← Ontario Building Code FAQ
OBC DIVISION B · PART 9 · SECTION 9.21 — MASONRY AND CONCRETE CHIMNEYS AND FLUES

What clearance does a masonry chimney need from combustible materials in Ontario?

Masonry chimneys generally require 50 mm (2 in.) clearance to combustible framing and materials for chimneys inside the building, reduced to about 12 mm (1/2 in.) for chimneys on an exterior wall, and the gap at floors and ceilings must be firestopped with noncombustible material.

Code reference
Combustible materials such as framing, sheathing and flooring must be kept clear of masonry and concrete chimneys, with a larger air space required where the chimney is within the building envelope and a smaller one permitted where the chimney is substantially outside. The clearance space at each floor and ceiling level must be sealed with noncombustible firestopping to block flame and smoke spread.
OBC Division B, Subsection 9.21.5.

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

What this means in practice

This clearance is about long-term pyrolysis: wood in sustained contact with warm masonry can char at surprisingly low temperatures over years. Typical violations found in renovations include joists notched into chimney masonry, subfloor run tight to the brick, and combustible framing packed against the chimney when chases are built. Trim and flooring rules at fireplaces are separate and covered under Section 9.22. When finishing a basement or attic around an existing chimney, frame a clearance gap, firestop it at each level with sheet metal or equivalent, and never use the chimney as a nailing surface.

Plain language

Keep wood framing about 50 mm away from an interior chimney, roughly 12 mm if the chimney runs up the outside wall, and seal the gap at each floor with metal or another material that cannot burn. Wood touching warm brick for years can slowly turn into a fire hazard.

Related questions

How high does a chimney have to extend above the roof in Ontario?What are the flue lining requirements for masonry chimneys in Ontario?

Get answers like this for any code question

BUILDAiD Assist searches the building code and answers in plain English with exact citations — in seconds.

Try BUILDAiD Beta

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.