No rule to measure yourself
A depth of snow says nothing about its weight. Fresh, packed, soaked by rain or turned to ice, the same depth weighs very differently. The roof's capacity depends on its design, its age, its condition and what has been added since.
A structural engineer, and only a structural engineer, can say what the roof can carry. Ask for that opinion before winter and add it to the preventive maintenance program. No one goes up to measure the snow to decide in the engineer's place.
Where the wind piles snow
On a large flat surface, the wind pushes snow into drifts in four places.
- Against a higher wall, where a low roof backs onto a taller part of the building.
- Along the edges, against the parapets.
- Around air conditioners and other units.
- In the lee of screens and signs.
In a warehouse or an industrial building, the structure spans long distances from one support to the next. These long spans tolerate an unevenly distributed load less well.
When the building speaks
These signs, seen or heard from inside, can indicate that the structure is under too much stress.
- A crack that was not there before, in a wall or a ceiling.
- A door that rubs or sticks when it used to close properly.
- Cracking sounds that are not usually heard.
- A ceiling or a beam that sags.
Faced with even one of these signs, evacuate the area and contact a structural engineer. Until the engineer has given an opinion, no one goes on the roof.
Trapped water, winter's other danger
The worst sometimes comes from water that can no longer drain. In a thaw or a downpour, a roof with frozen or buried drains holds a heavy pond, and that water seeps in at the first flaw in the membrane.
Before winter, ask us to mark the location of the drains so they can be found under the snow.
What sets a good snow removal contractor apart
Badly cleared, a roof can come out of winter damaged. The following points are checked before choosing. They are for questioning the contractor, not for doing the work yourself.
- Their tools. They work with plastic shovels, with no metal shovel or pick.
- How they finish. They leave a layer of snow on the membrane instead of scraping down to it.
- Their knowledge of the roof. Before starting, they know where the drains, vents and ducts are.
- Their method. They remove the snow gradually, without piling it in big heaps elsewhere on the roof.
For safety, the contractor marks off the area on the ground where snow falls, protects entrances and passers-by, locates electrical wires and equips the crew with fall protection. Check that the contractor is insured for this work. In spring, have us examine the membrane.
Frequently asked questions
How much snow can a commercial roof hold?
No single figure applies to every roof. The reference points that apply to your building come from a structural engineer.
Can building staff take care of it?
No, building staff do not clear snow from the roof. Working at height on an icy surface is dangerous and requires training and fall protection. An inexperienced person can also puncture the membrane. This work belongs to an equipped and insured contractor.
How should ice around a drain be handled?
Freeing the drain is a roofer's job, without striking the ice bonded to the membrane, since a blow from a pick can hole the roof. The roofer chooses the method based on the system in place.