Footing Forms

Written for Canada. Plans are metric, but many crews and suppliers work in feet and inches, so this site gives both: millimetres and metres, with feet and inches beside them. Codes are the National Building Code of Canada 2020 (NBC 2020) Part 9 as your province adopts it. A number marked Confirm has not been checked against every province, so check it with your own authority before you act on it.

Reviewed by a working tradesman with 17+ years in the field.

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The forms that set a footing's width, thickness, level and position, built right before the concrete goes in.

The big idea

A footing spreads the weight of the building over the soil. The forms decide its width, thickness, level and position, and once the concrete is in, none of those can be changed. A footing 50 mm (2 in) off centre, or 25 mm (1 in) low, shows up later as a crooked wall or a stepped slab. Take the time here.

Types of footings

• Strip footings: continuous footings under foundation walls.
• Pad footings: isolated footings under columns, posts or point loads.
• Grade beams: reinforced beams spanning between pads or piles. These are engineered.
• Thickened slab edges: slab edges or interior zones thickened to carry a load.

Check the drawings before you cut a board

For anything larger or unusual, footing size comes from the structural drawings. For a typical house, it comes from Part 9 of the building code in your province (based on the NBC). Either way, write the numbers on your layout and on the form itself so the next person does not guess.

The model code, Part 9, Section 9.15.3, says:

• Footings rest on undisturbed soil, rock or compacted granular fill (9.15.3.2).

• Thickness is at least the greater of 100 mm (4 in) and the footing's projection beyond the wall (9.15.3.8).
• Steps: the rise between level parts is no more than 600 mm (23 5/8 in), and the run between risers is at least 600 mm (9.15.3.9).
• Wet or loose ground: on gravel, sand or silt with the water table below the footing width, walls need twice the width and columns twice the area (9.15.3.4(3)).
• Extra width is added for masonry veneer, masonry walls and flat insulating concrete form walls (9.15.3.5).
• Depth below finished ground follows Table 9.12.2.2. For clay or unclassified soil it is generally 1.2 m (3 ft 11 in); other soils depend on frost and heating (9.12.2.2).

Working it out

Wall footing. A two-storey exterior wall, 200 mm thick, on a 350 mm footing: projection = (350 − 200) ÷ 2 = 75 mm (3 in), so the minimum thickness is the greater of 100 mm and 75 mm, which is 100 mm (4 in). The drawings usually call for more than the minimum.

Column pad. A one-storey column at 3 m spacing needs 0.4 m², so a square pad is √0.4 = 0.632 m (24 7/8 in) a side. Two floors at 0.75 m² need 0.866 m (34 in).

Footing layout

• Mark footing width and the wall centreline. The wall sits on the footing's centre unless the drawings say otherwise.
• Mark column centrelines and pad locations from the gridlines.
• Mark step-downs and elevation changes before forming starts.
• Plan anchor bolt and dowel positions so form hardware and rebar do not clash.
• Check the diagonals. Equal diagonals mean a square building (see Elevation and Layout).

Forming materials

• Lumber forms: 38 x 235 mm (2x10) or 38 x 286 mm (2x12) boards, 38 x 89 mm (2x4) stakes, spreaders and kickers. Use straight, sound lumber, without large knots or splits at the edge.
• Panel systems: steel or aluminum panels with corner pieces and strongbacks. Faster on long runs.
• Hybrid: panels for straight runs, lumber for steps and odd corners.

Building the forms

1. Snap lines. Mark footing edges and the centreline on the prepared base.
2. Set corner stakes. Establish the corners, then pull string lines to check straightness.
3. Set the form boards. To the snapped line and the elevation. The top of the board is the top of the footing.
4. Fasten to the stakes. Screws or nails, driven so the board is not pushed out of line.
5. Add spreaders. Hold the width along the full run, not just at the corners.
6. Brace with kickers. Wet concrete is heavy and pushes outward. Brace for that, not for the empty form.
7. Check the elevation. Laser or level at corners, mid-spans and steps, not just the corners.
8. Place the rebar. On chairs, with the cover the drawings call for (see Rebar Placement).
9. Walk it. Width, depth, elevation, rebar, bracing and a clean base. Fix problems now.

Footing base preparation

The soil under a footing matters as much as the concrete. Concrete cannot make up for a soft base.

• Strip topsoil, roots and organic material. The bottom of every excavation must be free of organic material (9.12.1.1).
• Remove loose soil and soft spots and replace them with compacted granular material, not loose dirt.
• Level the base so the footing thickness is even.
• Excavations must be kept free of standing water (9.12.1.2), and kept from freezing for the whole construction period (9.12.1.3). Never pour into mud, onto ice or onto frozen ground. It thaws and settles.

Pouring considerations

• Place concrete evenly along the run so one side is not pushed out of line.
• Consolidate with a vibrator or by rodding to remove air pockets.
• Watch forms and stakes during the pour and correct movement immediately.
• Plug leaks before you lose a lot of concrete.
• Keep rebar on its chairs. Do not let it get walked into the soil.

For commercial and civil footings, use heavier forms and bracing, follow the engineered formwork drawings, use anchor bolt templates for columns and steel, and coordinate critical positions with the surveyor.

Common mistakes

• Width or depth different from the drawings
• Wrong elevation, which then shows up in wall or slab heights
• Not enough bracing, leading to a blowout
• Rebar too low, touching the soil
• Missing dowels or wrong spacing
• Water under the footing reducing bearing
• Pouring on frozen or soft ground