Rebar Chairs and Spacers

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.

Some links on this site are affiliate links. If you buy through them we may earn a commission at no extra cost to you. Affiliate disclosure

The supports that hold the steel at the height and cover the drawings give, so it cannot sink, float or touch the form.

The big idea

Rebar chairs and spacers hold the steel where the drawings put it. Without them, bars sink into the base, float up, shift or touch the form. They cost very little compared to the problem they prevent.

Why use them

• Keep the required concrete cover
• Hold the steel at the right height
• Stop bars moving while concrete goes in
• Keep steel centred in a wall
• Protect the steel from corrosion

Types of chairs and spacers

• Plastic slab chairs: under mesh and bars in slabs and pads.
• Continuous chairs: a long support for mats and heavy bars.
• Concrete blocks (dobies): under footing bars, where they bear on the soil without sinking.
• Wall spacers: plastic or concrete units that keep bars off the form face.
• Rebar wheels: for round columns and curved forms.
• Beam bolsters: for heavy bars in grade beams and structural slabs.

Where steel sits next to an exposed concrete face, one municipal specification (City of Winnipeg, Section 03200) requires plastic-coated supports, and says broken concrete blocks and wood supports are not acceptable. Check what your drawings and specification require.

Matching the support to the cover

The support height is the cover. The cover comes from the drawings, and from CSA A23.1 and A23.3. Commonly quoted values are about 75 mm (3 in) for concrete cast against soil and as little as 20 mm (3/4 in) for interior slabs, but they are not a substitute for the drawing (see Rebar Placement).

Example. The drawings call for 75 mm (3 in) under footing steel. Dobies that are 65 mm (2 9/16 in) tall leave the steel 10 mm (3/8 in) low everywhere, so the cover is short and the footing fails inspection. Use 75 mm supports.

Counting chairs

For a rectangular slab on a square grid at spacing s: lines = length ÷ s, rounded up, plus 1, in each direction, and chairs = lines × lines.

Example. A 6 m × 6 m (19 ft 8 in) slab on a 0.8 m (2 ft 7 in) grid: 6 ÷ 0.8 = 7.5, round up to 8 spaces, so 9 lines each way. 9 × 9 = 81 chairs. Staggering the lines by about 200 mm (8 in) from their neighbours helps avoid a straight line of weakness, per the same supplier guide.

Using them properly

• Place chairs at regular spacing so the steel does not sag between them.
• Use continuous chairs for heavy mats.
• On soil, use a base that will not sink under the load. Plastic chairs on wet ground can punch in.
• On a vapour barrier, use chairs with wide bases that will not puncture it.
• Use wall spacers to keep bars centred in the form.
• Re-check height after tying. Ties can lift or pull a bar.
• Do not use wood, rocks, brick or scrap. Wood rots and rocks shift, and both create paths for water to reach the steel.

Where each one goes

Footings

Concrete blocks or approved supports under the bars, cover kept from the soil as drawn, and bars held so they cannot move during the pour.

Foundation walls

Wall spacers keep the bars off the form faces, evenly spaced along the vertical bars, with cover checked at corners and openings.

Slabs and pads

Plastic chairs under mesh and mats, sitting flat and stable, at the spacing the maker's data and drawings say.

Stairs and landings

Small chairs or blocks under landing mats, supports for bars in risers and treads, and cover kept at nosings.

Retaining walls

Heavy-duty spacers for thick walls, more cover on the earth side as drawn, and alignment checked before the pour.

Common mistakes

• Rebar touching soil or the form
• Debris used as supports
• Too few chairs, so the steel sags
• Chairs tipping or sinking during the pour
• Cover too small, which fails inspection