PART 9 — HOUSING AND SMALL BUILDINGSUpdated for the 2024 Ontario Building Code
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9.23.16.7.Thickness or Rating
(1) The thickness or rating of roof sheathing on a flat roof used as a walking deck shall conform to either Table
9.23.15.5.-A or 9.23.15.5.-B for subfloors.
(2) The thickness or rating of roof sheathing on a roof not used as a walking deck shall conform to either
Table 9.23.16.7.-A or Table 9.23.16.7.-B.
(3) Asphalt-coated or asphalt-impregnated fibreboard not less than 11.1 mm thick conforming to CAN/ULC-S706.1,
“Standard for Wood Fibre Insulating Boards for Buildings,” is permitted to be used as a roof sheathing over supports
spaced not more than 400 mm o.c., provided the roofing consists of
(a) a continuous sheet of galvanized steel not less than 0.33 mm in thickness, or
(b) a continuous sheet of aluminum not less than 0.61 mm in thickness.
(4) All edges of sheathing described in Sentence (3) shall be supported by blocking or framing.
Table 9.23.16.7.-A
Thickness of Roof Sheathing
Forming Part of Sentence 9.23.16.7.(2)
Minimum Thickness, mm
Maximum Spacing of
Plywood and OSB, O-2 Grade OSB, O-1 Grade and Waferboard, R-1 Grade
Supports, mm Lumber
Edges Supported Edges Unsupported Edges Supported Edges Unsupported
300 7.5 7.5 9.5 9.5 17.0
400 7.5 9.5 9.5 11.1 17.0
600 9.5 12.5 11.1 12.7 19.0
Table 9.23.16.7.-B
Rating for Roof Sheathing When Applying CSA O325
Forming Part of Sentence 9.23.16.7.(2)
Panel Mark
Maximum Spacing of Supports, mm
Edges Supported Edges Unsupported
400 2R16 1R16
500 2R20 1R20
600 2R24 1R24
This provision includes a table — formatting is preserved from the source; refer to the official code for the authoritative layout.
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Code text is reproduced for reference from the Ontario Building Code (O. Reg. 163/24, 2024 Building Code Compendium). This page is provided for general information and is not an official copy. Always verify requirements against the official Ontario Building Code and confirm with your local building department.