What Winter Does to Commercial Roofs in Markham

Winter changes three things on a Markham commercial roof. Ice blocks drains and backs water up behind parapets. Snow drifts against screens and higher walls and concentrates load. Cold restricts which membranes and adhesives can be installed at all. Most winter leaks are decided in October, by whether the drains and flashings were dealt with.

What kind of roofs does Markham actually have?

Markham's building stock is not uniform, and winter treats each type differently.

• Technology and corporate campuses along the Highway 404 and Woodbine corridors: low-rise office buildings with large flat roofs, heavy rooftop mechanical loads, screened equipment areas and interior drains. Occupancy is continuous, so any water entry lands on somebody's workstation.
• Milliken Mills and the older industrial areas: long-span single-storey units, steel deck, many of them still carrying built-up roofs with aggregate or older modified bitumen, sometimes with a second roof already over the first.
• Newer single-ply roofs on the more recently built industrial and flex space: mechanically fastened TPO or EPDM over long uninterrupted fields, with the perimeter and corner zones taking the wind.

The common factor is that every one of these is low-slope, drains internally or through scuppers, and depends on a small number of drainage points staying open through the coldest months of the year. Our coverage of the municipality is on the commercial roofing in Markham page.

Why do so many winter leaks start at the drains?

Because a flat roof in Markham does not shed snow. It holds it, and then melts it slowly from the warm side.

Heat escaping through the assembly, plus sun on a dark membrane, melts the underside of the snowpack. That meltwater runs across the roof toward the drains. The drains and the perimeter are the coldest parts of the roof, so the water refreezes as it arrives. Once the drain bowl and the surrounding sump are iced, the next melt has nowhere to go and backs up behind the ice.

Standing meltwater then finds whatever is weakest: an open lap, a shrunken pitch pocket, a base flashing that has pulled slightly away from a curb, the clamping ring on the drain itself. This is why a building that is watertight through a rainy autumn starts leaking during the first February thaw. The mechanism is described in more detail on our drainage guidance, and it is the reason autumn drain clearing is the highest value single task on the calendar.

Overflow scuppers matter here too. If they are set correctly they relieve water when the primary drains are blocked. If they were installed above the low point, or have been covered by later work, they do nothing at the moment they are needed.

Where does snow actually pile up on a Markham roof?

Not evenly. Wind redistributes it, and the load concentrates in predictable places:

• Against parapets, especially on the leeward side of the building.
• At the step where a lower roof meets a higher wall, which is common on office campuses with a taller core or a mechanical penthouse.
• Behind mechanical screens and around large rooftop units, where the equipment acts as a fence.
• In valleys between roof levels and against skylight and hatch curbs.

The Ontario Building Code sets design snow and rain loads through Supplementary Standard SB-1, and the code already accounts for drifting through accumulation factors, which is precisely why the numbers at a parapet are higher than in the open field. What that means practically is that the drift is where structural concern lives, and whether a particular accumulation is acceptable on a particular building is a question for a professional engineer, not for a roofing crew standing on it.

What a roofer can tell you is what the drift is doing to the roofing: crushing insulation under its weight, holding a permanent ice mass over a base flashing all winter, and leaving a saturated area that ponds every spring afterwards.

Can roof work be done in Markham in the winter at all?

Yes, with real limits, and the limits are chemistry rather than toughness.

• Self-adhered membranes and adhesives have minimum application temperatures. Below them the bond does not develop properly, and a seam that looks fine on the day releases later.
• Heat welded seams on TPO and PVC can be made in the cold, but the welder settings change with temperature and wind, so test welds and seam probing become more important, not less.
• Torch-applied SBS is workable in the cold, but rolls need to be stored warm, the substrate must be dry and free of frost, and hot work procedures and fire watch still apply.
• Nothing goes down over frost, ice or damp. Moisture trapped in an assembly closed on a cold day is a leak the following spring.
• Insulation is only installed in the area that can be closed in the same day, which shortens the productive day considerably when daylight is short.

So the honest answer is that winter is for stabilising, not usually for full tear-offs. Repairs, temporary dry-ins, flashing work in a mild window and emergency response all happen through the winter. A full flat roof replacement is normally scheduled for a workable season unless the roof cannot wait, in which case it is phased and detailed around the weather.

What makes winter roof access harder and more dangerous?

Everything on a snow-covered roof is hidden. Skylights, roof hatches, open drains, curb edges and the roof edge itself all disappear under the same white surface, and a snow-covered skylight will not hold a person. Fall protection under the Occupational Health and Safety Act and the working-at-heights requirements enforced by the Ministry of Labour, Immigration, Training and Skills Development applies the same in January as in July, except that anchor points may be buried and the walking surface is ice.

Practical consequences for a building operator: do not send facility staff onto the roof to investigate during or after a storm, do not let a parking lot snow contractor onto the roof unless roof work is explicitly in their scope and they are equipped for it, and keep the path to the roof hatch or ladder clear so a crew can actually get there when you need them.

If water is coming in now, containment inside the building is the first job and the roof is ours. Our emergency roof repair line runs 24/7 at 416-456-0777.

What should be done in the autumn to avoid all of this?

A short list, done before the first freeze, removes most of the winter risk on a Markham building:

1. Clear every drain, sump, scupper and overflow after the leaf fall finishes, and check that the strainers are present and intact.
2. Walk the perimeter. Cap flashing joints, missing fasteners, loose edge metal and open termination bars are what ice pries at all winter.
3. Check the details around rooftop units, especially anywhere a mechanical or electrical contractor has been on the roof since the last inspection.
4. Look for existing ponding areas and note them. Those are where ice will sit and where the spring leak will appear.
5. Confirm interior conditions that push moisture into the assembly, such as humid exhaust venting near a roof penetration.
6. Agree in advance who clears snow, at what trigger, with what equipment, and who is called if something is wrong.

That routine belongs in a flat roof maintenance program rather than in a scramble in December, and it is the cheapest work anyone does on a commercial roof all year.