Why there is no magic number
No snow depth alone tells you when to shovel. The weight of snow varies a lot depending on whether it is fresh, packed, soaked by rain or turned to ice. A roof's capacity depends on its design, its age, its condition and what has been added to it since.
Only a structural engineer can say what your roof can carry. Get that opinion before winter and include it in your preventive roof maintenance program.
Where snow builds up
On a large flat roof, wind piles snow into drifts:
- at the foot of taller walls, where a low roof meets a higher part of the building
- along the parapets
- around HVAC units and other equipment
- behind screens and signs
The structure of warehouses and industrial buildings spans long distances between supports. These long spans are more sensitive to uneven loads.
Signs that call for immediate action
Some signs inside the building must be taken seriously. They indicate the structure may be under too much strain:
- new cracks in walls or ceilings
- doors that start to rub or stick
- unusual creaking
- a ceiling or beam that sags
If you see any of these signs, get the occupants out of the area and call a structural engineer. Send no one onto the roof before you have the engineer's opinion.
Drains first
The biggest winter risk is sometimes water that can no longer drain. A thaw or rain on a roof whose drains are frozen or buried creates a heavy pond that seeks out the smallest defect in the membrane.
Have the drain locations marked before winter so they can be found under the snow.
Clearing snow without damaging the membrane
Careless snow removal damages the roof. A contractor who knows flat roofs:
- Uses plastic shovels, never metal shovels or picks.
- Leaves a layer of snow on the membrane rather than scraping down to the surface.
- Knows where the drains, vents and ducts are before starting.
- Removes snow gradually, without creating big piles elsewhere on the roof.
The contractor also sees to people's safety. That means setting up a perimeter on the ground where the snow falls, protecting entrances and passers-by, watching for power lines and equipping workers against falls.
Ask us for proof of insurance for this work, and have us inspect the membrane in the spring.
Frequently asked questions
How much snow before clearing a commercial roof?
No single depth applies to every roof. A structural engineer can give you benchmarks specific to your building.
Can my maintenance staff clear the roof?
No, leave roof snow removal to an equipped and insured contractor. Working at height on a slippery surface is dangerous and requires training and fall protection. Your staff could also puncture the membrane.
Should ice around the drains be removed?
The drains must be cleared, but without striking ice stuck to the membrane. Breaking ice with a pick punctures the roof. Ask us which method suits your system.