It’s Snow Joke!


Snow AccumulationIt’s that time of the year
again, you know it’s coming, that 4-letter word starting with ‘S’.

Snow, and particularly accumulation of snow, can affect buildings in many negative ways.

Heavy accumulations of snow can cause roofs to collapse.  Snow melting and re-freezing can create ice and cause ice dams which can result in moisture intrusion and damage to shingles and over-flashings.  This moisture can lead to structural concerns.

Snow can slide from sloped roofs, and ice-dams can break-off and fall onto people and property below and in extreme cases cause impassable exit doors from the property.

However, while many people look up for the problems that might be associated with snow, as Certified Home Inspectors we are taught to look in other places.Gas Meter in snow

One area of concern is the heavy accumulation of snow that can surround the gas meter to any building.

The bottom portions of the gas meter has a vent which allows the safe release of excess gas pressure to the atmosphere rather than permitting an excess pressure condition at any gas appliance in or on the building. 

This excess pressure can create a build-up of unburned fuel which could ignite, or lead to poor combustion, which in turn leads to a build-up of carbon monoxide.

Another issue at ground level is when Water Heaters, or Furnace vent pipes and exhausts come out near grade level from the basement.  This again can lead to either carbon monoxide build-up or a shut down of the appliance when automatic safety features are triggered by the inability of the appliance to “breath”.

Turning our attention back to the roof, the reason why we need to be conscious of the level of snow that falls on a roof, is the weight that snow carries with it.  Roofs that have been built to modern day codes are supposed to be able to withstand the dead-load pressures of snow from a usual winter.  But these codes are based upon the “usual” winter and balanced over the “average” roof locations.

Roof collapse from snowMost urban homes have wind eddies that remove snow from the roof, and thus limiting the level of damage that might occur.  In suburban and rural areas other things come into place that might affect the amount of snow that accumulates on the roof of your home.  It is this accumulation that can create serious damage if it is heavier than the design loads supported by the roof framing.

Snow doesn’t seem to be that heavy when you look at it closely, but the National Research Council of Canada estimates that 1″ of “regular” snow equals about 1 – 1 1/2 pounds per square foot of weight. So even a foot of fresh snow adds only 12-18 pounds per square foot to the roof.

If the snow is wet snow, this can be considerably more, and if the snow turns to ice, then more still.  A cubic foot of water weights around 62 pounds, so an ice dam can place a load on the roof some 4-5 times heavier than “regular” snow.

To avoid any potential problems you may want to remove the snow now. It is important that if you do this, you follow two very important rules:

  1. Ensure that if you are going to remove snow from the roof, you do it safely.
  2. To avoid large unbalanced snow loads, do not remove all the snow from one side of a gable roof, but take snow off from both sides gradually.

It is important that snow removal be carried out in a careful and safe manner. In 1995 in the Ottawa area one man was killed when his ladder slipped while he was clearing snow from his roof. Also many hospitals each year report numerous injuries from people clearing snow from their roofs.

Removing snow from one side of the roof can place huge forces on parts of, not just the roof frame, but the house frame, that it was not engineered for, in fact according to engineering reports, this load can be as high as 50% more for an unbalanced load on the roof.

This is also the case for if your house has a flat roof below the main slope.  Snow can fall from the main roof and create an instant concentrated load on the flat roof below, causing the flat roof to collapse.

When it comes to clearing the snow from your roof, try to use a long handled, or extending handled rake with the tines of the rake covered with cloth so it sticks to the snow, but doesn’t damage the shingles.

This is much safer than going up on the roof and shoveling the snow off with a broom, which should be done as a last resort, and always with safety harnesses and ties for when you slip.  Never use a shovel on the roof or you will damage the shingles.

6mil polythene sheetingA better idea is to prepare your roof for winter prior to the first snow fall, so that it becomes slick and the snow won’t stick to it.  Place sheets of polythene over the roof slope prior to the snow fall, tacking it at the top and bottom of the slope of the roof so the snow will slide right off when the weight gets too much.

Remember, make sure that as an when the snow comes off the roof, it’s going to fall down, just make sure that when it falls there is nothing important underneath it, like gas meters, air vents, furnace and water heater exhausts or people.

Not Clever!And if you have to go up a ladder to reach the roof, just remember, snow is heavier than you think, and an avalanche from a roof can be just as devastating as an avalanche down a mountain, if you are standing at the top of a ladder at the time it all decides to slip.

Have a safe winter!

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