Rebar Placement

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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Putting steel bars in the right size, place, length and cover before the concrete hides them.

The big idea

Concrete is strong when squeezed and weak when pulled. Rebar takes the pull. It only works if it is in the right place, the right size, the right length and covered with concrete on every side. Once the concrete is in, nobody can see it, so the inspection before the pour is the only check there ever is.

Purpose of rebar

• Takes tension that concrete cannot carry
• Controls cracking from shrinkage and load
• Ties footings, walls, slabs and stairs together
• Strengthens corners, openings and transitions

Rebar sizes and types in Canada

Canadian bars are named by an "M" number that is close to the diameter in millimetres, and the bars are made to CSA G30.18.

• 10M and 15M are the common house-and-small-building bars. 25M and up is commercial, industrial and civil work.
• Old US sizes on older drawings are not the same. A US #4 is 1/2 in (12.7 mm), smaller than 15M (16.0 mm). Do not swap them.
• Grade: normally 400 MPa, either weldable (400W) or regular (400R). Use what the drawings call for.
• Coated or special steel: epoxy-coated, galvanized or stainless where the drawings require it for corrosion.

Concrete cover

Cover is the concrete between the steel and the outside face. It stops water and air reaching the steel and protects it in a fire. More exposure means more cover.

• Cast against soil (footings): commonly 75 mm (3 in).
• Interior slabs: as little as 20 mm (3/4 in).
• Formed faces exposed to weather or soil (walls, exterior slabs): in between, depending on bar size and exposure.

Example. Bottom steel in a footing cast on soil with 75 mm cover sits on chairs 75 mm (3 in) high.

Bar supports

• Plastic chairs under mesh and bars in slabs
• Concrete blocks (dobies) under footing bars
• Continuous chairs for mats and heavy bars
• Wall spacers to keep bars off the form face
• Never use wood, rocks, bricks or debris (see Rebar Chairs and Spacers)

Counting bars and weight

Bars at a spacing along a length: spaces = length ÷ spacing, and bars = spaces + 1. Weight = total length × mass per metre.

Wall. A 12 m (39 ft 4 in) wall with vertical 15M bars at 400 mm (15 3/4 in) on centre: 12 ÷ 0.4 = 30 spaces, so 31 bars. At 2.4 m (7 ft 10 in) each that is 74.4 m, and 74.4 × 1.57 = 116.8 kg, about 257 lb.

Footing. Two continuous 15M bars along a 30 m (98 ft) footing: 60 m × 1.57 = 94.2 kg, about 208 lb, before laps and waste.

The spacing and bar size here are examples. The drawings or the bar schedule give yours.

Where the steel goes

Footings

Many house footings are unreinforced under Part 9, so do not add or leave out bars on your own. Follow the drawings: longitudinal bars in the middle of the width on supports, tied to the dowels that connect the wall, corner bars at every turn, splices at the length the drawings give.

Foundation walls

Vertical bars tied to the footing dowels, horizontal bars at the specified spacing, extra bars around openings and at corners, and spacers to hold cover on both faces.

Slabs and pads

Welded wire mesh or bars, as drawn, on chairs at the height the drawings say, with perimeter bars at thickened edges and dowels where the slab ties into a wall.

Stairs and landings

Bars in the treads and risers, landing mats tied into the stair steel, dowels to tie to slabs or walls, and extra bars at nosings.

Retaining walls

Vertical bars closer together near the bottom, horizontal bars as drawn, corner bars at turns and steps, and top bars for caps or slab connections.

Lap splices and tying

A splice passes the load from one bar to the next. A splice that is too short is a weak link you cannot see.

• Use the splice length on the drawings. It depends on bar size and concrete.
• Stagger splices where you can, and do not splice in high-stress locations unless designed.
• Tie the splice at both ends.
• Use tie wire and a tying tool, firm but not so tight the bar moves out of position.
• Keep wire ends turned away from the form face, and make sure bars cannot shift while concrete goes in.

Alignment and inspection

• Check spacing with a tape.
• Check cover at several points, at corners and at openings.
• Straight, level bars, held up by supports.
• Dowels aligned for the wall or column above.
• Look at every corner and opening.

Common mistakes

• Rebar touching soil or the form
• Wrong spacing
• Missing corner bars
• Splices too short
• Rebar floating or pushed down during the pour
• Missing or misaligned dowels
• Wrong bar size or grade