Winter weather will be upon us before you know it, and with it will be the stresses on buildings, infrastructure and roofs. Whether you’re a recent transplant or have been in Canada all your life, brushing up on the basics of winter loads ahead of the coming storms is sure to be smart.
A Matter of Live and Dead (Loads)
Structures are engineered to handle two primary types of loads: Live loads and dead loads.
Dead loads are the continuous, resting loads of the building – the weight of the structure and everything in it.
Live loads are the variable, temporary forces that a structure must resist. These include wind, rain, snow and ice. According to the National Research Council of Canada, a roof should be able to handle 21 pounds per square foot (22 in Alberta) of additional snow and ice live load.
The wide variety of climates in Canada makes for a similarly wide variety of snowfall amounts and consistencies across the nation. The range of snowfall weight – and therefore its strain on a roof – can be surprising. Snow weight depends on two primary factors. First, the amount of snow obviously determines the amount of weight on a roof. But even more important is the water content of that snow.
Powdery to Packed
While a foot of light, airy snow can weigh just a few pounds per square foot, wetter, packed snow can weigh nearly ten times as much. That means a few inches of packed, wet snow puts as much strain on your roof as many feet of light snow. But a single inch of ice can weigh a staggering 5 pounds per square foot. When you measure the accumulation on your roof, it’s crucial to be aware of how wet, packed and icy the snow is.
It’s also important to understand that your roof is an ever-changing, dynamic place. Snow typically doesn’t settle on it evenly in all places and stack up nicely. Different roof details, angles and wind patterns might result in a roof with a foot of snow in most areas, but a large, packed drift in one area. That uneven live load could be the straw that breaks your ridgeline’s back.
Ice Dam Dangers
If the roof itself is warmer than the air around your roof – due to ventilation leaks or other thermal breaks such as chimneys or exhaust – this can cause snowmelt on the roof itself. Melted snow runs down the roof to a colder area and re-freezes. This process can repeat itself, forming the dreaded ice dam.
The ice dam’s rim of ice traps water and snowmelt behind it, creating tremendous extra weight and forcing water up through any imperfections or gaps in your roof – even pinhole-sized ones. This can cause extensive water damage to ceilings, walls, insulation and building contents. It may sound counter-intuitive, but keeping your roof cold helps prevent ice dams from developing.
To accomplish this, you need continuous, high-performing insulation keeping your attic or roof cool. “Robertson has our own patented Robertson Thermal System to meet even more stringent thermal requirements,” explained Lou DiNardo, Robertson technical review manager. “In addition to conventional insulation, we have insulated metal panels that are very popular and capable to meet thermal requirements.” Both of these systems not only protect against the formation of ice dams, but also provide superior year-round thermal performance.
Snowfall to Snow Fall
All that snow on the roof has to come down one way or another. If conditions allow it to slide off on its own, this can be dangerous. Hundreds of pounds of snow sliding at once can shear off rooftop equipment and chimneys. Worse still, it could slide off onto someone, causing serious injury. Robertson’s ColorGard and X-Gard systems may require one or two rows of snow guards, depending on the amount of snow accumulation. These systems not only help protect people and surroundings but also keep eavestroughs from being torn off the eaves.
An Ounce of Prevention
Much of the prevention of dangerous rooftop winter loads is a matter of good design. Roofline shapes in relation to prevailing winds can drastically affect snow accumulation. “We design our buildings to have capacity for the snow loads per the building location,” said DiNardo. “However, roof slope and geometry can help mitigate the loads.” Complex rooflines accumulate more snow than simpler roofs like straight, single-ridge gable roofs, which shed snow and ice more quickly.
Wind can create drifts around dormers, valleys and chimneys. But a row of evergreen trees planted on the windward side of the building can create a windbreak. Pitch and simplicity also play an important role. “By increasing the roof pitch and considering the eave height based on any adjacent structures, we can best determine how to manage snow loads,” DiNardo said.
A Pound of Cure
While codes, loads and structure design are all intended to make buildings that can stand up to the elements, sometimes storms throw more at them than they can handle. Sometimes we inherit a building design that perhaps isn’t ideal for winter storms. In these cases, our roofs need our help.
Gradual shedding of snow and ice is the best approach. Roof rakes are a good solution to aid in this shedding but be sure to choose a rake with rollers or bumpers on the bottom to avoid damaging the roof. Shoveling is another solution but is hazardous and best left to professionals. Be sure shovelers don’t leave footprints of packed snow on the roof – these are a recipe for ice dam creation.
Another smart idea is to install heat cables in gutters that don’t drain well – or those that are prone to freezing up and blocking drainage. These can help move damaging water off the roof safely.
Best of Luck!
We hope these tips help keep you and your structures warm, safe and happy this winter. If we can help with anything at all, please don’t hesitate to reach out to your Robertson representative.