Rebar spacers: which one where and how many
Concrete cover is not “whatever turns out” but a design figure on which the service life of the structure depends. A plastic spacer costs pennies; redoing a monolith costs everything.
Why concrete cover matters
The layer of concrete between the rebar and the surface protects the steel from moisture and fire and transfers load. Typical design values: slabs 20–25 mm, beams and columns 25–35 mm, foundations over concrete blinding 40 mm, in the ground without blinding 70 mm. If the rebar lies on offcuts and stones, the cover wanders from 0 to 60 mm within one slab, and after a few years rust streaks and spalling appear.
Which spacer where
- Columns, walls, beams: a “star” on a vertical bar. The number in the name is the concrete cover, the second number the rebar diameter.
- Floor slabs, floors on the ground: a “chair” under the bottom mesh, which gives two pairs of heights depending on the side it is placed.
- A foundation slab on film waterproofing or sand: a wide-base spacer, so as not to punch through the film.
- Heavy cages, grillages, bars of 25–40 mm: the T series and K-type spacers.
- Top mesh of a slab: separate “frog” supports made of rebar, or tall chairs.
Consumption
For a slab, at least 4 chairs per square metre (the design sets the spacing, closer for thin rebar); with a 200 mm mesh pitch that is every fifth node. For a column, 4 stars per running metre around the perimeter, that is one on each corner bar every 0.8–1 m. Packs hold 500–1000 pieces, and a house with a 150 m² slab and eight columns uses one pack of chairs and one pack of stars.
Formwork alongside
For walls in panel formwork, take BEFAST ties, 25 mm tubes and cones together with the spacers: the tube goes on the tie rod, the cones close the holes, and after stripping the rod comes out and the channel is closed with a plug. It is calculated on a 40×40 cm grid: 6.25 ties per m² of panel (with a 50×50 cm grid, 4).
Mistakes
A spacer of the wrong diameter that slides down the bar. Chairs placed only along the edges of the mesh, so that the middle sags when the concrete is poured. Saving on wide bases on film, after which the film tears and the waterproofing does not work.
Updated 31 Oct 2026