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OBC DIVISION B · PART 9 · SECTION 9.27 — CLADDING

Do I need a rainscreen or air gap behind siding in Ontario?

Yes, in most cases. Exterior walls need a first plane of protection (the cladding) and a second plane (sheathing membrane and flashings), with a capillary break between them, commonly achieved with a drained and vented air space of at least 10 mm.

Code reference
Cladding assemblies must manage rain using two planes of protection: an outer cladding layer that sheds most water, and an inner drainage plane of sheathing membrane and flashing that intercepts what gets past. A capillary break, typically a drained and vented air space of not less than 10 mm or a cladding-and-material combination providing equivalent drainage, must separate the two planes so water cannot wick inward.
OBC Division B, Subsection 9.27.2.

Paraphrased for clarity — open this provision in the BUILDAiD Library for the exact wording.

What this means in practice

This is the rainscreen principle formalized in code. Strapping the wall with 10 mm or thicker furring over the building paper or housewrap creates the drainage cavity, and some drainage-wrap and drainage-mat products achieve the same function. Certain claddings, like vinyl siding, provide inherent drainage through their profile. The most common failures are blocked cavities at the bottom of walls, missing insect screening at openings, and flashings that dump water behind the membrane instead of out. Get the laps shingle-style, top over bottom, everywhere.

Plain language

Siding is not fully waterproof, so the code requires a backup layer behind it, usually housewrap plus a small air gap. Any rain that sneaks past the siding drains down and back outside instead of soaking the wall.

Related questions

How far above the ground does siding have to be in Ontario?What are the code rules for installing vinyl siding in Ontario?

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This page is provided for general information. Building code requirements depend on your specific project, occupancy and municipality, and code editions change. Always verify requirements against the current Ontario Building Code and confirm with your local building department.