Snow on the Roof

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Since the lastest storm, I'm scared to death about the snow that has built up on my roof -- both at home and on the flat roof at my business. The roof hasn't caved in -- yet -- but I have 4 questions: 1) When should I be concerned? 2)Which roof is more dangerous? 3) What can I do about the snow? 4) How about those gutter melting wires?

Chicken Little, Hyde Park

Dear Chicken,
In the old days, before insulation in the attic, so much heat escaped through people's attics that the snow barely had time to land before it melted off. As insulation gets better, snow can build up deeper.

Snow weighs differing amounts, depending on its water density. Eight inches of light, cold, powdery snow will weigh much less than six inches of wet, warmer temperature snow. What can bring the roof down is either a heavy load of snow- or a medium load of snow which then gets rained on heavily. The snow can soak up the rain and their combined weight can go through the roof -- literally.

The more pitched a roof is, the less likely the snow will cause any damage. The pressure exerted down causes the snow to overcome its friction against the roofing and slide off down the roof. Hudson Valley roofs are often built steep by design. Some older houses have slate roofs which are incredibly heavy and also very slippery when wet so that the snow can slide easily off. A granular asphalt roof is much stickier and if combined with a lower pitch, may create a condition in which a weak supporting structure is overcome by heavy snow weight. Likewise, flat roofs must deal with snow weight through brute strength.

This brings us to building inspection. As much as many people complain about having to go through the expense and trouble of filing for a building permit, this inspection process will give an expert a chance to review the type, pitch and strength of your roof to see if it meets building code and can withstand heavy snow without caving in.  A well-designed roof that has not been compromised by wood decay or insect infestation should withstand almost anything Mother Nature can dish up.

If your house or business has been built according to code, you shouldn't have too much to worry about. However, if you have questions about the quality of your roof or if you have a low pitched roof (less than 30 degrees) or a flat roof and the snow is heavy, I would consider having it professionally shoveled.

Note: Don't try to shovel the roof yourself. The top surface of snow on your roof may be soft and fluffy, offering excellent traction, but the undersurface (where your boots will actually end up) may be wet or even icy. Conditions can also change radically from location to location on your roof, depending on how the sun strikes it and the quality of the insulation underneath. For these same reasons, anyone you hire to shovel your roof should be wearing OSHA (Occupational Safety & Health Act) fall protection gear. And be sure your homeowner's insurance is paid up.

You can also buy a plastic shovel on a telescoping pole that will allow you to shovel some roofs from the ground or from a ladder beside the roof. The other important criteria in shoveling a roof (beside not shoveling yourself off in the process) is to not damage the roof while removing snow. Agressive shoveling can remove the aggregate on asphalt shingle roofs, can crack the slate or tile rooves, and can tear a flat roof membrane.

Those criss-cross wires on the lower sections of people's roofs and the wires in gutters are there to prevent the condition known as icedamming. As a roof melts its snow off, snow may run down to a gutter which is frozen. This water may then back up, leaving a little ridge which then allows run-off water to pool and works it way backward underneath roofing shingles and down into the house. People address this problem by running an electric element through their gutters, which keeps the gutter from freezing up and ensures that the water will always have a passage off the roof. Using the criss-cross coils on the last feet of roofing will prevent any water from backing up underneath the shingles by keeping it all liquid and running downhill. These wires should be controlled by a thermostat.

Roofs do collapse and surpisingly, it's often steel or concrete flat roofs that do. They don't have the ability to withstand the short-term stress that wooden roofs can cope with. However, just to put this all in perspective, I checked with Don Westermeyer who is the building inspector of Hyde Park. During last winter, in and around Duchess County, several old barn roofs collapsed, an older trailer home roof collapsed, and the roof of an auto parts store fell in. According to Mr. Westermeyer, a roof in good repair that was built to modern code should be able to withstand the forty pound snow load per square foot that the Hudson River Valley is rated for. He did note, however, that roof damming did cause many interior water leak problems.