Ontario Snow Loads: What They Mean for Your Steel Building

Ontario snow loads are one of the biggest forces a steel building has to carry, and one of the least understood by owners. The snow load sets how deep the rafters are, how heavy the purlins get, how far you can clear span and what the foundations see. Two identical buildings, one in Windsor and one near Georgian Bay, can need noticeably different steel. Here is what the numbers mean and what you should decide before your building is engineered.

What a snow load actually is

A snow load is the weight of snow and rain on the roof that the structure must be designed to carry safely. In Ontario it is set by the Ontario Building Code, which follows the same approach as the National Building Code of Canada. Each municipality has climatic data that includes a ground snow load and an associated rain load, based on long-term weather records for that location.

The ground snow load is a starting point, not the design load. The engineer converts it into a roof snow load using factors for the building's use, its exposure to wind, the roof slope and the roof shape. That's why your neighbour's building is not a reliable guide to yours. Our overview of building codes for steel buildings in Canada explains how these code requirements fit together.

Why snow loads vary so much across Ontario

Ontario covers a lot of climate. Lake-effect snow belts east of Lake Huron and Georgian Bay, the Muskoka and Haliburton highlands, and Northern Ontario all carry higher ground snow loads than much of the southwest and the Golden Horseshoe. Elevation matters too: a site on high ground can have a higher value than a town in the valley below it.

Because the climatic values are tied to a specific location, the engineer uses the data for your municipality, and the building department checks it at permit review. If your project is outside Ontario, the same method applies with that province's code and climatic data.

Drifting: where roofs actually get into trouble

The uniform snow load across a flat, open roof is rarely the worst case. Wind moves snow and piles it up in predictable places, and those drifts can be several times heavier than the snow on the rest of the roof. Common drift locations on commercial and industrial buildings:

• Roof steps. Where a lower roof meets a taller wall, such as an office wing beside a high-bay warehouse.
• Parapets and false fronts. Snow collects behind them.
• Rooftop units and screens. HVAC equipment and screens create local drifts.
• Adjacent buildings. A new low building placed beside an existing taller one picks up drift from it, and a new tall building can add drift to its lower neighbour.
• Valleys. Where two roof slopes meet in a multi-gable layout.

Drift loads are a design issue, not just a maintenance one. If you plan an addition later, tell your designer now. A building designed for a future roof step costs far less than reinforcing it after the fact. Our article on choosing a steel warehouse building covers planning for expansion.

Roof shape and slope

Most pre-engineered steel buildings use a low-slope gable roof. Steeper slopes can reduce the design snow load because snow slides off, but only where it can slide freely onto clear ground. Snow sliding off a roof onto a lower roof, a canopy or a walkway creates its own problems. Single-slope roofs simplify drainage but concentrate snow at the low eave and can create drifts against a high wall.

Snow guards, canopies over doors and where the eaves drain all deserve attention at design time, especially over truck doors, man doors and pedestrian routes.

Snow load plus everything else on the roof

The frames don't carry snow alone. The engineer combines snow with dead load, wind and collateral loads: sprinkler piping, lighting, ductwork, cable trays, suspended heaters and rooftop solar. In a cold climate, the snow load combined with these loads often governs the design of the roof members.

Give your designer a realistic list of what will hang from the roof, now and in the future. Adding a heavy rooftop unit or a sprinkler upgrade to a building that wasn't designed for it may need reinforcement and an engineer's review. If the building will have overhead cranes, those loads are combined with snow as well.

How snow loads affect span, cost and foundations

Higher snow loads mean deeper rafters, heavier purlins, sometimes closer frame spacing, and more load into the columns and footings. The effect grows with width: a wide clear span under a heavy snow load uses a lot more steel than the same span in a mild climate. Our guide to clear span steel buildings explains when a few interior columns make more sense.

That is also why a price quoted without a site address isn't worth much. Any per-square-foot figure is a general planning range until the building is engineered for its actual location. The building's importance category matters too: facilities that must stay operational after a major storm, like some municipal buildings, are designed to higher loads than a typical warehouse.

What owners should decide early

• Exact site location. The engineer needs the municipality and site to apply the right climatic data.
• Future additions. Lower wings, lean-tos or neighbouring buildings that could create drifts.
• Rooftop and hung equipment. HVAC, sprinklers, solar and process piping.
• Roof shape and drainage. Where snow and meltwater will go, and what is below the eaves.
• Use of the building. It sets the importance category and can change the design load.

Settle these before engineering starts. Changes afterwards mean revised frames and drawings, which our steel building timeline guide shows is one of the most common causes of delay. Your engineer and municipality confirm the final loads and requirements for your site.

Built for Canadian winters by one team

Meta Steel Buildings designs, fabricates and erects Canadian-made steel buildings engineered for the site they stand on, from snow-belt locations across Ontario to projects across Canada. One team handles consulting, 3D design, fabrication, foundations, permits and erection for commercial and industrial steel buildings. We're PEO registered and WSIB covered. Sketch your layout in the 3D building designer, then request a quote or call (888) 230-3363 with your site address.

Frequently Asked Questions

What is the snow load for a steel building in Ontario?

It depends on the location. The Ontario Building Code provides a ground snow load and rain load for each municipality, and the engineer converts these into a roof design load using factors for the building use, wind exposure, roof slope and shape. Drift areas near roof steps and parapets are designed for higher loads. Your engineer and municipality confirm the values for your site.

Do I need to clear snow off a steel building roof?

A properly engineered building is designed to carry the code snow load without clearing. Owners should still watch for unusual conditions, such as heavy drifts, ice build-up, blocked drains or snow sliding onto lower roofs, and follow the guidance of their engineer. Adding equipment to the roof without a review can reduce the margin the design was built with.

Does a higher snow load make a steel building more expensive?

Generally yes. Higher snow loads need deeper rafters, heavier purlins and larger footings, and the effect grows on wide clear spans. That is why a reliable price needs the site address and the building use. Treat any early cost figure as a general planning range until the building is engineered.