Canaan property ownership, winter roof leaks, and related design work has schooled me in the issues that bear on ice damming. Most people don't even know what it is. This is because they do not live in or visit an area with wintery conditions, or because they are house-science-ignorant (which is fine until you are shopping houses or considering major attic or roof work). DC area residents see a few inches of snow annually, so a DC winter that is delivering 50+ inches in a series of bigtime dumps has meted out more than a few object lessons about ice damming. This is so prevalent that it warranted an article in the Post on the subject (below).
What is ice damming? Ice damming occurs when the roof surface temperature is sufficiently warmer in some areas (typically over the living spaces) than in others (typically over the roof overhangs (soffits). Ice damming is the formation of ice or snow-ice, usually near soffits and in the roof valleys, which in turn holds back water or slush. The result is that areas of the roof experience submersion (unlike a rain storm) and water finds its way into the house.
Canaan and Davis are full of houses with ice damming challenges. I'll run through what I see as the lines of defense:
1. House design - mimimize valleys (sloped troughs created when perpendicular roof slopes intersect). Note that if you place roof ridge lines perpendicular to prevailing winds (from the West by Southwest in Canaan), there will be spindrift accumulation on the lee side of the ridge. Knowing this, you don't want to place gables with windows close to the roofing, nor do you want a roof valley on the lee side of a ridge if you can help it.
The roof temperature needs to be regulated by the design and insulation choices in your roof construction. A traditional unconditioned, ventilated, attic allows the roof to stay cool, but often the area of the roof that crosses over the exterior walls and hangs out from the house (soffit) is even cooler, making the traditional ventilated attic a marginal defense against ice damming. Another way to regulate roof temperature is to directly insulate the roof sheeting. This can be done by foaming with icynene open or closed cell foam from below; foaming must cover both the areas of roof over the attic or living areas as well as the areas of roof over your soffits (roof overhangs). Another approach is to sheet the outside surface of the roof with rigid insulation, or better yet, structured insulated panels (SIPs) that integrated foamboard with plywood. The R-value of one of these solutions will drive the ability of your roof to stay cool in winter. Whether or not to ventilate the attic or rafter cavities below this solution is a separate consideration. Most foaming solutions would suggest closing up the ventilated attic and conditioning the attic. Aside from avoiding ice damming, the benefits include: lower energy bills, cooler Summer afternoons in the house, less sound infiltration from airplanes, and if the attic is closed up and insulated you now have additional storage.
2. When Reroofing. When reroofing in Canaan, it is recommended that you use self-sealing, waterproof, roof wrap as the secondary barrier beneath the shingles or other roofing material. This prevents any breach of the roofing material from becoming a water issue INSIDE the house. Roofing materials should consider the high winds in certain areas of the Valley. Cedar roofing is not recommended because of the high precipitation, wind and temperature variations of the Valley. It is an old-fashioned solution based on the idea that cedar is robust in all conditions, but it is wood -- it does not perform as well as modern roofing materials. Not to mention that forests are never far, and wooden roofs catch fire. Minimize or completely eliminate gutters unless there are foundation drainage issues in conflict.
3. Snow Breaks. These are expensive, let me tell you. They hold snow on the roof rather than allow it to slide off. The strategy here is to hold snow back from the soffits, where ice damming is most prevelant. This holds off the gravitational pressure of snow-ice sliding down the roof toward the soffits. The second reason for snow breaks is to prevent sudden 'calving' of large blocks of snow or ice...a safety consideration with some building designs. In general, the most elegant functional design would eliminate the need for snow breaks by positioning pedestrian areas away from roof driplines and ensuring that the roof is so cool and regulated that it does not experience ice damming.
4. Heat tape. For existing homes that have ice damming issues. This is the best solution. Heat tape elements come on at freezing temps and keep a drainage area open at all times. This does not stop the formation of large ice formations but it keeps drainage areas open to gutters or elsewhere, thus preventing water from backing up through the roofing material and into the house.
Why care? Most don't when they are looking at the kitchen or the master suite. What do they know? It is summer, they have no idea what the implications of roof design entail for winter, or for lifetime ownership costs. But they should, particularly in Davis and Canaan. We are specifically designing the site orientation, roof lines, gable locations, and porches to perform their best. Then we are offering water barriers, insulation solutions, and roof materials that will have our roofs looking smart in the dead of winter, and you will know you were smart when you get your energy bill. You will be feeling even smarter 20 years hence, when the roof is holding up better than your neighbor's.
The DC blizzards have presented more than the obvious inconvenient truth of global climate change. Increasingly, home buyers in DC and in the second home market will look at the cost of ownership, not just the pattern in the kitchen's granite. I think that when it comes to notions of luxury and comfort, buyers will start to consider that it is comfortable not to have soaring cathedral ceilings; it is luxurious not to be worrying about your gutters; and it is downright sumptuous to be cool on the second floor on an August afternoon -- without the AC on.
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