material you want a waterproof membrane there to back it up. Synthetic underlay or tar paper (or worse; nothing) are not adequate in high risk areas. Many people forget that shingles are designed to shed water, but they are not completely water tight. The same can be said for a steel roof. An ice dam can easily pop the screws and create an entryway for water.
Ice dams are very difficult to fix in the winter and usually you have to wait until the spring to address the underlying issues. In the meantime, keeping snow off the roof or running warm water through a hose can break up ice dams. I don’t recommend throwing rock salt on an ice dam as it can damage shingles or corrode steel.
In the end, you should speak to a qualified professional and have them suggest a solution within your budget. Doing nothing could leak to a very costly problem down the road (pun intended).
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