Free Tool
Truss vs Rafter Span Checker
Pick your city, enter the roof span and see whether a site-built rafter works under the Ontario Building Code tables or whether the roof needs trusses.
Truss vs Rafter Span Checker
Result
We frame roofs across Ontario, site-built rafter roofs and engineered truss roofs, set and braced to the drawings.
Get a quote for roof framingPreliminary planning only. This tool does not replace the drawings. The designer or engineer of record and the building permit set govern every size, span and detail.
We frame roofs across Ontario, site-built rafter roofs and engineered truss roofs, set and braced to the drawings. Roof and truss framing. Request a free quote.
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Where the numbers come from
- Rafter spans. Tables 9.23.4.2.-F (specified roof snow loads 1.0 to 2.0 kPa) and 9.23.4.2.-G (2.5 and 3.0 kPa), Maximum Spans for Roof Rafters, 2024 Ontario Building Code (O. Reg. 163/24), Division B. (source)
- Snow load. Article 9.4.2.2., Specified Snow Loads: S = Cb x Ss + Sr, Cb 0.45 where the entire roof width does not exceed 4.3 m and 0.55 otherwise, not less than 1 kPa. Article 9.4.2.1. limits the clear span of any member to 12.2 m. (source)
- Ground snow and rain loads. SB-1 Table 2, Ss and Sr in kPa for each location. (source)
- Trusses. Article 9.23.14.11., Roof Trusses. (source)
- What the tables assume. Note A-9.23.4.2.: a rafter supports the roof sheathing and encloses an attic space but does not support a ceiling; spans are horizontal and snow loads are on the horizontal projection. (source)
Worked example
A Toronto roof spanning 15 ft (4.57 m) with Spruce-Pine-Fir No. 1 and No. 2 rafters at 400 mm. SB-1 gives Ss 0.9 kPa and Sr 0.4 kPa for the City Hall station. S = 0.55 x 0.9 + 0.4 = 0.90 kPa, raised to the 1.0 kPa minimum. In the 1.0 kPa column of Table 9.23.4.2.-F a 2x6 spans 4.45 m, too short, and a 2x8 spans 5.85 m, so a 2x8 rafter at 400 mm is allowed by the table for that span.
Snow columns
| Specified snow load S | Table and column used |
|---|---|
| Up to 1.0 kPa | Table 9.23.4.2.-F, 1.0 kPa |
| Over 1.0 to 1.5 kPa | Table 9.23.4.2.-F, 1.5 kPa |
| Over 1.5 to 2.0 kPa | Table 9.23.4.2.-F, 2.0 kPa |
| Over 2.0 to 2.5 kPa | Table 9.23.4.2.-G, 2.5 kPa |
| Over 2.5 to 3.0 kPa | Table 9.23.4.2.-G, 3.0 kPa |
| Over 3.0 kPa | Not in the tables: engineer or trusses |
Last checked 2026-11-09 against the 2024 Ontario Building Code (O. Reg. 163/24) compendium and SB-1 as published on ontario.ca.
This tool is for preliminary planning. The designer or engineer of record and the building permit set govern.
Frequently asked questions
How is the snow load found?
Article 9.4.2.2. gives the specified snow load as S = Cb x Ss + Sr, where Cb is 0.55 (0.45 if the whole roof width is 4.3 m or less), Ss is the 1-in-50 year ground snow load and Sr the associated rain load from SB-1 Table 2. It is never less than 1 kPa. The tool takes the next table column at or above that load: 1.0, 1.5, 2.0, 2.5 or 3.0 kPa.
What is the span?
The horizontal distance between the inside faces of the supports. Rafter tables measure spans horizontally, and the snow load is on the horizontal projection of the roof, so a steeper pitch does not shorten the span in the table.
When are trusses needed?
When no rafter size in Tables 9.23.4.2.-F and -G spans your roof at the spacing you chose, or the snow load is above 3.0 kPa, or the clear span passes 12.2 m. Wood trusses are designed under good engineering practice such as TPIC 2019, and the truss drawings govern (Article 9.23.14.11.).
Why not use roof joists?
Rafters enclose an attic and do not carry a ceiling. Roof joists that carry a ceiling use Tables 9.23.4.2.-D and -E. A roof with a ceiling hung from the rafters is a different case.
Why is my city not listed?
SB-1 warns that snow loads should not be interpolated for unlisted places. The tool lists the cities and towns we serve that appear in SB-1. For any other place, enter Ss and Sr from SB-1 Table 2 yourself.
Is this a quote?
No. It is a planning tool. Contact us with your plans and we will arrange a free site visit.
To see how a roof is laid out, read Roof Pitch and Rafter Length Calculator.
Related: Roof and truss framing, Roof Rafter Span Calculator, Roof Pitch and Rafter Length Calculator.
More free tools are on the framing tools page.
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We frame roofs across Ontario, site-built rafter roofs and engineered truss roofs, set and braced to the drawings.
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