Changing from asphalt shingles to metal

The winter surprise

Metal roofs are much better products compared to asphalt shingles when considering durability, performance, and resistance to the weather.  However, one aspect of metal roofs that often surprise new owners is the difference in behaviour during the winter, if the metal is a smooth metal like painted steel, painted aluminum, or native copper.

Depending on the roof slope, snow can accumulate on smooth metal roofs, and depending on the amount of heat escaping through the attic, can melt and then freeze onto the roof, just like it can freeze onto your car in winter.  And just as on your car, it can really hold on to the roof surface.  If the weather conditions are right, the metal roofs can often hold on to the snow cover for a while.  Being a good insulator, snow tends to trap the heat which works it way to the roof through insufficient insulation and ventilation, and gradually contribute to a growing crust of ice underneath the snow cover.  Eventually, it may become warm enough that there is sufficient melting of the ice at the roof surface, to allow gravity to pull the snow/ice mass, overcoming the surface friction.  At this point, an avalanche happens, and because of the ice crust that has grown underneath the snow, a large portion of the snow mass slides down.

Since snow weighs about 40 lbs per cubic foot (more if there’s ice), and given that we often see accumulations of about 6 inches in the Montreal area, we’re often dealing with a mass of snow and ice weighing about 2 to 4 tons depending on the roof surface area.  When that mass falls from a height of two stories or more, the effect on the things below can be devastating.  Also at danger are  gutters (eavestroughs) if they are in the path of the avalanche.  The eavestrough installers do NOT guarantee their work on roofs with smooth metal roofs.

People try to protect life and property from such damage by installing snow-fences and/or snow stoppers.  These work reasonably well, as long as they are installed properly (steel bolts fastened directly into the roof joists, at proper intervals and the right positon).  Of course, proper installation is more often NOT done, so we’ve seen snow stoppers torn off roofs (along with attendant roof damage).  Even snow fences have been damaged if they happen to lie in the path of a large mass of snow.

If the homes are relatively close together, and the roof slopes towards the neighbour’s house, another form of damage happens when the snow avalanche shoots across the space separating the homes and collides with the neighbour’s siding, or windows, or other property.  Your new shiny metal roof certainly makes an impression on the neighbours, but not the good kind.

Another interesting phenomenon happens when the garage roof slopes down towards the driveway.  Snow will often slide down at the end of the day (when it gets the warmest), and the soft wet snow will freeze overnight.  If you happened to park the car in the garage, you will get your full aerobic and resistance exercise the next morning, breaking up the frozen mass in front of your garage doors.  Just remember to do it early enough to get to work on time.

On the other hand, there are situations where smooth metal roofs are the perfect product to have, especially if you have a cottage which you visit only occasionally during winter.  With the smooth metal, the snow will sooner or later come off the roof, and if there is nothing below to damage and no-one to hurt, then this avalanche of snow is a good thing as it reduces the need to go to the cottage to remove the snow off the roof.

So if a smooth metal roof can create a danger during the winter, what alternatives do you have?  One very popular product that we install is the granulated steel shingle – which works like a metal roof, but holds snow as an asphalt roof would.  The product belongs to a whole family of products which come in various forms – resembling asphalt shingles, or clay tiles or wood shakes.  These come with 50-year warranties (on the product), and are excellent performers (when properly installed).

Therefore, if you are considering switching to metal, think carefully about how the behaviour of the roof will change, and whether a smooth roof surface or a granulated surface is the best for your situation.

Determining the appropriate product for one’s specific circumstance usually requires a consultation, as the product’s interaction with the rest of the roof system should also be considered.   Ice dams can form on metal roofs just as easily as they do on regular roofs, if the underlying insulation and ventilation are not sufficient.  The problem becomes severe once the weather warms up enough to allow the ice to lose its grip on the roof surface, and the ice mass hurtles off the roof.  The insurance company will usually pay to repair the damage ONCE.  After that, you either need to change the roof covering to something that will not allow such behaviour, or they will refuse to insure the property.  And how do we know this?  We’ve been hired to fix exactly these kinds of situations.

If you want to change from asphalt shingles to metal, you will be improving the performance of your roof.  Just remember to choose the right product for your situation.  If you’re not sure, call us for a consultation.

(c) 2016 Paul Grizenko



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