Footing and foundation sizing: BCIN Calculation Lab

Last updated October 22, 2026

When this shows up on the exam

Footing sizing is a favourite calculation on the House exam and the Small Buildings exam. A typical question describes a house or small building (number of floors, wall type, cladding, joist spans, soil) and asks for the minimum strip footing width, column footing area or footing thickness. The numbers are small, but every question hides one adjustment that is easy to miss. If you follow the same order every time, you will not miss it.

The method

Check that the table applies. The Section 9.15 footing tables are for stable soil with an allowable bearing pressure of 75 kPa or more, not on fill, peat or sensitive clay OBC Div. B 9.15.1.1.. They also assume joist spans of 4.9 m or less and a floor live load of 2.4 kPa or less OBC Div. B 9.15.3.3.. Outside those limits the footing is designed under Section 4.2 (or Section 9.4 for other soil cases, as 9.15.1.1. directs), with one exception for long joists, step 4.
Count the floors supported. Count the floors whose load comes down to this footing. A basement slab on ground sits on the soil, not on the footing, so it is not counted.
Read the base value from Table 9.15.3.4. Row = number of floors supported (1, 2 or 3). Column = exterior wall strip footing, interior wall strip footing, or column footing area (the column value assumes columns at 3 m o.c.) OBC Div. B Table 9.15.3.4.. Watch the columns: for 3 floors the interior wall footing is wider than the exterior one.
Long joists (light wood frame only). If any supported joist span exceeds 4.9 m, you may scale the strip footing width with the formula in OBC Div. B 9.15.3.4.(2): W = w × ∑sjs / (storeys × 4.9). For an exterior wall, ∑sjs adds the full joist span bearing on the wall on each storey. For an interior wall, take half the span on each side, on each storey, and add them all.
Add the exterior wall adjustments OBC Div. B 9.15.3.5., per storey supported: 65 mm for masonry veneer over wood frame, 130 mm for solid masonry, 150 mm for flat ICF wall construction. Count only the storeys that actually have that construction.
Add the interior wall adjustments OBC Div. B 9.15.3.6.: an interior loadbearing masonry wall gets 100 mm per storey of masonry. A non-loadbearing interior masonry wall needs 200 mm up to 5.5 m high, plus 100 mm for each additional 2.7 m.
Columns: scale for spacing. Column footing area is adjusted in proportion to the column spacing: area = table area × (spacing / 3 m) OBC Div. B 9.15.3.7..
High water table: double last. On gravel, sand or silt where the water table is closer below the bearing surface than the footing width, double the wall footing width (after all the adjustments above) and double the column footing area OBC Div. B 9.15.3.4.(3).
Thickness. Footing thickness is the greater of 100 mm and the projection beyond the supported wall, pier or column OBC Div. B 9.15.3.8.. For a centred wall, projection = (footing width minus wall thickness) / 2.

Worked example 1: veneer and thickness

A two-storey wood-frame house. The ground storey has brick veneer, the upper storey has vinyl siding. The exterior footing supports 2 floors, joist spans are 4.3 m and the footing carries a 250 mm concrete foundation wall, centred.

Step	Working	Result
Table 9.15.3.4	Row 2 floors, exterior wall column	350 mm
Veneer, 9.15.3.5	1 storey × 65 mm	+65 mm
Width	350 + 65	415 mm
Projection	(415 − 250) / 2	82.5 mm
Thickness, 9.15.3.8	greater of 100 mm and 82.5 mm	100 mm

Answer: 415 mm wide and 100 mm thick. The common wrong answer is 480 mm, from adding veneer for both storeys when only one has it.

Worked example 2: joist span over 4.9 m

A one-storey wood-frame bungalow. Floor joists bearing on the exterior wall span 6.1 m. No veneer.

Table 9.15.3.4, row 1 floor, exterior wall: w = 250 mm.
∑sjs = 6.1 m (one storey, full span on an exterior wall). Storeys = 1.
W = 250 × 6.1 / (1 × 4.9) = 250 × 1.245 = 311.2 mm.
Round up: minimum 312 mm OBC Div. B 9.15.3.4.(2).

Now an interior bearing wall on 2 floors, with joists of 5.4 m on one side and 4.4 m on the other on both floors. Per storey: 5.4/2 + 4.4/2 = 4.9 m. ∑sjs = 4.9 × 2 = 9.8 m. W = 350 × 9.8 / (2 × 4.9) = 350 mm. Taking half spans on each side brings it back to the table value. Using full spans would double it, which is the classic mistake.

Worked example 3: column pad with a high water table

A one-storey building on sand. Columns along the centre beam are 2.7 m apart. The water table is about 300 mm below the bearing surface, and the pad will be about 850 mm wide, so the water table is closer than the footing width.

Step	Working	Result
Table 9.15.3.4	Row 1 floor, column area (3 m o.c.)	0.40 m²
Spacing, 9.15.3.7	0.40 × 2.7 / 3	0.36 m²
Water table, 9.15.3.4.(3)	0.36 × 2	0.72 m²
Square pad side	√0.72 = 0.849 m	850 mm

If the pad carries a 250 mm square concrete pier, centred, the projection is (850 − 250) / 2 = 300 mm, so the pad must be at least 300 mm thick.

Worked example 4: weaker soil below the footing

A 1.0 m square pad on dense sand carries 140 kN, so the bearing pressure is 140 kPa, under the 150 kPa allowed for dense sand in OBC Div. B Table 9.4.4.1.. Soft clay starts 0.6 m below the pad, which is within twice the footing width (2.0 m), so it must be checked OBC Div. B 9.4.4.2.. The load spreads at 60° to the horizontal, so each side grows by 0.6 / tan 60° = 0.346 m. Side at the clay = 1.0 + 2 × 0.346 = 1.693 m. Area = 2.866 m². Pressure = 140 / 2.866 = 48.9 kPa. Soft clay allows 40 kPa, so this footing does not work and must be redesigned.

Foundation wall thickness in one line

For unreinforced walls, Table 9.15.4.2.-A gives the maximum height of finish ground above the basement floor for each wall thickness OBC Div. B 9.15.4.2.. Work out that height first (wall height minus the part above grade), pick the right material block of the table and the right lateral support column, then take the first thickness whose value is equal to or greater than your height. Never take the nearest value if it is smaller.

Common mistakes

Using the exterior column for an interior wall (or the reverse). They differ for 1 and 3 floors.
Counting the basement slab as a floor supported, which pushes you one row down the table.
Adding veneer or masonry for every storey when only some storeys have it.
Using the masonry rate (130 mm) for veneer (65 mm), or the exterior rate for an interior masonry wall (100 mm).
In the long joist formula: using full spans for an interior wall, or forgetting the storeys term in the denominator.
Column area: not scaling for spacing, or inverting the ratio (3 / spacing).
High water table: doubling the table value and then adding the veneer, instead of doubling the finished width.
Thickness: forgetting to halve (footing width minus wall), or forgetting the 100 mm minimum.
Frustum check: spreading at 45° instead of 60°, or dividing the load by the side length instead of the area.

Find it fast in the code PDF

Bookmark Division B > Part 9 > Section 9.15 > Subsection 9.15.3, then Ctrl+F:

"Number of Floors Supported" (Table 9.15.3.4)
"supported joist span exceeds 4.9 m" (the long joist formula)
"masonry veneer over wood-frame construction" (exterior adjustments)
"projection of the footing" (thickness)
"in proportion to the distance between columns" (column spacing)

For soil: Division B > Part 9 > Section 9.4 > Subsection 9.4.4, then Ctrl+F "frustum" or "Allowable Bearing Pressure".

Drill these questions

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Code cited in this method

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

OBC Div. B 9.15.1.1. General
(1)Except as provided in Articles 9.15.1.2. and 9.15.1.3., this Section applies to
(a)concrete or unit masonry foundation walls and concrete footings not subject to surcharge
(i)on stable soils with an allowable bearing pressure of 75 kPa or greater, and
(ii)for buildings of wood-frame or masonry construction,
(b)wood-frame foundation walls and wood or concrete footings not subject to surcharge
(i)on stable soils with an allowable bearing pressure of 75 kPa or greater, and
(ii)for buildings of wood-frame construction, and
(c)flat insulating concrete form foundation walls and concrete footings not subject to surcharge (See Note A-9.15.1.1.(1) (c) and 9.20.1.1.(1)(b))
(i)on stable soils with an allowable bearing pressure of 75 kPa or greater, and
(ii)for buildings of light-frame or flat insulating concrete form construction that are not more than 2 storeys in building height, with a maximum floor-to-floor height of 3 m.
(2)Foundations for applications other than as described in Sentence (1) shall be designed in accordance with Section 9.4.
(3)Where a foundation is erected on filled ground, peat or sensitive clay, the footing sizes shall be designed in conformance with Section 4.2.
(4)For the purpose of Sentence (3), sensitive clay means the grain size of the majority of the particles is smaller than 0.002 mm, including leda clay.
Open OBC Div. B 9.15.1.1. in the code viewer
OBC Div. B 9.15.3.3. Application of Footing Width and Area Requirements
(1)Except as provided in Sentence 9.15.3.4.(2), the minimum footing width or area requirements provided in Articles 9.15.3.4. to 9.15.3.7. shall apply to footings, where
(a)the footings support
(i)foundation walls of masonry, concrete, or flat insulating concrete form walls,
(ii)above ground walls of masonry, flat insulating concrete form walls or light wood-frame construction, and
(iii)floors and roofs of light wood-frame construction,
(b)the span of supported joists does not exceed 4.9 m, and
(c)the specified live load on any floor supported by the footing does not exceed 2.4 kPa. (See Table 4.1.5.3.)
(2)Except as provided in Sentence 9.15.3.4.(2), where the span of the supported joists exceeds 4.9 m, footings shall be designed in accordance with Section 4.2.
(3)Where the specified live load exceeds 2.4 kPa footings shall be designed in accordance with Section 4.2.
Open OBC Div. B 9.15.3.3. in the code viewer
OBC Div. B Table 9.15.3.4. Minimum Footing Sizes — Forming Part of Sentence 9.15.3.4.(1)
Table 9.15.3.4 — Minimum Footing Sizes — Forming Part of Sentence 9.15.3.4.(1) Forming Part of Sentence 9.15.3.4.(1) Number of Floors Supported | Minimum Width of Strip Footings Supporting Exterior Walls[2], mm | Minimum Width of Strip Footings Supporting Interior Walls[3], mm | Minimum Footing Area for Columns Spaced 3 m o.c.[1], m² 1 | 250 | 200 | 0.40 2 | 350 | 350 | 0.75 3 | 450 | 500 | 1.0 Notes to Table 9.15.3.4.: (1) See Sentence 9.15.3.7.(1). (2) See Sentences 9.15.3.5.(1). (3) See Sentence 9.15.3.6.(1).
Open OBC Div. B Table 9.15.3.4. in the code viewer
OBC Div. B 9.15.3.4. Basic Footing Widths and Areas
(1)Except as provided in Sentences (2) and (3) and in Articles 9.15.3.5. to 9.15.3.7., the minimum footing width or area shall comply with Table 9.15.3.4.
(2)Where the supported joist span exceeds 4.9 m in buildings with light wood-frame walls, floors and roofs, strip footing widths shall be determined according to
(a)Section 4.2., or
(b)the following formula:
W = w × [∑sjs/(storeys × 4.9)]
where
W = minimum footing width,
w = minimum width of footings supporting joists not exceeding 4.9 m, as defined by Table 9.15.3.4.,
∑ sjs = sum of the supported joist spans on each storey bearing on an exterior wall whose load is transferred to the footing, or sum of half of the supported joist spans on each storey bearing on both sides of an interior wall whose load is transferred to the footing, and
storeys = number of storeys supported by the footing. (See Note A-9.15.3.4.(2))
(3)Where a foundation rests on gravel, sand or silt in which the water table level is less than the width of the footings below the bearing surface,
(a)the footing width for walls shall be not less than twice the width required by Sentences (1) and (2), and Articles 9.15.3.5. and 9.15.3.6., and
(b)the footing area for columns shall be not less than twice the area required by Sentences (1) and (2) and Article 9.15.3.7.
Open OBC Div. B 9.15.3.4. in the code viewer
OBC Div. B 9.15.3.5. Adjustments to Footing Widths for Exterior Walls
(1)The strip footing widths for exterior walls shown in Table 9.15.3.4. shall be increased by
(a)65 mm for each storey of masonry veneer over wood-frame construction supported by the foundation wall,
(b)130 mm for each storey of masonry construction supported by the foundation wall, and
(c)150 mm for each storey of flat insulating concrete form wall construction supported by the foundation wall.
Open OBC Div. B 9.15.3.5. in the code viewer
OBC Div. B 9.15.3.6. Adjustments to Footing Widths for Interior Walls
(1)The minimum strip footing widths for interior loadbearing masonry walls shown in Table 9.15.3.4. shall be increased by 100 mm for each storey of masonry construction supported by the footing.
(2)Footings for interior non-loadbearing masonry walls shall be not less than 200 mm wide for walls up to 5.5 m high and the width shall be increased by 100 mm for each additional 2.7 m of height.
Open OBC Div. B 9.15.3.6. in the code viewer
OBC Div. B 9.15.3.7. Adjustments to Footing Area for Columns
(1)The footing area for column spacings other than shown in Table 9.15.3.4. shall be adjusted in proportion to the distance between columns.
Open OBC Div. B 9.15.3.7. in the code viewer
OBC Div. B 9.15.3.8. Footing Thickness
(1)Footing thickness shall be not less than the greater of
(a)100 mm, or
(b)the width of the projection of the footing beyond the supported element.
Open OBC Div. B 9.15.3.8. in the code viewer

Show all 11 articles

OBC Div. B Table 9.4.4.1. Allowable Bearing Pressure for Soil or Rock — Forming Part of Sentence 9.4.4.1.(1)
Table 9.4.4.1 — Allowable Bearing Pressure for Soil or Rock — Forming Part of Sentence 9.4.4.1.(1) Type and Condition of Soil or Rock | Maximum Allowable Bearing Pressure, kPa Dense or compact sand or gravel[1] | 150 Loose sand or gravel[1] | 50 Dense or compact silt[1] | 100 Stiff clay[1] | 150 Firm clay[1] | 75 Soft clay[1] | 40 Till | 200 Clay shale | 300 Sound rock | 500 Notes to Table 9.4.4.1.: (1) See Note A-Table 9.4.1.1.
Open OBC Div. B Table 9.4.4.1. in the code viewer
OBC Div. B 9.4.4.2. Foundation Capacity in Weaker Soil and Rock
(1)Where a soil or rock within a distance equal to twice the footing width below the bearing surface has a lower allowable bearing pressure than that at the bearing surface as shown in Article 9.4.4.1., the design capacity of the foundation shall not be greater than would cause the weakest soil or rock to be stressed beyond its allowable bearing pressure.
(2)In calculating subsurface pressures referred to in Sentence (1), the loads from the footings shall be assumed to be distributed uniformly over a horizontal plane within a frustum extending downward from the footing at an angle of 60° to the horizontal.
Open OBC Div. B 9.4.4.2. in the code viewer
OBC Div. B 9.15.4.2. Foundation Wall Thickness and Required Lateral Support
(1)Except as required in Sentence (2), the thickness of foundation walls made of unreinforced concrete block, concrete core in flat wall insulating concrete forms or solid concrete and subject to lateral earth pressure shall conform to Table 9.15.4.2.-A for walls not exceeding 3.0 m in unsupported height.
(2)The concrete core in flat insulating concrete form foundation walls shall be not less than the greater of
(a)150 mm, or
(b)the thickness of the concrete in the wall above.
(2.1)Foundation walls made of flat insulating concrete form units shall be laterally supported at the top and at the bottom.
(3)The thickness and reinforcing of foundation walls made of reinforced concrete block and subject to lateral earth pressure shall conform to Table 9.15.4.2.-B and Sentences (4) to (7), where
(a)the walls are laterally supported at the top,
(b)average stable soils are encountered, and
(c)wind loads on the exposed portion of the foundation are no greater than 0.70 kPa.
(4)For concrete block walls required to be reinforced, continuous vertical reinforcement shall
(a)be provided at wall corners, wall ends, wall intersections, at changes in wall height, at the jambs of all openings and at movement joints.
(b)extend from the top of the footing to the top of the foundation wall, and
(c)where foundation walls are laterally supported at the top, have not less than 50 mm embedment into the footing, if the floor slab does not provide lateral support at the wall base.
(5)For concrete block walls required to be reinforced, a continuous horizontal bond beam containing not less than one 15 M bar shall be installed
(a)along the top of the wall,
(b)at the sill and head of all openings greater than 1.20 m in width, and
(c)at structurally connected floors.
(6)In concrete block walls required to be reinforced, all vertical bar reinforcement shall be installed along the centre line of the wall.
(7)In concrete block walls required to be reinforced, ladder- or truss-type lateral reinforcement not less than 3.8 mm in diameter (No. 9 ASWG) shall be installed in the bed joint of every second masonry course.
Open OBC Div. B 9.15.4.2. in the code viewer