Foundation Wall 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 give a foundation wall its thickness, height and plumb, and why they have to be built for the pour.

The big idea

A foundation wall carries the floors, walls and roof above it down to the footing, and in a basement it also holds back the soil. The forms decide its thickness, height, straightness and plumb.

How hard does the concrete push?

Fresh concrete acts like a heavy liquid. At about 2,400 kg/m³ it weighs 23.5 kN per cubic metre, so the pressure on the form at depth h is 23.5 × h kPa. This is the fluid ceiling, reached when the concrete is placed fast. The form design or the manufacturer's pour-rate table sets the real design pressure.

• At the bottom of a 2.4 m (7 ft 10 in) wall: 23.5 × 2.4 = 56.5 kPa, about 1,180 lb/ft².
• Total push on each metre of wall length: ½ × 23.5 × 2.4² = 67.8 kN, about 4,650 lbf per foot, roughly 6.9 tonnes-force per metre.
• One snap tie at the bottom carrying a 0.6 m × 0.6 m (2 ft × 2 ft) patch: 0.36 m² × 56.5 kPa = 20.3 kN, about 4,570 lbf. Check that against the tie's rated load.

That is why ties, whalers and bracing come from a design, and why a pour rate matters: pouring faster keeps the concrete liquid higher up the form for longer.

Types of foundation walls

• Standard house walls: thickness set by the Part 9 tables for the wall height and the height of backfill. Common thicknesses are 150, 200, 250 and 300 mm (6, 8, 10 and 12 in).
• Engineered walls: thicker, reinforced, or both, for heavy loads or tall retained soil. Built to the drawings.
• Basement walls: need dampproofing or waterproofing and drainage, and a firm connection to the floor system before backfill.
• Retaining walls: resist soil pressure, covered on their own page.
• Commercial walls: usually steel or aluminum panel systems or gang forms, built to an engineered formwork design.

Under the model code (NBC Part 9, Section 9.15.4, read in the Vancouver Building Bylaw 2014):

• Table 9.15.4.2.A gives the minimum thickness of unreinforced concrete or block walls against the height of finished ground above the basement floor, for walls up to 3.0 m (9 ft 10 in) unsupported height. Read the table itself for your wall.
• A wall counts as supported at the top when the floor system is anchored to it with anchor bolts, joists are embedded in it, or it carries a solid masonry superstructure (9.15.4.3).
• It counts as unsupported where an opening is wider than 1.2 m (3 ft 11 in), or the openings total more than 25 per cent of the wall length, unless the wall around the opening is reinforced for earth pressure.
• Insulating concrete form (ICF) walls are at least the greater of 140 mm (5 1/2 in) or the thickness of the concrete above (9.15.4.2).
• Exterior foundation walls extend at least 150 mm (6 in) above finished ground (9.15.4.6).

Forming systems

• Lumber and plywood forms: form-grade plywood on studs, with whalers, strongbacks and kickers. Use sheets in good condition. Stud and whaler spacing comes from the form design, not from habit.
• Panel systems: steel or aluminum panels with pins, wedges, corners and ties. Follow the manufacturer's tie spacing and pour-rate tables.
• Hybrid systems: panels for straight runs, lumber for odd corners and renovation work.
• Gang forms: large panels lifted by crane. Engineered, and lifted by a trained crew only.

Wall layout and preparation

Every mark you make here ends up in the finished wall. Do it once and check it twice.

• Snap lines for both wall faces and confirm the wall is centred on the footing.
• Check the footing dowels for location and spacing before forming around them.
• Mark window and door openings, beam pockets, ledges and sleeves from the drawings.
• Confirm anchor bolt zones and spacing with the Anchoring pages and the drawings.
• On commercial work, align walls to the gridlines using survey control.

Building lumber wall forms

1. Set the base. A treated sill or form base on the footing, to the line.
2. Lay out the studs. At the spacing from the form design. Wall height and pour rate change it.
3. Fasten the plywood. Tight seams, screws or nails driven flush.
4. Add whalers. Horizontal stiffeners to stop bulging.
5. Add strongbacks. Vertical stiffeners for tall or long walls.
6. Install ties. At the specified spacing, with spreaders so the wall thickness is held.
7. Brace. Kickers and diagonals on both faces. Brace for the pour, not for the empty form.
8. Check plumb. Level or laser on every panel before and after bracing.

Building panel wall forms

• Set the first panel perfectly. It controls the whole wall.
• Pin the next panels tight to it with no gaps.
• Install ties at the engineered spacing for the height and pour rate.
• Install corners using the manufacturer's corner pieces.
• Add strongbacks and whalers where the layout calls for them.
• Check alignment with a string line or laser down the whole run.

Rebar

Reinforcement is set by the drawings or by the Part 9 table for your wall. Details are on the Rebar Placement page.

• Vertical bars tied to the footing dowels
• Horizontal bars at the specified spacing
• Corner bars so cracks do not start at turns
• Lap splices at the lengths the drawings give
• Extra bars around openings and beam pockets
• Cover kept on both faces so the steel does not touch the form

Openings and inserts

Openings are the most error-prone part of a wall form. They must be square, level and braced so they do not squeeze or shift.

• Window and door bucks, built to the rough opening plus allowance
• Beam and ledger pockets, sized for the member and its bearing
• Sleeves for services, set and capped so concrete cannot fill them
• Conduit stubs and inserts, fixed so they cannot float or move
• Vents and access openings as drawn

Pouring considerations

For a 30 m (98 ft) long, 2.4 m (7 ft 10 in) high, 200 mm (7 7/8 in) wall: 30 × 2.4 × 0.2 = 14.4 m³, about 18.8 yd³, before waste.

• Place concrete in even lifts around the wall, not all in one spot.
• Hold the pour rate the form design allows. Faster pours mean higher pressure.
• Vibrate to remove air pockets, without hitting the form or the rebar.
• Watch ties, whalers and openings for stress, and stop if anything moves.
• Keep cover on the rebar throughout.
• Check plumb again during the pour.

Common mistakes

• Wrong wall thickness
• Panels not plumb or not aligned
• Weak bracing leading to movement or a blowout
• Missing ties or wrong tie spacing
• Openings out of square
• Rebar too close to the form face
• Anchor bolts misplaced or missing
• Pouring too fast