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OBC DIVISION B · PART 9 · SECTION 9.23 — WOOD FRAME CONSTRUCTION

What are the notching and drilling limits for floor joists in Ontario?

Holes in wood floor joists must be no larger than one-quarter of the joist depth and kept at least 50 mm (2 in.) from the edges. Notches are only allowed on the top, no deeper than one-third the depth, and only near the bearing end, within about half the joist depth of the support.

Code reference
The OBC limits field modification of joists: drilled holes may not exceed one-quarter of the joist depth and must maintain not less than 50 mm of wood to any edge, while notches are restricted to the top of the member, to a depth of one-third of the joist depth, and to a zone close to the bearing point; members cut beyond these limits must be reinforced or sized accordingly.
OBC Division B, Subsection 9.23.5.

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

What this means in practice

These limits protect the parts of the joist doing the work: the top and bottom edges at midspan carry bending, the ends carry shear, so a modest hole through the neutral middle is tolerable while a bottom notch at midspan is a break waiting to happen. Plumbing stacks and HVAC trades are the usual offenders, especially 75 mm drains through 235 mm joists, which is already at the limit. Engineered I-joists follow their manufacturer's hole charts instead of these rules, and their flanges must never be cut. Doubling or adding plates can restore over-cut members, ideally with designer sign-off.

Plain language

Trades can drill holes through the middle of floor joists as long as the hole is no more than a quarter of the joist's depth and stays 50 mm from top and bottom. Cutting notches is far more restricted, top edge only, near the ends, never underneath in the middle of the span.

Related questions

What stud size and spacing does the Ontario Building Code require for walls?What are the wall bracing and sheathing requirements for houses 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.