Structural design · Snow actions

Canadian snow load calculator

Set the project location and climatic design data to begin your calculation.

Start with the project locationChoose a location listed in NBCC Table C-2. The climatic values will populate automatically, then complete the roof inputs below.

Step 1 of 4

01 · Project setup

Project and site selection

Climate data required

Project / calculation name

NBCC 2020 Table C-2 location

Start here. Choose the location named in the project design basis. Values are drawn from the imported NBCC Table C-2 records; no interpolation is used.

Province / territory

Table C-2 elevation (m)

NBCC 2020 climatic design data

Ground snow load, S_s (kPa)

Associated rain load, S_r (kPa)

Source: NBCC 2020 Appendix C, Table C-2 · Climatic design data for selected locations in Canada. Open official NRC code PDF

Data provenance: source pages C-11 to C-37 (PDF pages 649–675). The dropdown includes only listed Table C-2 locations.

Location applicability. All 680 listed Table C-2 locations are included. Each entry applies at its tabulated elevation and Gazetteer-defined latitude/longitude. NBCC cautions that, especially in complex terrain, values should not be interpolated from Table C-2 for unlisted locations; obtain site-specific values from Environment and Climate Change Canada.

Project status

Selected site

Awaiting site

Selected locationCANADA

No site selected

Select an NBCC 2020 Table C-2 location.

Climatic inputs

Ground snow load · S_s— kPa

Associated rain load · S_r— kPa

Site elevation— m

Setup progress

Site and climatic dataIncomplete

Base roof calculationPending

Obstruction calculationOptional

Location and climatic source are recorded in this browser session only.v0.12

BASE ROOF SNOW LOAD

02 · NBCC 4.1.6.2

Base roof snow load

Enter climatic data

Importance category

Limit state

Shorter roof plan dimension, w (m)

Horizontal plan dimension. Enter the shorter side.

Longer roof plan dimension, l (m)

Horizontal plan dimension. Enter the longer side.

Mean roof height above grade (m)

Use the mean roof height required by NBCC, not the parapet height.

Roof slope, α (degrees)

Wind exposure factor, C_w

Reduced values require confirmation of every condition in 4.1.6.2.(4).

Roof surface

I confirm that all conditions for the selected reduced C_w value under NBCC 4.1.6.2.(4) are satisfied for the roof area and expected site conditions.

Without this confirmation, the app applies C_w = 1.0. Drift loading adjacent to a roof projection is calculated separately with C_w = 1.0 in the drift zone.

Calculation output

Specified base roof load

Awaiting inputs

Base roof snow load · ULSCalculated

— kPa

Uniform specified snow + associated rain load, subject to the selected inputs.

C_b—

C_s—

I_s—

Calculation breakdown

Enter S_s and S_r on the Site & Project tab.

—

NBCC calculation formula

S = I_s [ S_s ( C_b C_w C_s C_a ) + S_r ]
Base case: C_a = 1.0

Code checks

Assumptions and load cases

Partial loading · 4.1.6.3.(2)

Flat and shed roofs, gable roofs ≤ 15°, and arch/curved roofs require a partial-load case: full specified uniform snow load on one portion and half on the remainder, arranged for the critical member effect. The prototype flags this requirement; it does not generate member-specific load patterns.

Mean roof height adjustment · 4.1.6.2.(2)(c)

Where mean roof height is less than 1 + S_s/γ, C_b is taken as 1.0. γ is calculated in accordance with Article 4.1.6.13.

Next: Check roof-mounted obstructionsUse this when equipment, penthouses, chimneys or similar projections are present.

03B · NBCC 4.1.6.5 and 4.1.6.6

Multi-level roof / adjacent higher building

Three source-area cases

Enter the relevant source-area dimensions for each NBCC wind case. The application calculates Case I, II and III separately and uses the highest peak accumulation factor. Dimensions are horizontal plan dimensions, not sloped roof lengths.

Source-area reference · NBCC Figure 4.1.6.5.-B

One schematic · three cases

Case I — Upper roof

Use the upper roof plan dimensions as the drifting source area. β = 1.0.

Case II — Windward drift

Wind crosses the entire lower roof toward the higher projecting wall. Use the entire lower roof extending away from the step as the source area; drift accumulates against the wall. β = 0.67.

Case III — Downwind-facing step

Use the lower-roof source area on the downwind-facing side of the step. β = 0.67.

Conceptual reference only — not to scale and not a roof-plan extractor. Confirm wind direction, step orientation and source-area boundaries against the actual roof plan. Enter horizontal plan dimensions (not sloped roof lengths) in the corresponding case fields below.

Roof step and lower roof

Height difference, h (m)

Horizontal gap, a (m)

Use 0 for directly adjacent roof levels.

Evaluation distance, x (m)

Measured from the upper roof edge. At a = 0, x = 0 shows the peak; if a > 0, enter x ≥ a.

Lower roof slope, α (degrees)

Lower roof has perimeter parapets?

Lower roof surface

Source-area wind exposure factor, C_ws

The NBCC F ≤ 5 cap applies when Cws = 1.0.

Use optional Case I method for h ≥ 5 m?

Case I · Upper roofUse the upper roof plan area as the source area · β = 1.0

Shorter source dimension, w_s1 (m)

Longer source dimension, l_s1 (m)

Source-area parapet height, h_p1 (m)

Case II · Windward driftUse the entire lower roof as the source area · β = 0.67

Wind blows across the lower roof toward the higher projecting wall. Use the full lower-roof plan area extending away from the step for w_s2 and l_s2; the wall causes snow to accumulate as a drift against the step.

Shorter source dimension, w_s2 (m)

Longer source dimension, l_s2 (m)

Source-area parapet height, h_p2 (m)

Case III · Downwind-facing stepUse the lower-roof source area on the downwind-facing side · β = 0.67

Shorter source dimension, w_s3 (m)

Longer source dimension, l_s3 (m)

Source-area parapet height, h_p3 (m)

The source areas shown in Figure 4.1.6.5.-B must be identified from the actual roof plan and wind direction. This calculator does not infer those regions automatically. Confirm which roof edges have parapets; h_p is taken as zero unless all source-area roof edges have parapets. This prototype assumes C_ws = 1.0 for the drifting source area and applies the corresponding F ≤ 5 limit.

Calculated geometry

Multi-level drift profile

Awaiting inputs

■ Base roof load■ Drift surcharge— Total load profile

Case comparison

Enter site and roof data.

—

Specified load at x

Multi-level roof result

Local specified roof loadMulti-level drift

— kPa

At x = — m from the upper roof edge.

Governing C_a0—

Drift length—

Local C_a—

Governing calculation

Awaiting valid inputs.

—

Show NBCC relationships and calculation method

l_cs = 2w_s − w_s²/l_s
h′_p = clamp[h_p − 0.8S_s/γ, 0, l_cs/5]
F = 0.35β√[γ(l_cs − 5h′_p)/S_s] + C_b
C_a0 = lesser of βγh/(C_bS_s) and F/C_b
x_d = 5C_bS_s(C_a0 − 1)/γ

Optional check · Corner drift comparison (NBCC 4.1.6.8)

Additional check · NBCC 4.1.6.8

Corner drift load selector

Two-face comparison

Outside corner

Enter the two face drift loads at the corresponding radial location. NBCC permits the lesser severe of the two face drift loads to be extended radially around the outside corner.

Inside corner

Enter the two face drift loads at a location where the drift fields overlap. Apply the higher of the two loads.

Corner configuration

Face A specified drift load (kPa)

Face B specified drift load (kPa)

Selected corner loadLesser face load

4.000 kPa

Outside corner: lesser of Face A and Face B.

This is a two-face load comparison helper, not an automated plan-based drift contour. The engineer must establish where the two drift fields lie, whether they overlap, and the corresponding face loads from the applicable roof geometry.

Engineering scope and limitations

Engineering review

Scope of drift modules

Current prototype scope

Roof-mounted projection drift (4.1.6.7); three-case multi-level roof drift (4.1.6.5); horizontal gap rule for adjacent higher roofs (4.1.6.6); base-load-plus-drift graphics.

Still to be implemented

Automated source-area extraction from roof plans, corner drift contour/overlap mapping (4.1.6.8), gable unbalanced load cases, sliding snow, curved roof cases, valleys and multiple interacting obstructions.

Next: Review the calculation packageExport a consolidated report with inputs, equations and results.

04 · Calculation package

Snow load calculation report

❄

SNOWSPEC CANADASTRUCTURAL ENGINEERING CALCULATION WORKSHEET

DESIGN CALCULATION
SNOW ACTIONS

National Building Code of Canada 2020 · Division B · Subsection 4.1.6

Roof Snow Load Calculation

Project details and calculation inputs will appear here.

DOCUMENT TYPEStructural calculation

CODE BASISNBCC 2020

ISSUE DATE—

APPLICATION VERSIONSnowSpec v0.14 · Prototype

Calculation summarySpecified snow actions calculated from current worksheet inputs

Summary values are specified loads in kPa. They are not factored ULS/SLS structural design effects. Apply the governing load combinations separately.

Project and climatic design dataSite selection and climatic values

Design inputs and assumptionsRoof geometry, importance category and exposure assumptions

Base roof snow loadNBCC 2020 · Article 4.1.6.2

S = I_s [ S_s ( C_b C_w C_s C_a ) + S_r ] · Base case: C_a = 1.0

Roof-mounted projection driftNBCC 2020 · Article 4.1.6.7

Base load with drift surcharge profile

Multi-level roof driftNBCC 2020 · Articles 4.1.6.5 and 4.1.6.6

Base load with multi-level roof drift surcharge

Source-area reference · three NBCC cases

Corner drift comparisonNBCC 2020 · Article 4.1.6.8 · Engineering comparison helper

Code notes, exclusions and engineering reviewLimitations that must be considered before design use

Loads shown are specified snow actions. They are not factored structural design loads. Apply the applicable NBCC load combinations and load factors separately.
Partial loading may be required under Article 4.1.6.3. This prototype flags the requirement but does not create member-specific partial-load patterns.
Roof-mounted projection drift and multi-level roof drift are separate cases. Review whether both apply and whether their effects interact for the actual roof geometry.
Corner drift is a two-face comparison helper only. Plan-based drift contour and overlap mapping are not automated.
Unbalanced gable loading, sliding snow, curved roofs, valleys and multiple interacting obstructions are not fully implemented in this prototype.
Climatic data are transcribed from NBCC 2020 Appendix C, Table C-2. Each entry applies at its tabulated elevation and Gazetteer-defined latitude/longitude; confirm the selected record against the source and applicable jurisdictional requirements.
For unlisted locations, particularly in complex terrain, do not interpolate from Table C-2. Obtain site-specific climatic values from Environment and Climate Change Canada.
Confirm provincial or territorial adoption and amendments applicable to the project.

Prepared by____________________________

Checked by____________________________

Project / calculation reference____________________________

Review date____________________________

DEVELOPMENT PROTOTYPE — INDEPENDENT ENGINEERING VERIFICATION REQUIRED. This software is not certified or validated for professional design use. Independently verify all climatic data, inputs, equations, assumptions, code interpretations and governing load cases before relying on any result for design, permitting, construction or life-safety decisions.

Need to make a change?Return to the relevant calculation tab. The report refreshes from the current inputs.

SnowSpec Canada · NBCC 2020 prototypePrototype calculation engine · Inputs are local to this browser

Canadian snow load calculator

Project and site selection

Selected site

Base roof snow load

Specified base roof load

Assumptions and load cases

Roof-mounted vertical projection

Local obstruction load

Multi-level roof / adjacent higher building

Multi-level drift profile

Multi-level roof result

Corner drift load selector

Scope of drift modules

Snow load calculation report

Roof Snow Load Calculation