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Snow Load and the Ontario Building Code

How to work out the specified snow load for a Part 9 roof under the 2024 Ontario Building Code: S = Cb x Ss + Sr, SB-1 town values, the 1 kPa minimum.
the Part 9 specified snow load formula
S = Cb x Ss + Sr
the Part 9 specified snow load formula
minimum specified snow load, any roof
1 kPa
minimum specified snow load, any roof
Cb for roofs wider than 4.3 m (0.45 if narrower)
0.55
Cb for roofs wider than 4.3 m (0.45 if narrower)
minimum deck load, or roof snow load if greater
1.9 kPa
minimum deck load, or roof snow load if greater

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Under the 2024 Ontario Building Code, the specified snow load on a Part 9 roof is S = Cb x Ss + Sr: the 1-in-50 year ground snow load for your location times a roof factor of 0.45 (whole roof 4.3 m wide or less) or 0.55 (all other roofs), plus the associated rain load (OBC Div. B 9.4.2.2.OBC Div. B 9.4.2.2. Specified Snow Loads(1) Except as provided in Sentences (2) and (4), specified snow loads shall be not less than those calculated using the following formula: S = C_bS_s + S_r where S = specified snow load, C_b = basic snow load roof factor, which is 0.45 where the entire width of a roof does not exceed 4.3 m and 0.55 for all other…). Ss and Sr for your town come from Table 2 of Supplementary Standard SB-1, and the result is never less than 1 kPa.

This page explains the formula, how to find your town, and when the simple Part 9 method stops and Part 4 takes over. Your building department confirms the climatic values it accepts for your site.

Snow load at a glance

Item What the code does Where to look
Formula for Part 9 roofs S = Cb x Ss + Sr 9.4.2.2.(1)
Cb, roof 4.3 m wide or less 0.45 9.4.2.2.(1)
Cb, all other roofs 0.55 9.4.2.2.(1)
Floor value S is never less than 1 kPa 9.4.2.2.(2)
Where Ss and Sr come from SB-1, Table 2, by location 9.4.1.1.(3)
Decks and balconies of one dwelling unit roof snow load or 1.9 kPa, whichever is greater 9.4.2.3.(1)
Big roof steps (over 2 m, large flat upper roof) 1.5 x S near the step 9.4.2.2.(4) and (5)
Long-span arch or bow string trusses (over 6 m) design to Part 4 snow rules 9.4.2.2.(3)

This is our summary of the articles. Read the full sentences before you rely on a number.

The formula in plain words

Start with Ss, the ground snow load that has a 1-in-50 chance of being exceeded in any year at your location. Roofs usually hold less snow than the ground because wind blows some of it away, so Part 9 multiplies Ss by the basic snow load roof factor, Cb: 0.45 where the entire width of the roof does not exceed 4.3 m, and 0.55 for every other roof (9.4.2.2.(1)OBC Div. B 9.4.2.2. Specified Snow Loads(1) Except as provided in Sentences (2) and (4), specified snow loads shall be not less than those calculated using the following formula: S = C_bS_s + S_r where S = specified snow load, C_b = basic snow load roof factor, which is 0.45 where the entire width of a roof does not exceed 4.3 m and 0.55 for all other…). Most house roofs are wider than 4.3 m, so 0.55 is the usual value; 0.45 suits narrow roofs such as a small porch or a lean-to.

Then add Sr, the 1-in-50 year rain load that falls on the snow pack. SB-1 explains that the heaviest loads often come when snow is soaked by rain, which is why Sr is added in full rather than factored (SB-1, snow loadsOBC SB-1 Snow LoadsThe roof of a building should be able to support the greatest weight of snow that is likely to accumulate on it in many years. Some observations of snow on roofs have been made in Canada, but not enough to form the basis for estimating roof snow loads throughout the country. Similarly, observations of the weight, or…).

Finally, check the floor: the specified snow load is never less than 1 kPa (9.4.2.2.(2)OBC Div. B 9.4.2.2. Specified Snow Loads(1) Except as provided in Sentences (2) and (4), specified snow loads shall be not less than those calculated using the following formula: S = C_bS_s + S_r where S = specified snow load, C_b = basic snow load roof factor, which is 0.45 where the entire width of a roof does not exceed 4.3 m and 0.55 for all other…). In the mildest parts of southern Ontario the formula comes out under 1 kPa, so 1 kPa governs.

Worked examples with SB-1 values

All values below are read from SB-1 Table 2, Climatic Design DataOBC SB-1 Table 2: Climatic Design DataTable 2: Climatic Design Data Location | Elevation, m | Design Temperature — January — 2.5%, °C | Design Temperature — January — 1%, °C | Design Temperature — July 2.5% — Dry, °C | Design Temperature — July 2.5% — Wet, °C | Degree Days Below 18°C | 15 Min Rain, mm | One Day Rain, 1/50, mm | Annual Rain, mm | Moisture…, for a roof wider than 4.3 m (Cb = 0.55).

Location in SB-1 Ss (kPa) Sr (kPa) 0.55 x Ss + Sr Specified snow load S
Toronto (City Hall) 0.9 0.4 0.895 1.0 kPa (the minimum governs)
Ottawa (City Hall) 2.4 0.4 1.72 1.72 kPa
Sault Ste. Marie 3.1 0.4 2.105 about 2.1 kPa

Same Ottawa house, but a narrow 4 m wide porch roof: 0.45 x 2.4 + 0.4 = 1.48 kPa. The roof factor alone changes the answer by about 0.24 kPa, which is exactly the kind of detail an exam question tests.

How to find your town in SB-1

  1. Open Volume 2 of the 2024 Building Code Compendium and go to SB-1, "Climatic and Seismic Data". Part 9 sends you there for snow, wind and seismic values (9.4.1.1.(3)OBC Div. B 9.4.1.1. General(1) Subject to the application limitations defined elsewhere in this Part, structural members and their connections shall (a) conform to requirements provided elsewhere in this Part, (b) be designed according to good engineering practice such as provided in the CWC 2014, "Engineering Guide for Wood Frame…).
  2. Go to Table 2, Climatic Design Data. Locations are listed alphabetically. Some cities have several rows: Ottawa has separate rows for City Hall, Barrhaven, Kanata, Orleans and the airport, and Toronto has rows for City Hall, Etobicoke, North York and Scarborough.
  3. Read the two snow columns near the right side of the table: Ss and Sr, both 1-in-50 values in kPa. Do not confuse them with the hourly wind pressure columns next to them.
  4. If your location is not listed, SB-1 says not to interpolate, especially in complex terrain, and points to Environment and Climate Change Canada for values. It also notes that municipal building authorities may accept values based on local experience (SB-1 introductionOBC SB-1 IntroductionThe great diversity of climate in Ontario has a considerable effect on the performance of buildings; consequently, building design must reflect this diversity. This Supplementary Standard briefly describes how climatic design values are computed and provides recommended design data for a number of cities, towns, and…). In practice, ask your building department which row it uses.

SB-1 also notes that the listed Ss values carry an uncertainty of about 20%, so use the row your building department accepts rather than a value from a neighbouring town.

What the snow load is used for

In Part 9, the specified snow load picks the column of the roof span tables. Roof joist and rafter tables are grouped by specified roof snow load: Tables 9.23.4.2.-D and -F cover 1.0, 1.5 and 2.0 kPa, and Tables -E and -G cover 2.5 and 3.0 kPa (9.23.4.2.OBC Div. B 9.23.4.2. Spans for Joists, Rafters and Beams(1) Except as required in Sentence (2) and Article 9.23.14.10., spans for wood joists and rafters shall conform to the spans shown in Span Tables 9.23.4.2.-A to 9.23.4.2-G for the uniform live loads shown in the Tables. (See Article 9.4.2.2.) (2) Spans for floor joists that are not selected from Span Tables…). Ridge beams and lintels in Table 9.23.4.2.-L are also read by snow load. The tables give steps, not every value, so use the block equal to S or the next one above it. In the Ottawa example, 1.72 kPa means reading the 2.0 kPa block. The joist, rafter and beam spans method walks through this step by step.

Decks are the other common use. A deck serving one dwelling unit must carry the specified roof snow load or 1.9 kPa, whichever is greater (9.4.2.3.(1)OBC Div. B 9.4.2.3. Platforms Subject to Snow and Occupancy Loads(1) Balconies, decks and other accessible exterior platforms intended for an occupancy and subject to snow loads shall be designed to carry the specified roof snow load or 1.9 kPa, whichever is greater, where the platform, or each segregated area of the platform, serves a single dwelling unit. (See Note A-9.4.2.3.(1))). In Sault Ste. Marie, 2.1 kPa beats 1.9 kPa. See deck building code in Ontario for footings, spans and guards.

Where Part 4 takes over

The simple formula is only for light-frame construction that fits 9.4.2.1.OBC Div. B 9.4.2.1. Application(1) This Subsection applies to light-frame construction whose wall, floor and roof planes are generally comprised of frames of small repetitive structural members, and where (a) the roof and wall planes are clad, sheathed or braced on at least one side, (b) the small repetitive structural members are spaced not more…: repetitive members at not more than 600 mm o.c., clear spans of not more than 12.2 m, total roof area of not more than 4,550 m2, roof and wall planes clad or braced on at least one side, and no closely spaced obstructions such as parapets on flat roofs. Outside those limits, and for anything that is not a Part 9 building, the designer uses Subsection 4.1.6.

Part 4's formula is fuller: S = Is[Ss(Cb x Cw x Cs x Ca) + Sr], adding an importance factor, a wind exposure factor, a slope factor and an accumulation factor for drifting (OBC Div. B 4.1.6.2.OBC Div. B 4.1.6.2. Specified Snow Load(1) The specified load, S, due to snow and associated rain accumulation on a roof or any other building surface subject to snow accumulation shall be calculated from the formula, S = I_s[S_s(C_bC_wC_sC_a) + S_r] where I_s = importance factor for snow load as provided in Table 4.1.6.2.-A, S_s = 1-in-50-year ground snow…). Its basic roof factor starts at 0.8 and depends on the roof size, rather than the fixed 0.45 or 0.55 of Part 9. Two Part 9 cases already reach for Part 4 rules: bow string, arch or semi-circular trusses spanning more than 6 m (9.4.2.2.(3)OBC Div. B 9.4.2.2. Specified Snow Loads(1) Except as provided in Sentences (2) and (4), specified snow loads shall be not less than those calculated using the following formula: S = C_bS_s + S_r where S = specified snow load, C_b = basic snow load roof factor, which is 0.45 where the entire width of a roof does not exceed 4.3 m and 0.55 for all other…), and a lower roof below a roof step more than 2 m high where the upper roof is nearly flat and larger than 600 m2, which needs 1.5 times S within the drift length (9.4.2.2.(4)OBC Div. B 9.4.2.2. Specified Snow Loads(1) Except as provided in Sentences (2) and (4), specified snow loads shall be not less than those calculated using the following formula: S = C_bS_s + S_r where S = specified snow load, C_b = basic snow load roof factor, which is 0.45 where the entire width of a roof does not exceed 4.3 m and 0.55 for all other…).

Common mistakes

  1. Using the ground snow load as the roof load. Ss is not S; multiply by Cb and add Sr.
  2. Forgetting the 1 kPa floor in Toronto, Windsor and other low-snow areas.
  3. Using 0.45 for a whole house. The 0.45 factor is only for roofs whose entire width is 4.3 m or less.
  4. Rounding down to a lower table block. 1.72 kPa needs the 2.0 kPa column, not 1.5.
  5. Reading the wrong SB-1 row for cities with several locations.
  6. Applying the Part 9 formula to a large or unusual roof that fails 9.4.2.1.

How to look it up yourself

In the 2024 Compendium, Volume 1, Division B, go to Part 9, Section 9.4, "Structural Requirements", Subsection 9.4.2. In Volume 2, open SB-1. Search terms that appear in the text:

  • "Specified Snow Loads" (the heading of 9.4.2.2.)
  • "basic snow load roof factor"
  • "1-in-50 year ground snow load"
  • "Climatic Design Data" (SB-1 Table 2)
  • "whichever is greater" together with "1.9 kPa" (decks, 9.4.2.3.)

BCIN exam angle

Snow load questions appear on the House and Small Buildings exams, usually as the first step of a roof span question: find the town in SB-1, apply Cb, add Sr, check 1 kPa, then pick the table column. Building Structural goes further into Part 4. Study the House structural chapter and the Small Buildings framing chapter, or the Building Structural snow, rain and wind chapter. Read the article itself in the code viewer at 9.4.2.2., then practise with the free quiz on the House exam or Small Buildings exam page.

Based on the official 2024 Building Code Compendium on ontario.ca.

© King's Printer for Ontario, 2024. Reproduced with permission.

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Frequently asked questions

How do you calculate snow load in Ontario?
For a Part 9 roof, multiply the ground snow load Ss for your location by the basic roof snow load factor Cb (0.45 if the whole roof is 4.3 m wide or less, 0.55 for every other roof), then add the rain load Sr. Ss and Sr come from Supplementary Standard SB-1. The result can never be less than 1 kPa (Division B, Article 9.4.2.2.).
What is the snow load in Toronto?
SB-1 lists Toronto (City Hall) with Ss = 0.9 kPa and Sr = 0.4 kPa. For a typical house roof, 0.55 x 0.9 + 0.4 works out to about 0.9 kPa, which is below the code's floor, so the specified snow load is 1 kPa. Other parts of Toronto (Etobicoke, North York, Scarborough) have slightly higher Ss values in SB-1.
Where do I find the snow load for my town?
In Table 2, Climatic Design Data, of MMAH Supplementary Standard SB-1, in Volume 2 of the 2024 Building Code Compendium. Each listed location has a ground snow load (Ss) and an associated rain load (Sr). SB-1 says not to interpolate for unlisted places, especially in complex terrain; values for unlisted locations come from Environment and Climate Change Canada, and your building department may use local values.
What snow load does a deck have to carry?
A deck, balcony or other exterior platform serving one dwelling unit must carry the specified roof snow load or 1.9 kPa, whichever is greater (Division B, Article 9.4.2.3.).
When do I need Part 4 for snow load?
The Part 9 snow formula applies only to light-frame construction within the limits of Article 9.4.2.1. (members at 600 mm o.c. or less, clear spans up to 12.2 m, total roof area up to 4,550 m2, and so on). Bow string, arch or semi-circular trusses spanning more than 6 m must use Subsection 4.1.6., and buildings outside Part 9 use the full Part 4 formula.