Snow Load Redux

We dug this one out of the blog archives as it is super timely in dealing with the piles of snow left on our roof tops.snow image

Ice damming is the accumulation of snow and ice at the down-slope eaves that backs up water on the roof. Most steep roofs are designed to shed water, and they are not covered with waterproof membranes. When water backs up on these roofs, the roofs leak unless special precautions are taken.

Ice weighs approximately 57 pounds per cubic foot. The unit weight of snow on a roof can be close to that of ice or as low as 4 pounds per cubic foot. Most snow on roofs that has been compacted with warming and cooling cycles weighs between 10 and 25 pounds per cubic foot. Ice dams extending out beyond the roof edge and large icicles hanging from them create a tremendous load on the eaves. Structurally, it is difficult to design for this load. It is much more appropriate to design roofs so that such loads do not occur.
A combination of two effects creates ice dams at eaves in regions with high snow accumulation.

The largest effect is associated with heat moving from inside a warm building, through the ceiling, attic or roof joist assembly into the snow on its roof. Depending on the amount and type of insulation, the amount of ventilation, the depth of snow and the outside temperature, this heat may melt the snow.

If the base of the snow pack warms to 32ºF, snow will slowly melt. The water produced runs down the slope of the roof. If the outside temperature is cold enough, generally below 22ºF, the water refreezes when it reaches the cold eave of the roof.

The second effect is related to solar heating and night time cooling. It is a minor effect at lower elevations but can be a powerful consideration in mountain areas due to intense solar radiation and the thin mountain air at high altitudes.

Intense solar radiation penetrates the snow and melts some of it, even when the ambient temperature may be below 32ºF. If the melt water flows into a shaded area, it may freeze.
Snow will only hold about 2 to 5 percent water. As the water drops through the snow, a small portion of the water refreezes, releasing enough energy to heat the surrounding snow to 32°F. From a thermal stand point, it takes approximately one British Thermal Unit (BTU) of heat to raise the temperature of 1 pound of snow by .5ºF. It takes 144 BTUs to turn 1 pound of ice to water. Very little water will raise the temperature of snow to 32°F.

It often gets very hot on clear days in the mountains and very cold on clear nights. Melt water produced during such days may refreeze on eaves because of the drastic temperature drop from day to night.

Ice accumulation at the eave can happen quickly on a cold night if there is snow on the roof and excessive heat loss from the building into the roof system. Ice dams have long icicles hanging down from them. Water ponds up slope from the ice dam. This water is insulated from the cold by the snow above, and it receives heat from the building below. This heat often prevents the water from freezing behind the ice dam. The water is located above an exterior wall. In high altitude areas, the water behind the ice dam will freeze at night if the heat loss from the building is low.

Ice dams that accumulate over a longer period of time from minor heat loss from the building or the effects of high altitude look more like the ice dam shown in the next illustration. Water freezes at the outer edge of the ice dam and backs up the additional water flowing down the roof. When the water overflows the buildup at the edge, additional water freezes and builds the ice dam a little higher and farther out.

Water ponded behind an ice dam penetrates at head and side laps. The water often turns to ice beneath the roofing material on the cold eaves. As ice forms, it expands; this action can loosen fasteners. Ice under the roofing material becomes part of the ice dam. The overhanging ice mass may overturn, taking the roofing material with it. It may also overload the eaves, causing them to fail.

Super Insulated Roofs

Super insulated roofs have a thermal resistance rating of R-45 or greater and a vapor retarder on the warm side of the insulation. Generally these systems are not vented or are vented only enough to avoid creating a vapor trap in the attic or roof joist space. A super insulated roof greatly reduces, but does not stop, heat escaping the building. In some places, super insulation without ventilation is enough to eliminate chronic icing at eaves.

It is almost impossible with standard building materials and construction techniques to prevent some vapor and heat loss into an attic or roof joist space to prevent melting snow. Generally architects design more complicated buildings that require more than four walls. For these reasons providing good venting space between the insulation and the roof deck is always recommended as a means to carry off any escaping heat. A super insulated roof is not recommended at low elevations. Further, a super insulated roof will not work in high elevation applications due to high solar radiation and thin air.

Cold Roofs

Cold roofs are recommended for most roofs in snow regions that drain to cold eaves where ice dams form. Cold roofs are vented well enough to prevent snow on them from melting when the outside ambient temperature is 22ºF or colder. When it is warmer than 22ºF, outside melt water usually does not refreeze on cold eaves. The next illustration shows a cold roof.
Cold roofs at high elevations above sea level need more ventilation than roofs in other cold snowy regions. Intense solar radiation and thin mountain air also promote the buildup of ice dams. Europeans have used dual airway systems of venting in high mountain areas for many years. Substantial insulation and a continuous vapor retarder are important features of such roofs.

Intense solar radiation will melt the snow on the roof even though the ambient temperature is less then 20°F, causing ice dams on the roof. Intense solar radiation can also mean higher temperatures and no snow. Elevation and latitude have a combined effect of changing the sun’s intensity radiating to the roof. Regions farther north are affected during spring, whereas regions to the south are affected during late winter.

Higher elevation has the additional effect of creating colder days and particularly colder nights as there is less atmosphere to hold the heat on the ground. The radiant heat of the sun is greater at higher elevations, but so is the heat loss caused by the emissivity of the earth’s surface heat to the sky. At night, the temperature of a tile roof, or any object exposed to the clear sky, will be colder than the surrounding air temperature.

Ice dam research for high mountain areas is limited. Ice dam prevention is based on low elevation ice dam research and visual observations of many different roof systems in high mountain areas. Tight air and vapor retarders, heavy insulation and a ventilation system are needed to avoid ice damming in the mountains.

Cathedral ceilings where airways exist between solid joists are hardest to adequately vent. The use of a scissors truss can avoid individual airways, promote cross ventilation and provide a large area for insulation and venting.

Shaded areas of a roof, such as roofs exposed to the sun over most of its surface but shaded at the lower roof edge by trees, are problematic in mountain areas. Melt water refreezes in the shaded areas. In the same way, dormers and other roof surface interruptions cause shadows or differing amounts of solar radiation on the roof. Ice may also form in these areas. Observations show that the problems created by shaded areas can be reduced by increasing roof ventilation.

With a cold roof, when the sun goes behind a cloud, the snow on the roof is returned to the temperature of the surrounding air from above and to a lesser extent from below. Water in the snow refreezes in small crystals, creating what can be referred to as ripened snow. This ripened snow provides a flow path for future melt water to run off.

Cold Roof Concerns

The concerns typically encountered with unsuccessful cold roofs include:
1. The airway is too small or blocked.
2. Ventilation in the attic is constricted or blocked at the outside walls where the vertical distance between the roof and the ceiling is minimal.
3. Airway inlet is inadequate.
4. Air flow is blocked in rafter spaces by cathedral ceilings, hips, valleys and roof penetrations such as chimneys and skylights.
5. Screen mesh openings at the eave or ridge are too small or clogged with snow or debris and do not allow adequate free air movement.
6. Ridge ventilation is inadequate or placed so that it becomes obstructed with snow.
7. Mechanical equipment blows warm air onto the snow.
8. Roof penetrations add heat to the cold ventilation space.
9. Air exfiltration allows excessive building heat and moisture to enter the roof.
10. Heat losses from the building are excessive due to inadequate or poorly installed insulation.
11. At high elevations above sea level, high intensity solar radiation shines more directly onto south facing roofs. On those that drain to a shaded area, the melt water refreezes. Generally, a simple roof makes it easier to create a design that will require less maintenance in a mountain environment. Skylights, chimneys and dormers create shaded areas and are difficult to vent around.

Snow Movement

Steep roofs should be designed to prevent the snow from moving on the roof. On lower sloped roofs and in areas that do not experience much snow, the tile surfaces may be able to prevent snow movement. Snow guards (fences or brackets) must be considered. A recommended slope of between 4:12 (4 inches vertically to 12 inches horizontally) and 9:12 should be considered for a reasonable number of snow retention devices. Steeper roofs increase the need for snow retention. Some snow retention systems are under designed, and snow rips them from the roof.

Consult a structural engineer or snow guard manufacturer for anchor placement and design. If insufficient snow retention devices are used, the snow may rip them from the roof.
Allowing snow to slide off a roof requires special considerations and is not recommended. Snow does not come off a roof in a controlled fashion. Variable weather conditions before, during and after a snow fall prevent predictable snow slides. Dormers, waste stacks and other roof plane breaks are difficult, if not impossible, to design around. The impact zone of falling snow prevents a good building design from incorporating trees or shrubs in the area. The buildup of snow along a building can deteriorate siding and direct melt water into the building. Pedestrian walkways, driveways, lower roofs, gas meters and garage entrances can be adversely affected by sliding snow.

Low-Slope Roofs

Low-slope roofs that drain to an eave or downspout on the exterior of a building are exposed to the cold and are prone to freezing. This can cause extensive areas of ponding and increase the chance for leaks. They are a contributing factor to many roof collapses. The roofs are designed to carry heavy wet snow and some ice load. Ice blocking the scupper or eave and backing up water may overload the roof. In addition, external drains ice up and overflow onto building siding, causing additional maintenance problems.

Low-slope roofs should drain the melt water to a low area in the roof located within the heated walls of the building. A drain should be installed and heated by the building’s heat, ensuring the drain will always be open to carry off water.

Condensation on cold interior roof drain piping can be a problem in a warm attic area. For this reason it is best to use drain piping which is wrapped with appropriate vapor sealing insulation on the warm side of the insulation. A plastic roof drain pipe may provide enough insulation to prevent condensation if the relative humidity in the attic is not too high. Consult with local building officials to determine if plastic pipe such as PVC or ABS is allowed.

As shown in the next illustration, low-slope, internally drained roofs can be used to keep water from a steep roof from reaching cold eaves. The low-slope roof acts as a heated gutter to drain off the water. The vertical offset between the two roofs should be close enough together that snow buildup will bridge the gap between the roofs and prevent ice dams from forming. The steep roof will usually require snow retention devices to prevent snow movement or even snow creep. Parapet walls in this type of design have been broken due to snow creep.

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