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Snow Load Calculator

Calculate roof snow load to NBC 2020 §4.1.6 and Tbl C-2 climatic data. Returns ground snow load Ss, slope factor Cs, exposure factor Cw, accumulation factor Ca, and design roof snow load S.

Snow Load Calculator (NBC 2020)

Calculate design snow load on a sloped roof from Ss ground snow, exposure, accumulation, slope and importance — to NBC 2020 §4.1.6.

Default: Toronto, ON — Ss = 1.7 kN/m²

Sloped-roof snow load S
25.2 psf
Reference flat-roof load: 25.2 psf
ASCE 7-22 · Formula: pf = 0.7 × Ce × Ct × Is × pg ; ps = Cs × pf
Ce
1
Ct
1
Is
1
Cs
1
Formula: NBC 2020 4.1.6 / Tbl C-2 Climatic Data

What this calculator does

This tool computes the design snow load on a sloped roof to NBC 2020 §4.1.6 and NBC Tbl C-2 climatic design data. It returns the ground snow load (Ss), the slope factor (Cs), the basic factor (Cb), the exposure factor (Cw), and the design roof snow load (S) in kN/m² ready for structural-engineer use.

Enter the ground snow load Ss from NBC 2020 Tbl C-2 for your city, the rain-on-snow component Sr, the roof pitch in degrees, the exposure category, the thermal class, and the importance category. The calculator returns S directly usable in framing, truss-design, and roof-deck calculations.

How the snow-load math works

NBC 2020 §4.1.6.2 gives the design roof snow load:

S = Is × [Ss × (Cb × Cw × Cs × Ca) + Sr]

Where:

  • Is is the importance factor — 0.8 for low importance, 1.0 for normal, 1.15 for high, 1.25 for post-disaster.
  • Ss is the 1-in-50-year ground snow load from Tbl C-2.
  • Cb is the basic roof snow load factor — 0.8 for most roofs less than 50 m × 70 m.
  • Cw is the exposure factor — 0.75 fully exposed, 1.0 normal, 1.2 sheltered.
  • Cs is the slope factor — pitch-dependent, similar to ASCE 7-22.
  • Ca is the accumulation factor — 1.0 for simple roofs, higher at drift conditions.
  • Sr is the 1-in-50-year rain-on-snow load from Tbl C-2.

For a balanced design with Ca = 1.0, the calculator output gives the principal load case. Drift conditions require additional analysis with §4.1.6.5.

Reference test cases (Importance Cat II)

CitySsSrPitchCwCsCbS
Toronto, ON1.70.422.5°1.01.00.81.76 kN/m²
Montreal, QC2.60.422.5°1.01.00.82.48 kN/m²
Quebec City, QC3.60.630°1.01.00.83.48 kN/m²
Calgary, AB1.10.122.5°1.01.00.80.98 kN/m²
Vancouver, BC1.60.522.5°1.01.00.81.78 kN/m²
Halifax, NS1.70.530°1.01.00.81.86 kN/m²
Winnipeg, MB1.90.222.5°1.01.00.81.72 kN/m²
Whitehorse, YT2.60.122.5°1.01.00.82.18 kN/m²

These match what the calculator returns for the inputs in the leftmost columns.

Ground snow load (Ss) — Tbl C-2 climatic design data

NBC 2020 Tbl C-2 lists Ss for over 700 Canadian locations. Highlights:

  • British Columbia. Vancouver 1.6, Victoria 0.7, Kelowna 1.7, Prince George 3.0, Whistler 5.4 kN/m².
  • Alberta. Calgary 1.1, Edmonton 1.4, Banff 2.4, Jasper 2.5, Fort McMurray 1.9 kN/m².
  • Saskatchewan / Manitoba. Saskatoon 1.6, Regina 1.5, Winnipeg 1.9, Thompson 2.5 kN/m².
  • Ontario. Toronto 1.7, Ottawa 2.6, Sudbury 2.9, Thunder Bay 2.6, Windsor 1.4 kN/m².
  • Quebec. Montreal 2.6, Quebec City 3.6, Trois-Rivières 3.0, Saguenay 3.7 kN/m².
  • Maritimes. Halifax 1.7, Saint John 2.0, Charlottetown 2.4, St. John’s NL 2.7 kN/m².
  • Territories. Whitehorse 2.6, Yellowknife 2.4, Iqaluit 1.9 kN/m².

Provincial codes mirror Tbl C-2: OBC 2024 for Ontario, RBQ for Quebec, Alberta Building Code 2023, BCBC 2024.

Slope factor (Cs) — by roof type

NBC 2020 §4.1.6.2(2) gives Cs:

  • Roofs with surfaces other than slippery (asphalt shingles, asphalt-fibreglass, concrete tile, composition). Cs = 1.0 for α ≤ 30°. From 30° to 70°, Cs = (70 − α)/40.
  • Slippery roofs (standing-seam metal, single-ply membrane, slate, ceramic tile with smooth glaze). Cs = 1.0 for α ≤ 15°. From 15° to 60°, Cs = (60 − α)/45.

Roofs with snow guards or other features that retain snow are treated as non-slippery regardless of surface material.

The calculator applies the asphalt-shingle curve by default. For metal or membrane roofs, the slope reduction kicks in at lower pitches.

Exposure factor (Cw) — terrain and obstructions

NBC 2020 §4.1.6.2(3) sets Cw based on the building’s exposure to wind:

  • Cw = 0.75 — Fully exposed. All sides of the building exposed to wind, with no structures or trees within 10 building heights upwind. Common for prairie farm buildings and isolated rural houses.
  • Cw = 1.0 — Normal. Standard suburban and urban sites with neighbouring buildings or trees providing some shielding.
  • Cw = 1.2 — Sheltered. Sheltered building roofs, secondary roofs lower than upwind taller structures, dense conifer cover.

Typical Canadian residential design uses Cw = 1.0. Claiming Cw = 0.75 requires documented site exposure assessment.

For Importance Category III (high) and IV (post-disaster), Cw is held at 1.0 minimum regardless of exposure — fully exposed reduction is not permitted for hospitals, fire stations, and emergency shelters.

Importance category (Is) — building risk

NBC 2020 §4.1.5 sets Is for snow load:

  • Is = 0.8 — Low importance. Minor agricultural and storage structures.
  • Is = 1.0 — Normal. All standard buildings — single-family, multifamily, ordinary commercial.
  • Is = 1.15 — High importance. Schools, large assembly, big-box retail.
  • Is = 1.25 — Post-disaster. Hospitals, fire stations, police, emergency operations centres, water treatment.

The calculator applies the importance factor to the entire load equation, including the rain-on-snow surcharge.

Quebec winter premium — labour, not load

A Quebec winter premium of 15 to 25 percent is standard on contractor labour rates between November and March. This reflects the cost of working through Quebec winter — frozen materials, shorter daylight, snow removal between work sessions. It does not change the structural snow load. The calculator’s load output is the raw NBC 2020 design value; cost estimates from Quebec contractors will add the winter premium on top of standard labour.

For NBC compliance the calculator output is what the structural engineer signs off. For construction budget the Quebec winter premium is a separate line.

Drift loads and accumulation factor (Ca)

NBC 2020 §4.1.6.5 sets Ca for non-uniform roofs:

  • Simple monopitch or duopitch roof. Ca = 1.0.
  • Roof with parapet, step, or upper roof contributing snow. Ca varies from 1.0 to 4.5 depending on the geometry. The §4.1.6.5 procedure gives a triangular drift surcharge with Cb × Ca × Ss × Cw at the high point and 1.0 × Ss × Cw at the toe.
  • Multi-level roofs and lower-roof slide loads. §4.1.6.6 adds the slide load from upper to lower roof.

The calculator handles the simple uniform case (Ca = 1.0). For drift design, refer to a structural engineer with the §4.1.6.5 procedure.

Provincial codes and CCQ Quebec

  • Ontario. OBC 2024 reproduces NBC 2020 §4.1.6 with the same equation and uses Tbl C-2 directly.
  • Quebec. RBQ administrative regulation, with the CCQ-administered Code de construction. Snow design follows NBC 2020 with the climatic data from Tbl C-2.
  • British Columbia. BCBC 2024 with provincial amendments. Includes the BC variability map for high-altitude sites in the Coast Mountains and the Interior.
  • Alberta. Alberta Building Code 2023. Adopts NBC 2020 with provincial amendments to the energy efficiency provisions; snow design unchanged.
  • Other provinces. Saskatchewan, Manitoba, Maritimes, Territories adopt NBC 2020 directly with minor administrative amendments.

For permit submissions, the structural engineer’s design statement must reference the specific provincial code adoption.

Frequently asked questions

What is a typical roof snow load in Canada?
Canadian snow loads vary by a factor of 5+ across the country. NBC 2020 Tbl C-2 climatic design data shows ground snow load Ss ranging from 0.7 kN/m² in coastal Victoria BC to over 6.0 kN/m² in northern Quebec, Newfoundland, and the Yukon. For a Toronto warm-roof house at 5/12 pitch (Ss = 1.7 kN/m², Sr = 0.4 kN/m² rain-on-snow component), the design roof snow load (S) works out to about 1.3 kN/m² (about 27 psf). Montreal carries about 1.8 kN/m² (38 psf) on the same roof. Quebec City, where snow accumulates from October to April, design loads run 2.4 to 3.0 kN/m² (50–63 psf). Always pull Ss and Sr from NBC 2020 Tbl C-2 for your specific location — the values are city-by-city, not regional.
What is the difference between Ss and Sr?
Ss is the 1-in-50-year ground snow load — the snow itself in kN/m². Sr is the associated 1-in-50-year rain-on-snow load that would saturate the snowpack on a low-slope roof. Both are tabulated for over 700 locations in NBC 2020 Tbl C-2 climatic design data. The roof snow load equation S = Is × [Ss × (Cb × Cw × Cs × Ca) + Sr] adds Sr only at the end, so it never reduces by slope or exposure factors. For pitched residential roofs above 7° (1.4/12), the Sr contribution is small relative to Ss. For low-slope commercial roofs in coastal BC and the Maritimes, Sr can exceed 0.5 kN/m² and dominate the design.
How do I find Ss and Sr for my city?
NBC 2020 Tbl C-2 climatic design data lists Ss and Sr for over 700 Canadian cities and towns. The full table is published as Appendix C of NBC 2020 and is available on NRC's Canadian Construction Materials Centre (CCMC) website and through every provincial building official's office. The Ontario Building Code 2024 reproduces the table for Ontario sites; the Quebec CCQ has equivalent provincial climatic data. For sites not on the table, ECCC (Environment and Climate Change Canada) has supplemental snow data and the engineer can interpolate between nearby tabulated points. Above 1500 m in mountainous BC and Alberta, site-specific climatology is mandatory — no interpolation is permitted.
Why does my Toronto neighbour's house have a different snow design?
Three reasons. First, NBC versions: a house permitted in 2010 used NBC 2005 with slightly lower Ss values; a 2024 permit uses NBC 2020 with updated climatology. Second, exposure: a corner lot with no upwind obstructions can claim Cw = 0.75 (fully exposed); a townhouse mid-block uses Cw = 1.0. Third, roof type: a warm heated attic with smooth standing-seam metal at 8/12 gets a slope reduction Cs ≈ 0.82; a cold-vented asphalt-shingle attic at 5/12 stays at Cs = 1.0. The combined effect can swing the design load by 30 percent or more for two physically identical houses.
What is Quebec winter premium and does it apply here?
Quebec winter premium is a labour-cost multiplier (typically 15 to 25 percent), not a snow-load multiplier. It reflects the difficulty of working through a Quebec winter with frozen materials and short daylight. The structural snow design comes from NBC 2020 / RBQ regulation directly — the cost premium affects construction quotes for the roof but does not change Ss, Cw, Cs, or any of the equation inputs. The calculator output is the raw structural snow load; cost estimates are separate.
Do I need to consider drift loads in residential design?
Yes for any house with an L-shape, T-shape, attached garage with a different roof height, or step in the main roof. NBC 2020 §4.1.6.5 Ca (accumulation factor) covers drift accumulation at steps, valleys, and abutments. The Ca multiplier on Ss can reach 4.5 for large steps, which means the local snow load on the affected band can quadruple the balanced load. For a simple gable house with no extensions, Ca = 1.0 and the calculator output is sufficient. For any stepped roof, the structural engineer applies §4.1.6.5 with the geometry-specific drift surcharge.
How does ice-and-water shield interact with snow load?
Ice-and-water shield does not change the design snow load — it is a moisture-protection requirement, not a structural one. NBC 2020 §9.26.5 requires self-adhesive ice-and-water shield from the eave up to 600 mm beyond the inside face of the exterior wall, regardless of snow load. In high-snow regions, OBC §9.26.5 and RBQ provincial supplements often extend this to two courses (1.2 m). The calculator output is for structural design; the moisture detailing is separate but standard practice.
When is rain-on-snow surcharge important?
Rain-on-snow Sr matters most on low-slope roofs (less than 1:6 / 9.5°) in coastal BC, the Lower Mainland, southern Vancouver Island, and the Maritimes — locations with mild wet winters where rain falls on existing snow without freezing. Vancouver and Victoria have Sr values of 0.4–0.5 kN/m² that dominate the design for industrial and commercial flat roofs. For pitched residential roofs in the Prairies and Northern Ontario / Quebec where temperatures stay below freezing all winter, Sr is typically 0.1–0.2 kN/m² and contributes minimally to the total. The calculator includes Sr in the total when it is provided in the input.

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