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Code guide

Basement Framing and the Ontario Building Code

Framing a basement under the 2024 Ontario Building Code: wood on concrete, bottom plates, stud sizes, notching limits, fire blocks, insulation and air barrier.
poly between untreated wood and concrete
0.05 mm
poly between untreated wood and concrete
height above ground with no separator needed
150 mm
height above ground with no separator needed
of a loadbearing stud's depth must stay intact
2/3
of a loadbearing stud's depth must stay intact
air space at joist ends in concrete
12 mm
air space at joist ends in concrete

Last updated

Framing a basement under the 2024 Ontario Building Code starts with keeping wood away from damp concrete: untreated wood on a slab or foundation in contact with the ground needs 0.05 mm polyethylene or Type S roll roofing under it, unless it is at least 150 mm above the ground (OBC Div. B 9.23.2.3.OBC Div. B 9.23.2.3. Protection from Dampness(1) Except as permitted in Sentence (2), wood framing members that are not pressure-treated with a wood preservative and that are supported on concrete in contact with the ground or fill shall be separated from the concrete by not less than 0.05 mm polyethylene film or Type S roll roofing. (2) Dampproofing material…). Studs follow Table 9.23.10.1.OBC Div. B Table 9.23.10.1. Size and Spacing of Studs — Forming Part of Sentence 9.23.10.1.(1)Table 9.23.10.1. Size and Spacing of Studs — Forming Part of Sentence 9.23.10.1.(1) Type of Wall | Supported Loads (including dead loads) | Minimum Stud Size, mm | Maximum Stud Spacing, mm | Maximum Unsupported Height, m Interior | No load | 38 × 38 | 400 | 2.4 Interior | No load | 38 × 89 flat[1] | 400 | 3.6 Interior…, every wall needs a bottom plate (9.23.11.2.OBC Div. B 9.23.11.2. Bottom Wall Plates(1) A bottom wall plate shall be provided in all cases. (2) The bottom plate in exterior walls shall not project more than one-third the plate width over the support.), and loadbearing studs can lose no more than one-third of their depth to notches and holes (9.23.5.3.OBC Div. B 9.23.5.3. Wall Studs(1) Wall studs shall not be notched, drilled or otherwise damaged so that the undamaged portion of the stud is less than two-thirds the depth of the stud if the stud is loadbearing or 40 mm if the stud is non-loadbearing, unless the weakened studs are suitably reinforced.).

This page covers framing for finishing a basement in a house. For ceiling heights, bedrooms, egress windows and alarms, see our basement requirements guide. Finishing a basement usually needs a permit, and your building department decides how the code applies.

Basement framing rules at a glance

Item Rule in short Where to look
Wood on concrete poly or Type S roll roofing, unless treated or 150 mm up 9.23.2.3.
Joist and beam ends in concrete treated, or a 12 mm air space 9.23.2.2.
Termite areas treated wood or 450 mm clearance; barriers behind insulation 9.3.2.9.
Bottom plate required in every wall 9.23.11.2.(1)
Plate size 38 mm thick, stud width; 19 mm bottom plate if non-loadbearing 9.23.11.1.
Stud size and spacing Table 9.23.10.1., by wall type and load 9.23.10.1.
Notching and drilling keep 2/3 of a loadbearing stud, 40 mm of a non-loadbearing stud 9.23.5.3.
Unfinished loadbearing walls mid-height blocking or strapping 9.23.10.2.(1)
Fire blocks between interior and exterior wall concealed spaces 9.10.16.1.(1)
Insulation full height of the foundation wall 9.25.2.3.(4)
Air barrier continuous throughout the basement 9.25.3.1.(2)

This is our summary of the articles, not a copy of the code. Read the full text before you frame.

Keep wood off damp concrete

Concrete in contact with the ground wicks moisture. Wood framing that is not pressure-treated and is supported on that concrete must be separated from it by at least 0.05 mm polyethylene film or Type S roll roofing (9.23.2.3.(1)OBC Div. B 9.23.2.3. Protection from Dampness(1) Except as permitted in Sentence (2), wood framing members that are not pressure-treated with a wood preservative and that are supported on concrete in contact with the ground or fill shall be separated from the concrete by not less than 0.05 mm polyethylene film or Type S roll roofing. (2) Dampproofing material…). No separator is needed where the wood is at least 150 mm above the ground (9.23.2.3.(2)OBC Div. B 9.23.2.3. Protection from Dampness(1) Except as permitted in Sentence (2), wood framing members that are not pressure-treated with a wood preservative and that are supported on concrete in contact with the ground or fill shall be separated from the concrete by not less than 0.05 mm polyethylene film or Type S roll roofing. (2) Dampproofing material…). Many framers simply use a pressure-treated bottom plate, which meets the rule on its own.

Where joists or beams frame into a concrete or masonry wall with their bottom at or below ground level, the ends must be treated against decay or have a 12 mm air space at the end and sides, and insulation or vapour barrier must not block that space (9.23.2.2.OBC Div. B 9.23.2.2. Protection from Decay(1) Ends of wood joists, beams and other members framing into masonry or concrete shall be treated to prevent decay where the bottom of the member is at or below ground level, or a 12 mm air space shall be provided at the end and sides of the member. (2) Air spaces required in Sentence (1) shall not be blocked by…).

In areas where termites are known to occur, wood near the ground needs pressure treatment or clearance, and finished or insulated foundations need a termite barrier (9.3.2.9.OBC Div. B 9.3.2.9. Termite and Decay Protection(1) In localities where termites are known to occur, (a) clearance between structural wood elements and the finished ground level directly below them shall be not less than 450 mm and, except as provided in Sentence (2), all sides of the supporting elements shall be visible to permit inspection, or (b) structural wood…). Ask your building department whether your area is one of them.

Plates and studs

Every wall needs a bottom plate (9.23.11.2.(1)OBC Div. B 9.23.11.2. Bottom Wall Plates(1) A bottom wall plate shall be provided in all cases. (2) The bottom plate in exterior walls shall not project more than one-third the plate width over the support.). Plates must be at least 38 mm thick and at least as wide as the studs, but a non-loadbearing wall may use a 19 mm bottom plate (9.23.11.1.OBC Div. B 9.23.11.1. Size of Wall Plates(1) Except as provided in Sentence (2), wall plates shall be (a) not less than 38 mm thick, and (b) not less than the required width of the wall studs. (2) In non-loadbearing walls and in loadbearing walls where the studs are located directly over framing members, the bottom wall plate may be 19 mm thick.).

Stud size and spacing come from Table 9.23.10.1.OBC Div. B Table 9.23.10.1. Size and Spacing of Studs — Forming Part of Sentence 9.23.10.1.(1)Table 9.23.10.1. Size and Spacing of Studs — Forming Part of Sentence 9.23.10.1.(1) Type of Wall | Supported Loads (including dead loads) | Minimum Stud Size, mm | Maximum Stud Spacing, mm | Maximum Unsupported Height, m Interior | No load | 38 × 38 | 400 | 2.4 Interior | No load | 38 × 89 flat[1] | 400 | 3.6 Interior…. For an interior wall with no load, it allows 38 × 38 mm studs at 400 mm spacing up to 2.4 m high, or 38 × 89 mm studs laid flat at 400 mm up to 3.6 m. Most basement partitions use 38 × 64 mm or 38 × 89 mm studs on edge for strength and room for wiring. Loadbearing walls, such as a new wall that replaces a beam or post, need the larger sizes and closer spacing the table gives for their loads, and changing a structural support usually needs drawings.

Interior loadbearing walls that are not finished with drywall or panels need blocking or strapping at mid-height so the studs cannot buckle sideways (9.23.10.2.(1)OBC Div. B 9.23.10.2. Bracing and Lateral Support(0.1) Except as provided in Sentence (0.2), each exterior wall in each storey shall be braced with at least one diagonal brace conforming to Sentence (0.3). (0.2) Bracing is not required where the walls (a) have an interior finish conforming to the requirements of Section 9.29., or (b) where the walls are (i) clad…).

Notching and drilling for services

Running wires, pipes and ducts through studs is allowed within limits. The undamaged part of a loadbearing stud must stay at least two-thirds of its depth, and of a non-loadbearing stud at least 40 mm, unless the stud is reinforced (9.23.5.3.OBC Div. B 9.23.5.3. Wall Studs(1) Wall studs shall not be notched, drilled or otherwise damaged so that the undamaged portion of the stud is less than two-thirds the depth of the stud if the stud is loadbearing or 40 mm if the stud is non-loadbearing, unless the weakened studs are suitably reinforced.). A 38 × 89 mm loadbearing stud can lose about 29 mm at most.

Fire blocks

Concealed spaces in interior walls, ceilings and floors must be separated by fire blocks from concealed spaces in exterior walls and attic or roof spaces (9.10.16.1.(1)OBC Div. B 9.10.16.1. Required Fire Blocks in Concealed Spaces(1) Concealed spaces in interior walls, ceilings, floors and crawl spaces shall be separated by fire blocks from concealed spaces in exterior walls and attic or roof spaces. (2) Reserved. (3) Fire blocks shall be provided at all interconnections between concealed vertical and horizontal spaces in interior coved…). In a basement, that matters where a furred-out exterior wall opens into the floor joist space above, and at bulkheads and soffits that connect to wall cavities. Fire blocks stop a fire spreading through hidden cavities.

Insulation, vapour barrier and air barrier

Framing an exterior basement wall usually means insulating it:

  • Insulation must cover the full height of a foundation wall enclosing a basement (9.25.2.3.(4)OBC Div. B 9.25.2.3. Installation of Thermal Insulation(1) Insulation shall be installed so that there is a reasonably uniform insulating value over the entire face of the insulated area. (2) Insulation shall be applied to the full width and length of the space between furring or framing. (3) Except where the insulation provides the principal resistance to air leakage…), with R-values from SB-12. See the basement insulation guide.
  • The air barrier must be continuous and extend throughout the basement (9.25.3.1.(2)OBC Div. B 9.25.3.1. Required Barrier to Air Leakage(1) Wall, ceiling and floor assemblies that separate conditioned space from unconditioned space or from the ground shall be constructed so as to include an air barrier system that will provide a continuous barrier to air leakage (a) from the interior of the building into wall, floor, attic or roof spaces sufficient to…), sealed at the rim joist and the slab.
  • A vapour barrier goes on the warm side, and insulated foundation walls may use a variable-permeance membrane. See the vapour barrier guide.
  • In finished areas, cover insulation and vapour barrier with gypsum board or a similar finish (9.25.2.3.(7)OBC Div. B 9.25.2.3. Installation of Thermal Insulation(1) Insulation shall be installed so that there is a reasonably uniform insulating value over the entire face of the insulated area. (2) Insulation shall be applied to the full width and length of the space between furring or framing. (3) Except where the insulation provides the principal resistance to air leakage…).

Common mistakes

  1. Untreated plates straight on the slab. Add poly or roll roofing, or use treated lumber.
  2. Insulation packed against joist ends where the code needs a 12 mm air space.
  3. Over-drilled studs for plumbing in a loadbearing wall.
  4. No fire block where a furred wall meets the joist space.
  5. Removing a post or beam without a design and a permit.

How to look it up yourself

Open Division B, Part 9 of the 2024 Compendium. Section 9.23 covers wood-frame construction. Search terms that appear in the code text:

  • "Protection from Dampness" (9.23.2.3.)
  • "Size and Spacing of Studs" (Table 9.23.10.1.)
  • "Bottom Wall Plates" (9.23.11.2.)
  • "Wall Studs" (9.23.5.3.)
  • "Required Fire Blocks in Concealed Spaces" (9.10.16.1.)

Read the article in the code viewer alongside the stud table.

BCIN exam angle

Wood-to-concrete separation, stud notching limits and the stud table are common House exam questions. Study the House chapter on wood-frame and steel stud construction, then try the free quiz on the House exam page.

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

© King's Printer for Ontario, 2024. Reproduced with permission.

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Frequently asked questions

Do basement bottom plates have to be pressure treated in Ontario?
Not necessarily. Wood that is not pressure-treated and sits on concrete in contact with the ground must be separated from the concrete by at least 0.05 mm polyethylene or Type S roll roofing, unless the wood is at least 150 mm above the ground (Division B, Article 9.23.2.3.). Pressure-treated plates need no separator.
What size studs can I use for basement walls?
For interior walls carrying no load, Table 9.23.10.1. allows studs as small as 38 × 38 mm at 400 mm spacing up to 2.4 m high, though 38 × 64 mm or 38 × 89 mm is more common. Loadbearing walls need larger studs or closer spacing, depending on the loads they carry (Division B, Article 9.23.10.1.).
How much can I notch or drill a stud?
The undamaged part of a loadbearing stud must stay at least two-thirds of its depth, and of a non-loadbearing stud at least 40 mm, unless the stud is reinforced (Division B, Article 9.23.5.3.).
Do basement walls need fire blocking?
Concealed spaces in interior walls, ceilings and floors must be separated by fire blocks from concealed spaces in exterior walls and attic or roof spaces (Division B, Sentence 9.10.16.1.(1)). Where a furred-out exterior wall meets the floor joist space above, a fire block is usually needed at the top of the wall.
Does a basement wall need a bottom plate?
Yes. A bottom wall plate is required in all cases. Plates must be at least 38 mm thick and as wide as the studs, but a non-loadbearing wall may use a 19 mm bottom plate (Division B, Articles 9.23.11.1. and 9.23.11.2.).