This saga began in the fall of 1989 when we added a 12'x18' sunroom on the rear (south) and a 6'x8'
entry hall on the front (north) of our 1952 Lachine bungalow. The project
included replacement of the asphalt shingles over the whole roof.|
One result of these extensions was the creation of roof valleys which had not previously existed. It later became apparent that severe ice damming took place in the winter at the eaves below the newly created valleys.
In order to minimize the ice damming I began to remove snow from the lower portion of the roof each time it snowed, using a "roof rake". This became somewhat tiresome, so I investigated the cause of ice damming and found that it is generally attributed to poor attic ventilation. Further investigation revealed that the soffits were solid plywood covered by vented aluminum, so there was in fact no attic ventilation, except in the newly built extensions. (This had not been a problem in the 37 years before the extensions were added, but there were previously no major valleys in the roof.)
To provide ventilation, one roofer suggested removing the shingles and the roof deck near the soffits in order to cut out the plywood, and replace all the shingles at a cost of about $6000. Another roofer suggested removing the aluminum soffits, cutting out the plywood and replacing the soffits at a cost of about $3000. I opted for the latter, which was done, but the ice damming continued, so I returned to roof raking.
In the summer of 2006 I noticed that the shingles on the south side of the house were severely curled and needed to be replaced (17 years after installation).
Curled shingles, worn from roof raking?
Quotations for replacement asphalt shingles ranged from about $5000 to $8000, but around this time a brochure arrived in the mail from Permanent Roofing Systems (PRS), a local supplier / installer of aluminum shingles.The attributes looked interesting:
50 year warranty
snow slides off by itself (no roof raking!)
So I called them and described my problems. Owner Paul Grizenko described the features of their aluminum shingles and gave the standard solution for ice damming – improved attic ventilation. His installer discovered that the ventilation provided by the previous roofer consisted simply of small holes cut in the plywood above the aluminum soffits. So improved ventilation, combined with the snow sliding of the roof by itself, seemed like the answer to my roof raking and ice damming problems. I therefore decided to invest in aluminum shingles at a cost of about $25000, some three to four times the cost of asphalt shingles. This included some non- standard work, including:
Install Grace Ice and Water Shield over the whole roof (this is usually suggested for the lower three feet over the soffits to prevent leakage if ice damming does occur, but I decided to go whole hog).
Install using the "high wind" option, with secured flashing at the roof edges and double nailing to give a wind resistance rating of 160 kph. (Asphalt shingle suppliers such as Building Products and Iko advertise a high wind installation option and I asked for it, but none of the installers who quoted asphalt shingles offered this option.)
Build a protective enclosure over the heat pump to prevent it from being damaged by snow falling off the roof.
Some shots of the final installation. The shingles are secured
by the flashing at the edge of the roof for resistance to strong winds.
The finished product looks similar to an asphalt shingle roof.
The initial sheen dulls quickly.
One of a number of snow avalanches that landed on the deck in 2007/2008,
and the snow pile shoveled onto the lawn.
At the front, the snow slides off the roof,
and lands on the front walk.
I'm not sure. When I'm out there shoveling yet another avalanche I say "Why did I do it?", but when I have had some time to get refreshed I say "Maybe it was not such a bad idea."
Perhaps a winter of normal snow fall will remove the doubt.
The 2008/2009 snowfall was more normal than the first year, so I had to shovel the front walk and rear deck less frequently. Ice damming also occurred, but less seriously than the first year. So I guess I can live with it (don't have much choice).
There is a product available called Enviroshake (www.enviroshake.com) which looks interesting. It consists of shingles with the appearance of weathered cedar shakes, but is made from recycled plastic, tires, etc. It has a 50 year non-prorated warranty and is more environmentally friendly than asphalt shingles which go to landfill every time they are replaced. I understand that the installed cost of the Enviroshake shingles is about double that of asphalt shingles (so less than aluminum), but they last 50 years so the long term cost is lower. If I were doing it again I would give this product serious consideration.
Some Ice Buildup Thoughts
Ice buildup occurs when the temperature at the snow / roof interface reaches the melting point, allowing water to run down to the eaves. If the temperature at the eaves is below freezing the water freezes when it reaches the eaves. If this happens repeatedly, the ice builds up to form a dam. Water then collects behind the dam and backs up under the shingles to leak through the roof unless prevented by a membrane such as Grace Ice and Water Shield.
Theory has it that a well-ventilated attic will keep the attic temperature the same as the outdoor temperature so the melt / freeze condition will not occur. For the attic temperature to be the same as the outdoor temperature, however, there has to be a flow of air from the eaves to the roof ventilators. Unless exhaust fans are installed, the only way for a flow to be created is for the air in the attic to be warmer than the air outside. This occurs when heat from the sun through the snow, or from the house through the insulation, heats the attic creating a flow of air to carry away the heat until an equilibrium condition is reached, in which the attic temperature is always warmer than the outside temperature.
Our attic has eight inches of fiberglass batt insulation between the joists plus six inches cross wise for a total depth of fourteen inches giving a theoretical R value in excess of forty. Even with this amount of insulation, our attic temperature in winter exceeds the outside temperature by two to ten degrees (as measured by a transmitting sensor in the attic and the outdoor sensor on our heat pump). The temperature difference reaches maximum in mid-afternoon and minimum in early morning, which implies that the attic heating is caused by the sun rather than by heat loss from the house to the attic. (On a sunny day in summer, the afternoon attic temperature exceeds the outside temperature by as much as twenty degrees.) Interestingly, wind does not seem to affect the attic temperature significantly.
When the outside temperature is in the range from zero to minus ten celsius, therefore, conditions are ripe for ice buildup and damming to occur. This temperature range might also be expected to cause the snow to slide off, but this usually occurs only when the outside temperature goes above freezing for some time, so significant ice dams can build up during a period of mild weather before the snow slides off or the temperature drops again.
An Ice Buildup Theory
Based on our experience that ice buildup has become worse as eave venting increased, here is a theory to explain this phenomenon: With closed soffits, the air in the soffit area is at approximately the same temperature as the air in the rest of the attic, so the temperature of the roof deck is similar throughout. Therefore if the roof becomes warm enough for snow to melt, it is at the same temperature over the eaves, so the water does not freeze as it reaches the eaves.
With open eaves, the temperature of the air in the area of the eaves is similar to the outdoor temperature, while the temperature in the attic above can be significantly warmer. Thus, if the roof over the attic becomes warm enough for the snow to melt, there is a good chance that the air over the eaves is below freezing, causing ice buildup.
To test this theory, the temperature sensor previously located half way up the roof has been re-located to the area just above the eaves. From past results, it is known that the attic temperature exceeds the ambient by up to 10 degrees (20 in summer). If the temperature above the eaves stays near ambient, the above theory may be substantiated.
With the advent of polyurethane foam insulation, unvented (closed) attics with foam insulation on the inside of the roof deck may better prevent ice damming, as well as reducing heat loss as compared to conventional insulation. A feature which I have not seen mentioned would be to make the eaves very small, so that any melting that does occur on the roof would have nowhere to freeze.
We have had very little snow this year, so only one set of avalanches to deal with, the day we came home from a cruise December 18.
Regardless, ice damming persists, as shown in the first two photos. The third photo shows our neighbour's roof the same day. Her roof configuration is very similar to ours, so why do we have ice damming and she does not?
We have temperature sensors in the attic and at the eave where the ice damming is most serious. As expected, the temperature at the eave stays very close to the outdoor temperature, whereas the attic temperature is always warmer, from 2 degrees on a cloudy day to 6 degrees on a sunny day; more in summer. The early morning attic temperature is at its minimum relative to the outdoor temperature, so the major heat buildup is not coming through the attic insulation.
Photo 3: Our neighbour's roof the same day.
Some internet references to unvented attics: (Some of these take a few minutes to load)
Unvented Roof Assemblies For All Climates
Insulating Unvented Attics with Spray Foam
Foam Insulation for Vented or Unvented Attic Spaces
Our neighbour had her roof re-shingled last summer, but we did not get a chance to determine whether her soffits are open or closed, so my icing theory remains a theory.
We have had only a few small snowfalls so far this winter, but each has been followed by a mild spell, so there have been half a dozen avalanches to shovel away, resulting in a fairly significant snow pile opposite the deck.
Last summer I contacted our roofing supplier, Permanent Roofing Systems (PRS) to ask if there is a way to prevent the avalanches. He said they had recently been using Siegar Snow Guards, which cost $30 each to install. Assuming ten units for each of the three problem valleys, I'm guessing about $1000.
Before I got around to having snow guards installed, we decided to sell the house to move to Toronto, where our families live. We are currently in the process of putting our house on the market, so I guess this saga will have to be continued by someone else.