The following newsletter, directed to WETT wood heating professionals, provides some very sound advice regarding snow loads and chimney bracing.
Advice for avoiding damage from ice and snow
(WETT) Wood Energy Technology Transfer "FireWire" Newsletter, Dec. 2018
If you install in areas where heavy snow loads can be common, this article offers solutions you can use to reduce the likelihood of damage due to sliding snow & ice.
It is basic human nature: we equate water as being heavy, but snow as being light. Too often, people underestimate the weight of wet snow and they fail to understand the potential damage that could be caused if it were to slide off a roof all at once. Those who have witnessed this phenomenon are amazed by the noise, velocity and sheer force on impact. From inside the home, occupants who have experienced it report hearing a soft “whooshing” sound that quickly amplified to a thunderous, rumbling roar ending with a loud, window-shaking “WHUMPH!!!”
These events are regular occurrences in snow country. The steep pitch of the roof and selection of slick, metal cladding are often built right into a home’s design to facilitate it. Old-timers take these occurrences in stride because they expect snow & ice to slide off the roof from time to time. They plan for it by ensuring that nothing of value is left beneath the eaves that could get damaged by the weight of falling snow or jagged chunks of ice coming down off the roof. And they certainly would not even think about placing a chimney within this frozen freight train’s pathway down the roof – would they?? Unfortunately, it does happen a lot more than it should. The inexperienced think that installing a roof brace kit on the chimney might offer adequate protection, or that a snow-break placed just above the chimney will shield their investment.
There are problems with both approaches.
First; roof braces are not intended to protect against sliding snow & ice — they are actually intended to stabilize the top of the stack against the effects of wind.
Secondly; while some snow-breaks might work with new snow, these same designs do not work well with ice or hard-packed snow. Instead, the snow-break acts as a jumping ramp that launches accumulations directly onto the chimney! This is because corrugations built into the metal roof cladding can hold sliding snow & ice in place just as if it were on tracks. In these instances, the ice simply rides up over the snow break, falls back in place, then carries on its merry way. Another common defect with the snow-break is that it is only as strong as the anchor screws holding it to the roof. If screws are only biting into sheet metal or an underlying layer of ½” plywood, it can be yanked easily out of place by the sheering force of sliding accumulations. Heavy layers of ice can also build up around the chimney or around brace anchors. If left alone, the accumulating weight can slowly drag the chimney over and down.
Disagree? Industry veterans will have plenty of stories to share about homeowners that came in looking for bits and pieces to patch up a chimney that had been wiped-out by sliding snow and ice. Many of these folks did have snow-breaks, “crickets” or other devices in place to no avail. If you listen closely for a pattern in these stories, you will find that rapidly warming weather conditions frequently played a part. During major winter storms, moisture-laden clouds and warming temperatures can quickly dump large quantities of snow. As temperatures rise, the snow can shift to rain. The snow continues to absorb moisture until a critical point is met and then the whole works come sliding down the roof all at once. The other typical pattern can occur slowly over days or even weeks. During sunny days, radiant heat from the sun melts snow on the roof even though the outdoor ambient temperature is well below zero. The meltwater trickles down through accumulated snow then freezes as it hits the cold metal roof. As the days pass by, this layer of ice can continue to grow in both thickness and weight. Sooner or later, it will surely come down. A really warm day might be all it takes to trigger this dangerous chain of events.
Now that we are more aware of the problem, how can we deal with it effectively? Here are three solutions to consider:
Solution #1: Regularly remove accumulations of snow and ice before they can become a problem. This solution will require someone to climb up on the roof with a shovel. Of course, this brings its own set of potential problems. If going with this route, precautions must be taken to prevent slip and fall hazards. This is not necessarily a convenient, reliable, or a safe approach.
Solution #2: Install hardware that has been properly designed to hold snow and ice in place until it can gradually melt or be physically removed. The key is to keep accumulation from sliding as, once it begins to move and build momentum, all bets are off. There are many commercial systems available that have been properly engineered for this purpose. “Snow fences,” snow rails, or clusters of strategically placed “snow jacks” can provide a good level of protection if installed according to a manufacturer’s instructions. Keep in mind, however, that these devices could still fail to provide adequate protection in extreme weather events.
Solution #3: By far the best approach is to eliminate the potential for this problem by being smart with your installation planning. Do this by simply placing the chimney where it will be out of harm’s way. Bring the stack up through the roof close to the ridge cap. This will put the chimney above any sliding ice and snow. Another solution that can work quite effectively is to locate the chimney on the gable end of the roof. While this may not fit a homeowner’s preferred floor plan, he or she might be willing to reconsider if you can clearly explain your concerns. After all, you will be doing him or her a favour that will save a pile of unnecessary stress and unwanted repair expenses down the road.
Wood Heating Technology Transfer
Gil Strachan is a professional home inspector, representing Electrospec Home Inspection Services in east-central Ontario since 1994.