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Heavy Accumulations Of Snow On Roofs Are Hazardous

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With heavy accumulations of snow on roofs there is likely to be a a large amount of snow building up on roofs which can also create ice dams.
Given this potential, we urge our clients to keep track of how much snow is piling up on their roofs during the storm, and then keep track of these same areas in case ice dams begin to form as the snow begins to melt.

Warning signs of Heavy Accumulations of snow on roofs

heavy snowfall collapses roofTo identify potential structural overloading and associated hazards during and immediately after the snow, some symptoms may include (but are not limited) to the following:

  • Cracks in the ceiling or walls that have just appeared.
  • Squeaking or groaning from walls or your attic that are a result of lumber under strain and rubbing against nails or other lumber.
  • Water stains in the ceilings or walls.

If you notice any of these issues at your home or office, we recommend that you leave the property immediately, and if you are in a multi level property, notify the people adjacent to and below you as well. Then when it’s safe, you should consult Kipp Gaynor from the Structural Rehabilitation Group, a local Structural Engineer at 301-300-8700. Or visit their web site at: http://SRG-LLC.net

How much snow will your roof hold safely?

Snow Drift Loads on roofsUnfortunately the minimum weight load codes in Maryland are not as high as you would expect. Plus with high wind and drifting snow most of the roofs will have unbalanced roof loads, as shown in this photo.

Here are some specifications that I found and links to them that I thought were a little alarming:

Montgomery County Maryland

In Montgomery County Maryland Residential homes can vary between 20-25 Per Square Foot depending on the roof area and pitch of the roof. Commercial minimum design loads are 30 Per Square Foot.
1608.3 Flat roof snow loads.
The flat roof snow load, pf , on a roof with a slope equal to or less than 5 degrees (0.09 rad) (1 inch per foot = 4.76 degrees) shall be calculated in accordance with Section 7.3 of ASCE 7.1608.4 Sloped roof snow loads.
The snow load, ps, on a roof with a slope greater than 5 degrees (0.09 rad) (1 inch per foot = 4.76 degrees) shall be calculated in accordance with Section 7.4 of ASCE 7.1608.5 Partial loading.
The effect of not having the balanced snow load over the entire loaded roof area shall be analyzed in accordance with Section 7.5 of ASCE 7.1608.6 Unbalanced snow loads.
Unbalanced roof snow loads shall be determined in accordance with Section 7.6 of ASCE 7. Winds from all directions shall be accounted for when establishing unbalanced snow loads.
Visit this site for some photos and an explanation of the ASCE 7-05 specifications on roof snow loads, unbalanced snow loads and sloped roof snow loads.

Here are some specifications on roof loads in other areas in Maryland

  • Federalsburg Maryland

    1607.12 Minimum Roof Live Loads. Roofs shall be designed for a minimum live load of 20 pounds per square foot or designed for the minimum snow load, whichever is greater.

  • Baltimore County Minimum snow load is 30 pounds per square feet
  • Queen Annes County
    1607.11.2 Minimum roof live loads. Add new paragraph to existing subsection as follows: “Roofs shall be designed for a minimum live load of 30 pounds per square foot or designed for the minimum snow load, whichever is greater.”
  • Prince Georges County The minimum snow load for roof design is 25 pounds per square foot.

Now here is a quick calculation that explains why property owners in Maryland should be concerned about heavy accumulations of snow on roofs.Quick Snow Load calculation

This calculation is based on a 25% moisture density which may be conservative for the current snow fall. As a rule of thumb, saturated snow weighs approximately 20 pounds per cubic foot. The moisture content of snow can range from approximately 1% to 33%, which relates to snow potentially weighing from 1 pound per cubic foot to over 21 pounds per cubic foot for really “wet” snow.

Calculation: Snow x 1.25 = P where:

Snow = Inches of snow on the roof (depth)
1.25 = Weight of 1 sq ft of snow for each 1-inch of depth
P = Pounds per square foot (lbs/sq ft)

So if the snow on the roof is only 20-inches deep it could weight up to 25 lbs per sq ft of snow, which is pretty close to Maryland’s minimum roof load!

This isn’t the only concern you should have about heavy accumulations of snow on roofs either. After the storm passes, the next thing you need to be on the lookout for is ice dams forming on the roof.

What Is An Ice Dam?

An ice dam is a buildup of water that is formed from Heavy Accumulations Of Snow On Roofs as it melts, then runs down the slope of the roof and then forms into ice near the outside wall of the house. This is usually at the soffits where there is slightly cooler air because its located past the heated areas of the home, but where cooler air is drawn into the attic from the vents in the soffit. It may only be a difference of 1 Degree but if temperatures are low enough, that’s all that’s needed for the water to freeze. Through constant fluctuations in temperature, a wall of ice is formed across the roof and water begins to build up behind it. Once the pond of water behind the ice dam is deep enough, it starts to back flow into the exterior walls of the house. Ice dams are usually formed very slowly, and it usually requires Heavy Accumulations Of Snow On Roofs for them to form over a day or two. Then the water that is leaking into the walls starts to become noticeable. When this happens it can cause water damage to the walls and ceilings of every floor of the house below it, so you should contact us for snow and ice dam removal.

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