Roof Snow Load Concerns in Fawnskin


Above: Roof Snow Load Example

How do you know if your roof is in danger of failing under a snow load?

Recently I’ve been asked (several times) just how much snow is too much snow accumulation on a roof.

Beats me!

Probably the best thing to do is check with the local building department but I did some research on the topic and promised to report back so here are my findings.

Two main variables influence how much snow on the roof is too much.

  1. Where you live and the building code requirements is one major influence. Obviously if you live in a state that regularly gets a lot of snow, the requirements are in alignment with that unique requirement, but here in Southern California it can be a bit tricky since heavy snow fall has not been a problem in a long while. Also the type of wood used to build a roof is an influencing factor.
  2. Next, the other influence is the actual weight of the snow–which can vary greatly.

The weight of snow is estimated at pounds per square foot (or cubic foot).

Dry snow that is light and powdery might weigh about seven pounds per square foot while some of the sources I found estimate that the average snow weighs in at 15-20 pounds per square foot.

Then you have complicating factors to take into consideration such as compacted snow (or ice) and drift snow that may exceed 20 pounds per square foot.

Drift build ups come from surrounding buildings or trees and any multiple level roofs that can accumulate deep snow drifts, too.

How much snow weight can most roofs take?

Snow loads for many buildings in snow prone states are estimated to be about 20 lb per square foot but that figure does not take into account the weight of the actual wood used in roof support, nor does it take into account other factors.

How much does roof snow weigh?

The general rule of thumb is five point two pounds per square foot for a one inch layer of water or ice.

Most locals know that 12 inches of snow is equivalent to one inch of water.

BUT wet snow weighs more and so that means the load that could be withstood is less AND although the roof might be able to support the maximum snow load for a period of time (few days to a few week) wood can fail under prolonged load bearing.

What is a safe amount of snow to have on your roof over an extended period of time?

From what I read, a safe assumption would be to estimate about half of the estimated 20 pound design load of 20 pounds per square foot.

This translates into about two feet of snow, or one inch of ice and one foot of snow.

Dr. Jarl VonDevender and Doug Petty of the University of Arkansas Division of Agriculture Cooperative Extension Service wrote a snow load analysis and published this in March of 2006:

The water content of snow may range from 3% for very dry snow to 33% for a wet, heavy snow, to nearly 100% for ice. An inch of water depth weighs 5.2 lbs. per square foot. Thus, a roof designed to carry a snow load of 20 lbs. per horizontal square foot is expected to support nearly 12 inches of wet, heavy snow.”

In all likelihood, your roof is probably designed for a snow load of 20 lbs. per horizontal square foot. But keep in mind that poor materials and poor building construction methods can contribute to a lower rating. Ice is the real danger as it weighs nearly the same as liquid water. A cubic foot of solid ice weighs 92% of what a cubic foot of liquid water. This means that your roof can only support a few inches of ice.

Roof dangers and considerations to ponder in snow season include:

  • A roof pitch of 3/12 (or less) tends to prevent snow or ice from sliding off,
  • wind blown snow can create more snowdrift accumulation and uneven snow load,
  • porch roofs and other low roofs accumulate additional build up as snow or ice sliding off roofs above them,
  • shingle roofs (or other materials) that snow clings to don’t drop the accumulation as easily as metal roofs,
  • roof valleys can collect a lot of snow.

When removing snow from the roof, keep it mind that

  • shingles maybe brittle during cold temps,
  • dislodging the pebbles from the surface of the shingles can shorten the roof life,
  • any build up might be slippery and dangerous if you climb onto the roof to remove snow.

What do you do if you have too much snow on your roof?

Hopefully you will remove as soon as the weather clears. Most experts indicated that you do have some time between a large snowfall and possible structural failure.

Ways to remove snow include:

  • hiring someone to clear it,
  • raking the roof (taking care not to damage the exterior roofing materials),
  • adding heated conduits that help to melt the snow so it slides off.

So, how do you make the decision to shovel off a roof?

Visually inspect the roof line, rafters and trusses. If they are bending or sagging downward or flexing one way or the other, your roof may be compromised.

I’d say it is better to be safe than sorry!



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This entry was posted on Friday, February 12th, 2010 and is filed under Mountain Lake Resort.

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