There seems to be a lot of talk lately, and certainly media coverage, on roof collapses due to snow loads. I’ve been involved in many conversations these past few days asking me whether I feel folks should shovel off their roof or not.
The fact of the matter is, you can’t simply look at a roof and determine whether it is susceptible to collapse. It’s way more complicated than that and often involves consulting a structural engineer.
The critical factor in determining whether you have an excessive snow load on your roof isn’t the depth of the snow; it’s the weight of that particular snow on your particular roof.
Modern residential framed homes are designed for snow loads specific to the town or city you live in. Across the state, building codes for residential roof snow load ranges from 35 to 65 pounds per square foot (PSF). Due to the age of many existing homes, however, it’s very likely your home wasn’t designed for the current standards. The weight of one foot of fresh snow ranges from 3 PSF for light, dry snow to 21 pounds for the wet, heavy stuff, according to the Federal Emergency Management Agency. How about ice? It’s a whopping 57 PSF.
Massachusetts building codes and municipal planning departments have tables that relate truss size, pitch, and materials relative to the PSF of pressure from snow or other loading factors.
Not all roofs are the same
There is always a concern about how well the roof was constructed and the condition and strength of the materials used. Keep in mind that there are many variables to consider, such as poor materials and inferior construction methods. The type and grade of the lumber used to frame the roof can contribute to, or undermine, its strength.
Factor in the snow weight
Is the snow light and fluffy or heavy and wet? Is rain in the forecast? The answers to these questions may affect your decision to shovel off a roof.
In reality, unless you get a recommendation from a structural engineer or a building official in your city, the decision about whether to shovel off a roof will be based on an educated guess, on a case-by-case basis.
I reached out to my friend Todd Fratzel , a structural engineer, for advice. Todd is the principal engineer at United Construction Corp. and editor of HomeConstructionImprovement.com. “All roofs are not created equal, regardless of pitch, so it’s important to realize that pitch alone does not necessarily mean the roof is safe with deep snow,” Fratzel said.
TIP Remove snow that’s 18 inches high or more on low-pitched roofs.
Todd shared this general rule of thumb. I love rules of thumb!!!! Todd feels that low-pitch roofs are most prone to overloading because they hold onto snow more easily than steeper ones.
It’s a good idea, he said, to remove snow from low-pitch roofs when the snow depth is 18 inches or more. This is especially true if the snow is wet or if rain/ice are forecast.
In addition to low-pitch roofs, valleys, protruding elements like skylights and dormers, and north-facing roofs are also of concern due to snowdrifts and the inability of the snow to slide off.
Warning signs that a roof is about to collapse
Roof-rafter sag is one obvious sign of excessive snow weight. Other indicators are creaking, cracking, and/or popping sounds. If you hear these sounds, you should leave the building and call the fire department for inspection.
How to clear a roof
A roof rake is the perfect tool for helping to eliminate ice dams, but it can also be used for removing snow from a steep roof. A roof rake has a long handle, which enables you to pull snow off the roof while on the ground.
Use a snow blower, snowshoes, or trample the snow down around the perimeter to allow you access.
Remove snow from the edge first and work your way into the roof using downward strokes. Not all snow needs to be removed, however; a thin layer can protect the roof from damage while snow is being removed.
Be careful of falling ice chunks as you rake the roof. A friend of mine got hit in the face with a chuck of ice and almost knocked himself out.
Safety, safety, safety
Don’t get electrocuted. Avoid overhead utility lines.
We just discussed that the real threats are with the low-pitched roofs. In these situations, it is best to hire skilled professionals who have equipment that will prevent them from falling off your roof and who are properly insured.
Working on a snow- and ice-covered roof is hazardous, and I am not advocating you do this. Think twice before you go up there. More people are injured or killed every year trying to shovel roofs than from roofs falling on them. A professional roofer or contractor is the best person to handle these issues for you.
Garages, decks, and outbuildings
Many outbuildings and garages were built many years ago to minimal standards, regulations for structures where people are not meant to live. I see heavy snow damage to these buildings the most. If it is safe to use a roof rake on these structures, do so. Do you have a deck, porch, or carport? Shovel those off, too, if it safe to do so, using only a roof rake on the carport.
Replacement, reinforcement, or permanent repair, however, is often the best way to address an old, rickety, or underbuilt roof. As a temporary situation, you can always add extra vertical support posts to the roof ridge or rafters.