Columns, crawl spaces, roof spaces, masonry, chimneys and fireplaces (House)

Last updated October 22, 2026

Values this chapter turns on

roof vent ratio, low slope or roof joists: 1/150; roof vent ratio, low slope or roof joists
roof vent ratio, normal slope: 1/300; roof vent ratio, normal slope
chimney height above roof contact, minimum: 900 mm; chimney height above roof contact, minimum
air space behind veneer on wood frame: 25 mm; air space behind veneer on wood frame

Sections 9.17 to 9.22 are short, number-heavy rules: columns, crawl space and attic venting, hatches, masonry, chimneys and site-built fireplaces. Most questions are lookups or a one-step calculation such as a vent area or hearth size.

Columns (9.17)

Section 9.17 covers columns under beams that carry not more than 2 wood-frame floors, where the joists bearing on the beam are not longer than 5 m and the live load does not exceed 2.4 kPa. It also covers columns for decks and carport roofs. Anything else is designed under Part 4 OBC Div. B 9.17.1.1..

Column type	Minimum size
Steel pipe	73 mm outside diameter, 4.76 mm wall
Wood, rectangular	140 mm x 140 mm, and not narrower than the supported member
Wood, round	184 mm diameter
Unit masonry	290 mm x 290 mm or 240 mm x 380 mm
Concrete	200 mm x 200 mm, or 230 mm diameter

Sources: 9.17.3.1., 9.17.4.1., 9.17.5.2., 9.17.6.2.. Smaller columns need calculations or proven capacity. Steel columns get end plates of at least 100 mm x 100 mm x 6.35 mm. Built-up wood columns use full-length 38 mm members, bolted or nailed. Separate wood columns from concrete in contact with the ground with 0.05 mm polyethylene or Type S roll roofing. Every column sits centred on a footing.

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Crawl spaces (9.18)

A crawl space is an enclosed space between the underside of a floor and the ground cover below, with a clearance less than 1,800 mm in height OBC Div. B 9.18.1.1.. It counts as heated if it is used as a warm air plenum, holds uninsulated heating ducts or pipes, or is not separated from heated space under Section 9.25.

Access opening: 500 mm x 700 mm when the crawl space serves one dwelling unit, 550 mm x 900 mm otherwise OBC Div. B 9.18.2.1..
Unheated crawl spaces vented naturally need 0.1 m² of unobstructed vent area for every 50 m² of floor area, spread on opposite sides OBC Div. B 9.18.3.1..
Access way to serviceable equipment: at least 600 mm high and wide.
Ground cover, unheated: 50 mm asphalt, 100 mm of 15 MPa concrete, Type S roll roofing or 0.10 mm polyethylene, with sheet joints lapped 100 mm. Heated: 0.15 mm polyethylene as part of the air barrier, joints lapped 300 mm, sealed at the perimeter OBC Div. B 9.18.6.2..
Only crawl spaces under 1-storey portions of a dwelling unit can be warm air plenums.

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Roof spaces (9.19)

Where insulation sits between a ceiling and the roof sheathing, leave a space above the insulation and vent it OBC Div. B 9.19.1.1.. The vent area depends on slope OBC Div. B 9.19.1.2.:

Roof	Unobstructed vent area
Normal slope	1/300 of the insulated ceiling area
Slope less than 1 in 6, or roofs built with roof joists	1/150 of the insulated ceiling area

Place at least 25% of the required vent area at the top of the space and 25% at the bottom, on opposite sides. Keep at least 63 mm between the insulation and the sheathing in a vented roof joist space. Where baffles are used at the eaves, they give at least a 25 mm air space and rise 50 mm above the insulation. The lower part of a mansard or gambrel roof need not be vented.

Attic access OBC Div. B 9.19.2.1.: a hatch is needed where the attic is at least 10 m² in area, 1,000 mm in length or width and 600 mm high, or where it contains a fuel-fired appliance. The hatch is 550 mm x 900 mm, but for a single dwelling unit it can drop to 0.32 m² with no side under 500 mm.

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Masonry and ICF walls (9.20)

Section 9.20 applies to unreinforced masonry and veneer not more than 11 m high on the foundation, and to flat ICF walls in buildings of not more than 2 storeys OBC Div. B 9.20.1.1.. A solid masonry unit has a net solid area of at least 75% of its gross area.

Item	Rule
Mortar joint thickness	10 mm
Masonry veneer	solid units at least 70 mm thick
Air space behind veneer on wood frame	at least 25 mm
Cavity width in cavity walls	50 mm to 150 mm
Weep hole spacing	not more than 800 mm
Steel lintel end bearing	at least 150 mm
End bearing of beams on masonry	at least 90 mm
Mortar and masonry temperature	at least 5°C, kept for 48 h

Sources: 9.20.4.1., 9.20.6.4., 9.20.6.2., 9.20.13.8., 9.20.8.3., 9.20.14.1.. Veneer ties are corrosion-resistant straps spaced per Table 9.20.9.5.: the wider the vertical spacing, the tighter the horizontal spacing (for example 400 mm vertical with 800 mm horizontal). Exterior masonry walls of 1-storey buildings and top storeys are at least 140 mm thick; lower storeys of 2-storey buildings are at least 190 mm. Laterally support masonry walls at no more than 20 times their thickness for loadbearing and exterior walls.

Mortar type follows Table 9.20.3.2.-A: Type S for loadbearing exterior walls and for foundation walls and chimneys at or below ground. Flashing goes under weep holes and sills and over openings where the frame head is further below the eave than one quarter of the overhang.

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Masonry chimneys (9.21)

Section 9.21 covers site-built masonry or concrete chimneys up to 12 m high serving fireplaces or appliances totalling 120 kW or less OBC Div. B 9.21.1.1.. A flue pipe is the pipe from the appliance flue collar to the chimney. A chimney liner is the conduit that lines the masonry.

A flue serving a fireplace serves nothing else. Appliances sharing a flue must be on the same storey.
Clay liners are at least 15.9 mm thick. Leave a 10 mm unfilled space between liner and masonry. Liners run from 200 mm below the lowest flue pipe connection to 50 mm to 100 mm above the cap.
Chimney walls are solid masonry at least 70 mm thick.
Height: at least 900 mm above the highest point of contact with the roof, and at least 600 mm above any roof surface or structure within 3 m OBC Div. B 9.21.4.4..
Clearance to combustible framing: 50 mm for interior chimneys, 12 mm for exterior ones; 150 mm from a cleanout OBC Div. B 9.21.5.1..

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Fireplaces (9.22)

Every solid fuel fireplace needs outdoor combustion air through a noncombustible duct of at least 100 mm diameter with a damper OBC Div. B 9.22.1.4.. Firebrick liners are at least 50 mm on the sides and back, 25 mm on the floor. The fire chamber is at least 300 mm deep.

Hearth extension OBC Div. B 9.22.5.1.: at least 400 mm in front of the opening and 200 mm past each side. If the fire chamber floor is raised more than 150 mm above the hearth, add 50 mm for a rise up to 300 mm, plus 25 mm for each further 50 mm. A rise of 400 mm gives 400 + 50 + 50 = 500 mm.

Clearances: no combustibles within 150 mm of the opening (300 mm above it where they project more than 38 mm). Keep 100 mm between the back and sides of the fireplace and combustible framing, 50 mm for the smoke chamber OBC Div. B 9.22.9.3..

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Common traps

Using the 1/300 vent ratio on a low-slope roof or one built with roof joists.
Quoting 550 mm x 900 mm for a house crawl space access.
Forgetting the 600 mm chimney rule for roofs within 3 m.
Confusing the 25 mm veneer air space with the 50 mm minimum cavity.

Chapter review

Chapter at a glance

One line per section: the idea to carry into the exam. Then the answers that most often catch candidates out.

Chapter at a glance: the key idea in each section
Section	Key idea
Columns (9.17)	Section 9.17 covers columns under beams that carry not more than 2 wood-frame floors, where the joists bearing on the beam are not longer than 5 m and the live load does not…
Crawl spaces (9.18)	A crawl space is an enclosed space between the underside of a floor and the ground cover below, with a clearance less than 1,800 mm in height.
Roof spaces (9.19)	Where insulation sits between a ceiling and the roof sheathing, leave a space above the insulation and vent it.
Masonry and ICF walls (9.20)	Section 9.20 applies to unreinforced masonry and veneer not more than 11 m high on the foundation, and to flat ICF walls in buildings of not more than 2 storeys.
Masonry chimneys (9.21)	Section 9.21 covers site-built masonry or concrete chimneys up to 12 m high serving fireplaces or appliances totalling 120 kW or less.
Fireplaces (9.22)	Every solid fuel fireplace needs outdoor combustion air through a noncombustible duct of at least 100 mm diameter with a damper.

Columns (9.17)
Section 9.17 covers columns under beams that carry not more than 2 wood-frame floors, where the joists bearing on the beam are not longer than 5 m and the live load does not…
Crawl spaces (9.18)
A crawl space is an enclosed space between the underside of a floor and the ground cover below, with a clearance less than 1,800 mm in height.
Roof spaces (9.19)
Where insulation sits between a ceiling and the roof sheathing, leave a space above the insulation and vent it.
Masonry and ICF walls (9.20)
Section 9.20 applies to unreinforced masonry and veneer not more than 11 m high on the foundation, and to flat ICF walls in buildings of not more than 2 storeys.
Masonry chimneys (9.21)
Section 9.21 covers site-built masonry or concrete chimneys up to 12 m high serving fireplaces or appliances totalling 120 kW or less.
Fireplaces (9.22)
Every solid fuel fireplace needs outdoor combustion air through a noncombustible duct of at least 100 mm diameter with a damper.

Usual exam traps

Using the 1/300 vent ratio on a low-slope roof or one built with roof joists.
Quoting 550 mm x 900 mm for a house crawl space access.
Forgetting the 600 mm chimney rule for roofs within 3 m.
Confusing the 25 mm veneer air space with the 50 mm minimum cavity.

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Questions on Columns, crawl spaces, roof spaces, masonry, chimneys and fireplaces, in the exam format, every answer explained and cited to its article. Sign in to start; a free account gets the free questions.

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Code cited in this chapter

Based on the official 2024 Building Code Compendium on ontario.ca.

OBC Div. B 9.17.1.1. Application
(1)This Section applies to columns used to support
(a)beams carrying loads from not more than 2 wood-frame floors where
(i)the supported length of joists bearing on such beams does not exceed 5 m, and
(ii)the live load on any floor does not exceed 2.4 kPa, (See Table 4.1.5.3.)
(b)beams or header joists carrying loads from not more than 2 levels of wood-frame balconies, decks or other accessible exterior platforms, or 1 level plus the roof, where
(i)the supported length of joists bearing on such beams or joists does not exceed 5 m,
(ii)the sum of the specified snow and occupancy loads does not exceed 4.8 kPa for the determination of load on platform-type constructions, and (See Sentence 9.4.2.3.(1))
(iii)the platform serves only a single suite of residential occupancy, or
(c)carport roofs. (See Section 9.35.)
(2)Columns for applications other than as described in Sentence (1) shall be designed in accordance with Part 4.
Open OBC Div. B 9.17.1.1. in the code viewer
OBC Div. B 9.17.3.1. Size and Thickness
(1)Except as permitted in Sentence (2), steel pipe columns shall have an outside diameter of not less than 73 mm and a wall thickness of not less than 4.76 mm.
(2)Columns of sizes other than as specified in Sentence (1) are permitted to be used where the loadbearing capacities are shown to be adequate.
Open OBC Div. B 9.17.3.1. in the code viewer
OBC Div. B 9.17.4.1. Column Sizes
(1)The width or diameter of a wood column shall be not less than the width of the supported member.
(2)Except as provided in Article 9.35.4.2., columns shall be not less than 184 mm for round columns and 140 mm by 140 mm for rectangular columns, unless calculations are provided to show that lesser sizes are adequate.
Open OBC Div. B 9.17.4.1. in the code viewer
OBC Div. B 9.17.5.2. Sizes
(1)Unit masonry columns shall be not less than 290 mm by 290 mm or 240 mm by 380 mm in size.
Open OBC Div. B 9.17.5.2. in the code viewer
OBC Div. B 9.17.6.2. Sizes
(1)Concrete columns shall be not less than 200 mm by 200 mm for rectangular columns and 230 mm diam for circular columns.
Open OBC Div. B 9.17.6.2. in the code viewer
OBC Div. B 9.18.1.1. Application
(1)In this Section, a crawl space refers to an enclosed space between the underside of a floor assembly and the ground cover directly below, with a clearance less than 1,800 mm in height.
Open OBC Div. B 9.18.1.1. in the code viewer
OBC Div. B 9.18.2.1. Access Openings
(1)An access opening of not less than 500 mm by 700 mm shall be provided to each crawl space where the crawl space serves a single dwelling unit, and not less than 550 mm by 900 mm for other crawl spaces.
(2)Access openings shall be fitted with a door or hatch, except when the crawl space is heated and the access opening into the crawl space is from an adjacent heated space.
Open OBC Div. B 9.18.2.1. in the code viewer
OBC Div. B 9.18.3.1. Ventilation of Unheated Crawl Spaces
(1)Unheated crawl spaces shall be ventilated by natural or mechanical means.
(2)Where an unheated crawl space is ventilated by natural means, ventilation shall be provided to the outside air by not less than 0.1 m² of unobstructed vent area for every 50 m² of floor area.
(3)Vents shall be
(a)uniformly distributed on opposite sides of the building, and
(b)designed to prevent the entry of snow, rain and insects.
Open OBC Div. B 9.18.3.1. in the code viewer

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OBC Div. B 9.18.6.2. Ground Cover in Heated Crawl Spaces
(1)Where a crawl space is heated, a ground cover consisting of not less than 0.15 mm polyethylene sheet conforming to CAN/CGSB-51.34-M, " Vapour Barrier, Polyethylene Sheet, for Use in Building Construction," shall be installed as part of an air barrier system in accordance with Subsection 9.25.3.
(2)The ground cover required in Sentence (1) shall have its joints lapped not less than 300 mm, and
(a)be sealed and evenly weighted down, or
(b)be covered with concrete not less than 50 mm thick.
(3)The perimeter of the ground cover required in Sentence (1) shall be sealed to the foundation wall. (See Notes A-9.13.4., A-9.25.3.4. and 9.25.3.6., and A-9.25.3.6.(2) and (3))
(4)All penetrations of the ground cover required in Sentence (1) shall be sealed against air leakage. (See Subsection 9.25.3.)
Open OBC Div. B 9.18.6.2. in the code viewer
OBC Div. B 9.19.1.1. Required Venting
(1)Except where it can be shown to be unnecessary, where insulation is installed between a ceiling and the underside of the roof sheathing, a space shall be provided between the insulation and the sheathing, and vents shall be installed to permit the movement of air from the space to the exterior. (See Note A-9.19.1.1.(1))
Open OBC Div. B 9.19.1.1. in the code viewer
OBC Div. B 9.19.1.2. Vent Requirements
(1)Except as provided in Sentence (2), the unobstructed vent area shall be not less than 1/300 of the insulated ceiling area.
(2)Where the roof slope is less than 1 in 6 or in roofs that are constructed with roof joists, the unobstructed vent area shall be not less than 1/150 of the insulated ceiling area.
(3)Required vents may be roof type, eave type, gable-end type or any combination thereof, and shall be distributed
(a)uniformly on opposite sides of the building,
(b)with not less than 25% of the required openings located at the top of the space, and
(c)with not less than 25% of the required openings located at the bottom of the space.
(4)Except where each roof joist space referred to in Sentence (2) is separately vented, roof joist spaces shall be interconnected by installing purlins not less than 38 mm by 38 mm on the top of the roof joists.
(5)Vents shall comply with CAN3-A93-M, "Natural Airflow Ventilators for Buildings."
Open OBC Div. B 9.19.1.2. in the code viewer
OBC Div. B 9.19.2.1. Access
(1)Every attic or roof space shall be provided with an access hatch where the attic or roof space
(a)measures not less than
(i)10 m² in area,
(ii)1,000 mm in length or width, and
(iii)600 mm in height over at least the area described in Subclauses (i) and (ii), or
(b)contains a fuel-fired appliance. (See Note A-9.19.2.1.(1))
(2)The hatch required in Sentence (1) shall be not less than 550 mm by 900 mm except that, where the hatch serves not more than one dwelling unit, the hatch may be reduced to 0.32 m² in area with no dimension less than 500 mm. (See Note A-9.19.2.1.(2))
(3)Hatchways to attic or roof spaces shall be fitted with doors or covers.
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OBC Div. B 9.20.1.1. General
(1)Except as provided in Article 9.20.1.2., this Section applies to
(a)unreinforced masonry and masonry veneer walls not in contact with the ground, where
(i)the height of the walls constructed on the foundation walls does not exceed 11 m, and
(ii)the roof or floor assembly above the first storey is not of concrete construction, and
(b)flat insulating concrete form walls not in contact with the ground that (See Note A-9.15.1.1.(1) (c) and 9.20.1.1.(1)(b))
(i)have a maximum floor-to-floor height of 3 m,
(ii)are erected in buildings not more than 2 storeys in building height, and
(iii)are erected in locations where the seismic spectral response acceleration, S_a(0.2) , is not greater than 0.4. (See Note A-9.20.1.2.)
(2)For walls other than those described in Sentence (1), or where the masonry walls or insulating concrete form walls not in contact with the ground are designed for specified loads on the basis of ultimate and serviceability limit states, Subsection 4.3.2. shall apply.
Open OBC Div. B 9.20.1.1. in the code viewer
OBC Div. B 9.20.4.1. Thickness
(1)Except as provided in Sentence (2), mortar joint thickness for burned clay brick and concrete masonry units shall be 10 mm.
(2)Permitted tolerances in head and bed joints shall be not more than -5 mm to +10 mm.
Open OBC Div. B 9.20.4.1. in the code viewer
OBC Div. B 9.20.6.4. Masonry Veneer
(1)Except for masonry veneer where each masonry unit is supported individually by the structural backing, masonry veneer shall consist of solid masonry units not less than 70 mm thick.
(2)Veneer described in Sentence (1) over wood-frame walls shall have not less than a 25 mm air space behind the veneer.
(3)Masonry veneer less than 90 mm thick shall have unraked joints.
(4)Masonry veneer shall conform to Subsection 4.3.2. where the masonry units are required to be individually supported by the structural backing.
Open OBC Div. B 9.20.6.4. in the code viewer
OBC Div. B 9.20.6.2. Cavity Walls
(1)Cavity walls shall be made with not less than 90 mm wide units if the joints are raked and not less than 75 mm wide units if the joints are not raked.
(2)The width of a cavity in a cavity wall shall be not less than 50 mm and not greater than 150 mm.
(3)The minimum thickness of cavity walls above the supporting base shall be 230 mm for the top 7.6 m and 330 mm for the remaining portion, except that where 75 mm wide units are used, the wall height above the top of the foundation wall shall not exceed 6 m.
Open OBC Div. B 9.20.6.2. in the code viewer
OBC Div. B 9.20.13.8. Required Weep Holes
(1)Weep holes spaced not more than 800 mm apart shall be provided at the bottom of
(a)cavities in cavity walls, and
(b)cavities or air spaces in masonry veneer walls.
(2)The cavities or air spaces described in Sentence (1) shall include those above lintels over window and door openings required to be flashed in conformance with Article 9.20.13.3.
(3)The weep holes required in Sentence (1) shall be in a location such that any water that collects in the cavity or space will be directed to the exterior of the building.
Open OBC Div. B 9.20.13.8. in the code viewer
OBC Div. B 9.20.8.3. Bearing of Beams and Joists
(1)The bearing area under beams and joists shall be sufficient to carry the supported load.
(2)In no case shall the minimum length of end bearing of beams supported on masonry be less than 90 mm.
(3)The length of end bearing of floor, roof or ceiling joists supported on masonry shall be not less than 40 mm.
Open OBC Div. B 9.20.8.3. in the code viewer
OBC Div. B 9.20.14.1. Laying Temperature of Mortar and Masonry
(1)Mortar and masonry shall be maintained at a temperature not below 5°C during installation and for not less than 48 h after installation.
(2)No frozen material shall be used in mortar mix.
Open OBC Div. B 9.20.14.1. in the code viewer
OBC Div. B Table 9.20.9.5. Veneer Tie Spacing - Forming Part of Sentence 9.20.9.5.(1)
Table 9.20.9.5. Veneer Tie Spacing - Forming Part of Sentence 9.20.9.5.(1) Forming Part of Sentence 9.20.9.5.(1) Maximum Vertical Spacing, mm | Maximum Horizontal Spacing, mm 400 | 800 500 | 600 600 | 400
Open OBC Div. B Table 9.20.9.5. in the code viewer
OBC Div. B Table 9.20.3.2-A. Mortar Use — Forming Part of Sentence 9.20.3.2.(1)
Table 9.20.3.2.-A — Mortar Use — Forming Part of Sentence 9.20.3.2.(1) Forming Part of Sentence 9.20.3.2.(1) Location | Building Element | Mortar Type Exterior, Above Ground | Loadbearing walls and columns | S Exterior, Above Ground | Non-loadbearing walls and columns | N or S Exterior, Above Ground | Parapets, chimneys and masonry veneer | N or S Exterior, At or Below Ground | Foundation walls and chimneys | S Interior | Loadbearing walls and columns | N Interior | Non-loadbearing walls and columns | N
Open OBC Div. B Table 9.20.3.2-A. in the code viewer
OBC Div. B 9.21.1.1. Application
(1)This Section applies to,
(a)rectangular masonry or concrete chimneys not more than 12 m in height serving fireplaces or serving appliances having a combined total rated heat output of 120 kW or less, and
(b)flue pipes serving solid fuel-burning appliances.
(2)Reserved.
(3)Except as provided in Sentence 9.21.1.3.(1), chimneys (other than those described in Sentence (1) and Sentence 9.21.1.2.(1)), gas vents and flue pipes serving gas-, oil- or solid fuel-burning appliances and associated equipment shall conform to Section 6.3.
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OBC Div. B 9.21.4.4. Height of Chimney Flues
(1)A chimney flue shall extend not less than
(a)900 mm above the highest point at which the chimney comes in contact with the roof, and
(b)600 mm above the highest roof surface or structure within 3 m of the chimney. (See Note A-9.21.4.4.(1))
Open OBC Div. B 9.21.4.4. in the code viewer
OBC Div. B 9.21.5.1. Clearance from CombustibleMaterials
(1)The clearance between masonry or concrete chimneys and combustible framing material shall be not less than,
(a)50 mm for interior chimneys, and
(b)12 mm for exterior chimneys. (See Note A-9.21.5.1.(1))
(2)A clearance of not less than 150 mm shall be provided between a cleanout opening and combustible material.
(3)Combustible flooring, subflooring and ceiling finishes shall have not less than a 12 mm clearance from masonry or concrete chimneys.
Open OBC Div. B 9.21.5.1. in the code viewer
OBC Div. B 9.22.1.4. Combustion Air
(0.1)Every solid fuel-fired fireplace, including a factory-built fireplace, shall have a supply of combustion air from outdoors in accordance with Sentences (0.2) to (1).
(0.2)The combustion air shall be supplied by a noncombustible and corrosion-resistant supply duct.
(0.3)The supply duct shall have
(a)a diameter of not less than 100 mm or equivalent area, and
(b)an exterior intake for entry of air from the outdoors.
(0.4)The supply duct shall contain a tight-fitting damper that shall be located close to the interior outlet and be operable from the room containing the fireplace.
(0.5)The operating mechanism shall clearly indicate the actual position of the damper.
(0.6)The interior outlet shall
(a)be located as close as possible to the opening in the face of the fireplace, and
(b)be designed to prevent embers from entering the supply duct.
(1)Where a supply of combustion air is provided directly to the fire chamber of a fireplace, including a factory-built fireplace, the installation shall comply with the "Outdoor Air Supply" requirements provided in CAN/CSA-A405-M, "Design and Construction of Masonry Chimneys and Fireplaces."
Open OBC Div. B 9.22.1.4. in the code viewer
OBC Div. B 9.22.5.1. Hearth Extension
(1)Except as required in Sentence (2), fireplaces shall have a noncombustible hearth extending not less than 400 mm in front of the fireplace opening measured from the facing, and not less than 200 mm beyond each side of the fireplace opening.
(2)Where the fire chamber floor is elevated more than 150 mm above the hearth, the dimension of the hearth measured perpendicular to the plane of the fireplace opening shall be increased by not less than
(a)50 mm for an elevation above 150 mm and not more than 300 mm, and
(b)an additional 25 mm for every 50 mm in elevation above 300 mm.
Open OBC Div. B 9.22.5.1. in the code viewer
OBC Div. B 9.22.9.3. Clearance to Combustible Framing
(1)Not less than a 100 mm clearance shall be provided between the back and sides of a fireplace and combustible framing, except that a 50 mm clearance is permitted where the fireplace is located in an exterior wall.
(2)Not less than a 50 mm clearance shall be provided between the back and sides of the smoke chamber of a fireplace and combustible framing, except that a 25 mm clearance is permitted where the fireplace is located in an exterior wall.
Open OBC Div. B 9.22.9.3. in the code viewer