Snow Loads in Ontario

1) What are snow loads?

A snow load is the downward force on a roof caused by accumulated snow and ice. Engineers express it as a pressure (often in kPa in Canada). And yes, “snow load” is a polite way of saying: “How much winter can your roof hold before it starts negotiating?”

Snow loads matter for four very practical reasons:

• Structural safety: under-design can lead to collapse, damage, injury, or worse.
• Code compliance: Ontario requires snow load design to follow specific rules and climate data.
• Insurance & liability: if something fails, the first question is “Was it designed correctly?”
• Durability: even without collapse, repeated heavy loading can cause long-term deflection and cracking.

2) Ontario’s snow load geography: why the numbers swing so much

Ontario isn’t one climate. It’s a buffet of climates. Southern areas near the Great Lakes often have milder conditions, while central and northern regions (and snow belts) can see much higher snow loads. Lake-effect snow, elevation changes, prevailing wind patterns, and storm tracks all play a role.

A practical “ballpark” list (illustrative)

Here’s a simplified snapshot of how ground snow loads can vary across Ontario communities. Always verify your exact location using SB-1 climatic data (local micro-zones and elevation differences can change the value).

Region	Examples (illustrative)	Why it happens
Southern Ontario (lower)	Windsor, Hamilton, Toronto, London	Lake moderation, warmer winter swings, less persistent accumulation
Central Ontario (moderate)	Kitchener-Waterloo, Oshawa, Kingston, Barrie, Ottawa	More sustained cold periods + snow belt effects (especially Georgian Bay)
Northern Ontario (higher)	Sudbury, North Bay, Thunder Bay, Sault Ste. Marie	Colder temperatures + heavier, longer-lasting accumulation + wind effects

If you build in the Georgian Bay snow belt (Barrie/Collingwood/Owen Sound area), you already know: your roof doesn’t get “a little snow.” It gets “a winter internship.”

3) The Ontario Building Code approach: where snow load values come from

Ontario’s code references standardized climatic data (often from SB-1 “Climatic and Seismic Data”) and then applies factors to translate ground snow into roof snow. Why not just use ground snow directly? Because roofs behave differently: wind scours, drifting piles snow in certain zones, slope sheds snow, and rain-on-snow can add weight fast.

If you want to keep up with how Ontario updates requirements and commentary, this page is a useful anchor for structural changes and snow drift considerations: Ontario.ca – Part 4 Structural Integrity changes.

4) The OBC snow load formula (explained like a human)

For engineered design (Part 4), the specified roof snow load is commonly represented like this:

What each part means (plain language)

• I_s (Importance factor): how serious the consequences are if the building fails. Houses are usually “normal.” Hospitals/fire stations get higher factors.
• S_s (Ground snow load): your location’s base value from climatic data.
• C_b (Basic roof factor): adjusts ground snow to roof snow, influenced by roof size/geometry (Part 9 uses simplified rules).
• C_w (Wind exposure): wind can reduce or redistribute snow, but reductions are only allowed in specific conditions (open exposure, low obstructions, etc.).
• C_s (Slope factor): steeper roofs shed snow more readily (depending on roof surface type).
• C_a (Shape factor): accounts for drifting, valleys, roof steps, odd shapes, sliding snow accumulation zones, etc.
• S_r (Associated rain load): because wet snow and rain-on-snow can be dramatically heavier than dry powder.

5) Practical calculation example (Toronto-style, step-by-step)

Let’s walk through a simplified example — not to replace engineering, but to show how the parts fit together. Assume:

• Location: Toronto (illustrative S_s and S_r values based on standard climatic data assumptions)
• Building: single-family house → I_s = 1.0
• Roof: simple gable → C_a = 1.0
• Exposure: typical suburban → C_w = 1.0
• Roof pitch: around 8/12 (moderate steepness)
• Roof width: typical house width (C_b depends on Part 9 vs Part 4 conditions)

In real practice, you pull the exact S_s and S_r for your community, confirm the correct roof factors, compute C_s from slope (and roof surface), and then evaluate special cases like drifting and unbalanced loads. The important lesson: the “snow load number” isn’t a single guess — it’s a structured calculation.

6) The advanced stuff that causes real failures

Most collapses aren’t “uniform snow everywhere on a perfect gable roof.” They happen because loads become non-uniform — drifting, sliding, valleys, and ponding. This is where engineering earns its keep.

A) Drift loads (the sneaky overload)

Wind doesn’t politely spread snow evenly. It moves it. If you have a higher roof dumping wind and snow onto a lower roof, the lower roof can see loads far above the “uniform” case.

Classic Ontario example: a two-storey house with an attached single-storey garage. The garage roof next to the house wall is a drift magnet. That strip of roof can carry significantly more load than the rest of the garage roof.

B) Unbalanced loads on gable roofs

Wind can scour one side and pile snow on the other. That creates torsion and unbalanced forces in ridge beams, hips, and load paths. It’s not just “how much weight,” it’s “where the weight sits.”

C) Valleys (where snow goes to live)

Valleys collect snow. Snow slides into them. They can build up faster, compact, and hold meltwater. Valley rafters and valley zones often require heavier design because the accumulation can be much higher than surrounding areas.

D) Sliding snow (especially on metal roofs)

Metal roofs can shed snow suddenly. That sliding mass can accumulate at eaves or onto lower roofs. It can also become a safety hazard near entrances and walkways. Solutions can include snow guards, design allowances for accumulation zones, and smart roof geometry planning.

E) Ponding on low-slope roofs

Ponding is a progressive problem: load causes deflection, deflection creates a low spot, low spot holds more water/snow, which increases load, which increases deflection… you get the idea. Flat and low-slope roofs need special attention to slope, stiffness, and drainage.

7) When you need a structural engineer in Ontario

In Ontario, engineering is required in many scenarios (and smart in even more). You almost certainly want a P.Eng. involved when you have:

• Commercial buildings, assembly buildings, or anything with high occupant loads
• Multi-unit residential (apartments/condos)
• Complex roof shapes: multiple roof levels, big valleys, parapets, roof steps
• Long spans, open concepts, cathedral ceilings, exposed beams
• Major renovations that change load paths (removing walls, adding large openings)
• Heavy roofing materials (tile/slate) plus high snow loads

If your project touches structure, it may also touch permitting. This page explains the process in homeowner-friendly terms: How to Get a Building Permit in Ontario.

8) Warning signs homeowners should never ignore

If your roof is overloaded, you’ll often get clues. Watch for:

• Sagging rooflines or new dips that weren’t there before
• New cracks in ceilings/walls appearing during heavy snow periods
• Doors/windows sticking (frame distortion can show up in odd ways)
• Popping/creaking that’s new and repetitive under load
• Bowing sheathing visible from the attic (between rafters/trusses)

9) Snow removal: when it’s worth doing (and how not to make it worse)

Snow removal is risk management. It’s most worth considering when:

• You’ve had multiple storms with no melting cycle
• Snow is wet and heavy (or rain is saturating it)
• You have a flat/low-slope roof
• You see any warning signs listed above

Safety first: unbalanced removal can create unbalanced loading. Don’t clear one side completely and leave the other side packed. When in doubt, hire insured professionals.

10) Climate variability: why “average winter” is the wrong mental model

Ontario winters are increasingly variable: freeze-thaw cycles, rain-on-snow events, and heavier “shoulder season” storms can produce loads that feel surprising compared to the old “steady cold” pattern. Engineers and code bodies are paying more attention to changing climatic conditions and projected design variables.

If you like seeing how professionals think about projected climatic design variables, this NRC overview is a useful read: NRC – climatic design variables and projections.

11) Builder tips: design choices that can make snow loads easier to live with

• Roof pitch matters: steeper roofs can shed snow more effectively (but watch sliding snow zones).
• Simple geometry helps: fewer roof steps and valleys means fewer drift/accumulation traps.
• Drainage matters: especially for low-slope roofs—ponding is a real failure mechanism.
• Ventilation and insulation matter: reduce ice dam risk and keep the roof system stable over the season.
• Engineer early: it’s faster and cheaper than engineering after the plans are “final.”

And if you’re planning a custom build, durability starts with the structure and envelope — not just finishes. That’s the difference between “looks good” and “still looks good after 25 Ontario winters.” You can see the kind of high-performance builds we do at ICFhome.ca.

FAQ: Snow loads in Ontario

Link rule check (unique URLs): 3 internal (snow loads calculator, building permit, OBC changes) + 3 external (ICFhome.ca, Ontario.ca structural changes page, NRC climate design variables page). The calculator link appears twice by request.