Snow Avalanches from Metal Roofs (and what to do about them).

The problem

Metal roofs with smooth surfaces (native metal like copper or zinc, or painted metal) are generally quite slippery when wet.  In winter, under the right conditions, this allows accumulated snow (and ice) to avalanche off roofs.  If the fall area has no traffic or things that can get damaged, that’s probably a good thing, as there is less weight on the roof.  If the snow ends up hitting people (your kids, your postman/woman, your visiting friends), or sensitive property (your car, your expensive BBQ, your heat pump, your eavestroughs/gutters), then that’s not a good thing.  Unfortunately, if the snow control methods were not part of the original roof design and installation, then intervening after the roof is installed is often an area ripe with unintended consequences.

Snow control mechanisms

There are at least five ways that control over snow avalanches can be achieved.

  1. Snow stoppers
  2. Snow fences
  3. Snow deflectors
  4. Non-slippery surface
  5. Manual snow removal

Let’s review what these look like.

1- Snow Stoppers

These devices are manufactured in a variety of forms, materials, and methods on installation.  Their primary purpose is to hold the snow in place and reduce the chances of an avalanche.  Their ability to do their job depends on proper selection, proper placement, sufficient numbers, and proper installation.  Furthermore, if they are added after-the-fact, it is quite common to see various unintended consequences, such as leakage, creation of ice dams where there were none before,  damage to the roof surface through mechanical  shear, galvanic corrosion, or damage to the roof structure itself.  In terms of effectiveness, they are not as effective as snow fences (see section following), but they are often perceived as being more aesthetically pleasing.


2 – Snow Fences

These are found often on commercial or institutional properties where the possibility of falling snow is a danger to everyone around the structure.  They are most commonly manufactured from various forms of iron, and consist of a series of vertical ribs that support one or more horizontal retaining rods.  Given that they are responsible for holding back a considerable mass of snow (and ice), these need to be properly engineered and installed.  Most people find them rather ugly for residential application, but there are circumstances where they are the only reliable way to prevent damage from snow avalanches.


3 – Snow Deflectors

These take the form of various structures whose purpose is to deflect the snow so that it falls in areas that are not dangerous to those below.  An example of a deflector is a peaked porch overhang that directs the snow to the two sides of an entrance.  Sometimes, the deflectors take the shape of a strip of metal that guides the snow to fall in “safe” areas.  These are almost never suitable for after-the-fact installation, unless one is considering rebuilding the roof system.

4 – Non-slippery surface

Another way to reduce or eliminate the risk of snow avalanches, is to replace the smooth and slippery surface by one which is neither smooth nor slippery.  This generally is done as a last resort, when other forms of snow control were not successful.  For example, we have removed aluminum shingles and replaced them with granulated-steel shingles in several situations where the homeowners were threatened with loss of insurance coverage due to persistent instances of damaging snow avalanches.  In general when we see thse kinds of situations, it is because the wrong materials were chosen for the specific installation.  This is one reason why, when asked to recommend a roofing product, we would first do an on-site inspection to determine what risks would be present if a smooth surface material is installed at that location.  If it is clear that there will be a persistent danger of snow avalanches, then we would recommend a non-slippery surface material.

5 – Manual Snow Removal

Sometimes, the least expensive way of preventing snow avalanches, is to pro-actively remove the snow from the susceptible areas with tools such as the snow rake.  This can be done IF the roof surface is close enough to the ground, and IF the person doing this snow removal is in a safe place (because being buried in a snow avalanche that you initiate is not considered good practice).  This approach is usually an interim step, before a more permanent solution is found.  It also has the risk of damaging the roof surface by scratching or abrading the roof surface with the snow rake.

Unintended Consequences

The unintended consequences arise from two main reasons:

  • Snow is heavy
  • Secure anchorage of the snow devices requires piercing the roof surface.

Snow, that light and fluffy stuff that is so pretty on branches, and is a white yet scintillating blanket on our landscape, weighs about 40 lbs per cubic foot when dry, and up to 55-60 lbs. if wet or icy.  If you have a roof surface that is (say) 20 ft. from eave to ridge, and you have a snow depth of 6″, then one foot of horizontal exposure has about 400-600 pounds of mass behind it.  The snow device you choose has to resist that force, and it does that (usually) by having the device anchored with steel bolts into the roofing supports that can resist the shear or pull-out forces.  If you install into roof sheathing, there is an excellent chance that the snow device will be ripped out, with damage to the roof structure, and to whatever was below.

The other issue is that to fasten the snow control device to the roof, you have to pierce the surface with the steel bolts mentioned in the previous paragraph.  This now becomes an entry point for water, and no matter how well the device is caulked from the outside, the mechanical stresses on the snow device risk breaking the waterproof seal that the caulking is supposed to create.  This means that the secondary line of defense against water entry is the presence of a waterproof layer UNDER the roof surface.

Depending on the installation practices of the company that installed the metal roof, this layer may be non-existent, or non-functional.  Many installers of metal roofing install their product OVER the existing roofing, and thereby render any waterproofing method essentially ineffective.  If snow control devices need to be installed on such an installation, then the only truly reliable way to accomplish this is to remove the metal roof, strip the old roofing down to the roof deck, reinforce the roof deck (if necessary, this will depend on the type, age, and condition of the roof sheathing), install a proper water-proofing membrane onto the roof deck, and reinstall the metal roofing.  Now the snow control devices can be installed, with the knowledge that if water gets past the caulking on the roof surface, it will still be blocked from entering the roof structure by the water-proofing membrane.

Doing it right, the first time.

If the companies selling metal roof solutions are conscientious and honest, they would ensure that they would evaluate each installation for the avalanche risk before quoting a roof installation.  Unfortunately, this is something that many salespeople have little experience with, and often lack the necessary analytical approach to properly assess the risk and the necessary steps to include in the quotation as a solution.

Metal roofs are a wonderful product, when properly chosen, designed and installed.  But to make an informed choice, you have to know the type of things that can go wrong, and you need to know what steps should be taken to reduce the risks.  If you are uncertain of how to proceed, call me for a consultation, hopefully BEFORE you start experiencing the adrenalin rush that comes from escaping from a snow avalanche.

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