Clearing Snow From a Commercial Flat Roof Without Damaging It

Snow is cleared from a commercial flat roof with plastic shovels and plastic-edged equipment, leaving a layer of snow on the surface so no tool ever touches the membrane. It is removed in strips to avoid concentrating load, never chipped as ice, never salted, and the drains and overflows are opened first.

Does the snow need to come off at all?

Often it does not, and clearing a roof unnecessarily creates risk of its own. Commercial roofs in Ontario are designed to carry snow, with design loads set through the Ontario Building Code and the values for each location published in Supplementary Standard SB-1. A uniform blanket across the field of a properly designed roof is normally what the structure expects.

The situations that do call for removal:

• Drifting. Snow piled against a parapet, at a step where a lower roof meets a higher wall, or behind a mechanical screen concentrates load well above the field value. Drift is where structural concern actually lives.
• Rain on snow. A thaw or a rain event on a deep snowpack adds weight rapidly, and if the drains are iced the water has nowhere to go.
• Blocked drainage. Iced drains and scuppers with meltwater standing behind them, particularly when more precipitation is forecast.
• Signs from the structure. Deflection, new cracking, doors binding, sprinkler lines out of level, unfamiliar noises. Evacuate first and call a professional engineer.
• Access for repair. Clearing a working area so an active leak can be dried in.

Whether a specific accumulation on a specific building is acceptable is a structural determination and belongs to a professional engineer, not to whoever is holding the shovel.

What actually damages the roof during clearing?

In our experience most of the membrane damage we repair in the spring was done by snow removal rather than by snow.

• Metal tools. Steel shovels, scrapers and ice chippers cut membrane, gouge flashings and split laps. They have no place on a roof.
• Chipping ice. Ice bonds to the membrane surface. Removing it mechanically takes surfacing, granules and sometimes the sheet with it.
• Clearing to the surface. A tool that reaches the membrane will eventually catch a lap, a fastener head, a walkway pad edge or a seam.
• De-icing salt and chlorides. They corrode metal flashings, drains and fasteners, damage some membranes, and end up in the drainage system and on the landscaping below.
• Powered equipment. Snow blowers and small machines on a roof are a puncture risk and a load problem in their own right, and are only appropriate with plastic contact edges, a specific plan and the right operator.
• Hidden features. Skylights, drains, curbs and the roof edge disappear under snow. A snow-covered skylight will not hold a person.

How is a roof cleared properly?

The method is not complicated. It is discipline.

1. Open the drainage first. Clear the drains, scuppers and overflows so meltwater has somewhere to go before anything else happens. Half of all winter roof problems end here.
2. Leave a layer. Clear down to roughly fifty to a hundred millimetres of snow across the field and stop. That residual layer is what keeps every tool off the membrane, and it weighs very little.
3. Plastic only. Plastic shovels, plastic sleigh scoops, plastic-edged equipment. Nothing metal touches the roof.
4. Work in strips. Clear in parallel bands across the roof rather than emptying one area completely. Removing all the snow from one bay while the next stays loaded creates an unbalanced load, which is a structural condition in itself.
5. Start at the drift, not at the middle. The reason for being up there is usually the drift against the parapet or the higher wall, so that is where the load comes off first.
6. Never pile it. Snow taken off one area and heaped somewhere else on the roof has not been removed, it has been concentrated. If it cannot go off the roof, it is spread thin over a wide area.
7. Watch what is below. Where snow goes over a parapet or through a scupper, the ground below is cleared and controlled first.
8. Leave ice alone. Bonded ice is not chipped. Where it must go, it is dealt with by clearing a melt path, by hot water in a controlled way, or by a calcium-free ice melt product that the membrane manufacturer permits, applied sparingly.

What does the safety side require?

A snow-covered commercial roof is one of the more dangerous places anyone will work all year, and Ontario treats it as such. Working at heights on a roof falls under the Occupational Health and Safety Act, with the working-at-heights training and fall protection requirements enforced by the Ministry of Labour, Immigration, Training and Skills Development.

The winter complications are specific. Anchor points and tie-off locations may be buried and have to be located before work starts. The walking surface is ice under snow. Guardrails may be hidden or, on some roofs, absent entirely. The roof edge, skylights, hatches, open drains and equipment curbs are all invisible.

Practically, this means snow clearing on a commercial roof is a planned operation by a crew equipped for it, with the roof's hazards mapped in advance. It is not a job for whoever is available, and it is not something to send maintenance staff up to do during a storm.

It is also worth being explicit with your parking lot contractor: unless roof work is written into their scope, with the right equipment and the right training, the lot is where they stay.

What about ice at the parapets and drains?

This is the specifically Ontario problem, and it is not really a snow load problem at all.

Heat escaping through the assembly and sun on the membrane melt the underside of the snowpack. The meltwater runs across the roof toward the drains and the perimeter, which are the coldest parts of the roof, and refreezes there. The result is an ice dam at the parapet and an iced drain bowl, with water standing behind both.

That standing meltwater finds whatever is weakest: an open lap, a shrunken pitch pocket, base flashing that has pulled slightly away from the wall, the clamping ring at the drain. It is why buildings that were watertight all autumn start leaking during the first thaw.

Prevention is a maintenance question rather than a snow question. Drains and overflows clear before the first freeze, sound flashings and terminations at the parapet, and ponding areas corrected rather than tolerated. If water is already coming in, our emergency roof repair line runs at 416-456-0777.

What should the roof get after the snow is off?

A look, while the surface is visible. Clearing exposes the roof at exactly the moment it has been under the most stress, and it is a good opportunity to check:

• Whether the drains and scuppers are running once cleared.
• Membrane damage at the edges of the cleared area, at walkway pads and at any lap where a tool may have caught.
• Flashings at the parapet where ice has been sitting and prying.
• Soft areas underfoot, which suggest wet insulation.
• Anything on the roof that moved during a wind event, including loose cap metal and equipment covers.

Anything found gets recorded and dealt with in the spring, which is what the post-thaw walk is for. Our flat roof maintenance program covers that visit, and the wider checkpoint list is in our post on inspection checkpoints for aging flat roofs.

How should a building plan for winter before it arrives?

Decide these six things in the autumn and the winter becomes a routine rather than an emergency:

1. Who clears the roof, and are they equipped and trained for roof work specifically.
2. What triggers a clearing: a snow depth, a forecast rain-on-snow event, or an engineer's advice for that building.
3. Where the snow goes, and what is below that location.
4. Where the roof hazards are, mapped and given to the crew: skylights, hatches, drains, low parapets, unguarded edges, fragile covers.
5. Where the anchor points are and how they will be found under snow.
6. Who is called if something is found, and how they get roof access out of hours.

Add to that a drain clearing after the leaf fall and a walk of the perimeter details before the first freeze, and most of the winter risk on a commercial roof is gone. To set that up on a building or a portfolio, call 416-456-0777 or use the contact page.