Cover blocks and chairs for a slab: how thick, how many, how tall
The structural drawing sets the cover. The blocks and chairs on site decide whether the slab gets it. This guide covers how thick each block should be, how many a slab and its beams take, how tall to bend the chairs for the top steel, and which blocks to send back before the pour.
By Bulan Sarkar · Updated
Make each cover block exactly as thick as the nominal cover written on the structural drawing, and put blocks and chairs no more than 1 m apart, the maximum in IS 456:2000 clause 12.3.2. Under a slab that comes to about one block per square metre, so a 10 m by 9.3 m roof (about 1,000 sq ft) takes 100 blocks, with more under and beside the beams. The blocks must be concrete of the same strength as the slab, or PVC. A chair for the top steel is as tall as the slab thickness minus both covers and all four bar layers: 51 mm in a 125 mm slab with 20 mm cover and 10, 8, 8 and 8 mm bars.
Source: IS 456:2000 Plain and Reinforced Concrete, Code of Practice, clauses 12.2 to 12.3.2 and 26.4 with Table 16, checked Oct 2026.
How thick: the block matches the cover on the drawing
Cover is the concrete between the steel and the outside face of the slab, beam or column. It keeps air and water away from the bars so they don't rust. A cover block (or spacer) holds that gap open while the concrete goes in. Once the slab is cast, nobody can see whether it worked.
IS 456:2000 clause 12.3.2 says the spacers between the bars (or the links, in a beam or column) and the formwork should be the same size as the nominal cover. The nominal cover is the figure your structural engineer writes on the drawing. Clause 26.4.1 says it can never be less than the diameter of the bar, and Table 16 sets a minimum for each exposure condition in Table 3:
| Exposure | Where the concrete is (Table 3, shortened) | Cover, at least |
|---|---|---|
| Mild | Protected against weather or aggressive conditions, not in a coastal area | 20 mm |
| Moderate | Sheltered from severe rain; buried in non-aggressive soil; sheltered from salt air on the coast | 30 mm |
| Severe | Exposed to severe rain or alternate wetting and drying; exposed to the coastal environment | 45 mm |
| Very severe | Sea water spray, corrosive fumes, aggressive soil or ground water | 50 mm |
| Extreme | Tidal zone; direct contact with aggressive chemicals | 75 mm |
Note 1 to Table 16 lets the mild figure drop by 5 mm, to 15 mm, for main bars up to 12 mm. That is why many house drawings show 15 mm for the slab. Note 3 allows 5 mm less for severe and very severe exposure when the concrete is M35 or stronger, which is rare on a house.
Two more rules change the block size on the same site. Column bars get at least 40 mm, or 25 mm in a column no wider than 200 mm whose bars are 12 mm or less (cl. 26.4.2.1). Footings get at least 50 mm (cl. 26.4.2.2). So one house usually needs blocks in three or four thicknesses.
The cover that comes out may be up to 10 mm more than the nominal figure, and never less (cl. 12.3.2, Table 16 Note 2). A 20 mm block that has crumbled to 15 mm fails. A 25 mm block where the drawing says 20 mm stays inside the tolerance, but it lifts the bars and changes the slab's effective depth, so ask the engineer before swapping sizes.
How many blocks: no more than 1 m apart
Clause 12.3.2 says spacers or chairs should be placed at a maximum spacing of 1 m, and that closer spacing may sometimes be needed. The code gives no count, so the count is arithmetic. Our rule for a rectangular slab: lay the blocks on a grid with no gap over 1 m and the outer rows half a gap in from the edge. Then the number in each direction is the length in metres, rounded up:
Blocks under a slab = (length in m, rounded up) × (width in m, rounded up)
A 10 m by 9.3 m roof takes 10 × 10 = 100 blocks, about one per square metre. Count each separate panel, such as a balcony or a sunshade, the same way.
Beams need blocks too, because clause 12.3.2 puts the spacers between the links and the formwork. A beam cage needs them under its bottom and against both side shutters. We count one position for each metre of beam, rounded up, and three blocks at each position. The drawing often gives beams a different cover from the slab, so count beam blocks as a separate size.
Then add a tenth for breakage, our own margin. Blocks crack when bars are dropped on them or when people walk across the mesh, which is one reason clause 12.3 warns against rough handling and dropping the steel.
Chairs for the top steel: how tall and how many
A slab carries top steel over its supports: bars over the beams and walls, and the main bars of a cantilever such as a balcony. SP 34:1987, the BIS handbook on reinforcement detailing, says the top layer in a slab usually rests on the bottom layer on chairs (cl. 13.4.4.3). It describes a chair the fixer can bend on site from a bar of about 12 mm (cl. 13.4.3.1 a). Clause 12.3.2 of IS 456 puts chairs under the same 1 m maximum spacing as blocks.
The chair's height comes from the drawing, by subtraction:
Chair height = slab thickness − bottom cover − top cover − the two bottom bar layers − the two top bar layers
In a 125 mm slab with 20 mm cover top and bottom, 10 mm main bars under 8 mm distribution bars, and 8 mm bars both ways on top, that is 125 − 20 − 20 − 10 − 8 − 8 − 8 = 51 mm. Bend chairs a little short rather than tall. A short chair adds a millimetre or two to the top cover, which the +10 mm tolerance allows. A tall one leaves less top cover than the drawing asks for, and the tolerance allows nothing below it.
Count chairs like blocks, over the area of each band of top bars shown on the drawing. For a cantilever, SP 34 cl. 9.5 says the spacing of chairs under the top steel should preferably be specified on the detailing drawing, so ask for it if the balcony sheet leaves it out. On slab day, lay walking planks across the top steel so the crew's weight stays off the chairs.
Concrete, PVC or mortar: which blocks to accept
IS 456 clause 12.3.2 allows two materials for spacers and cover blocks: concrete of the same strength as the member, or PVC. For an M20 slab that means blocks at least as strong as M20 concrete, not soft sand-and-cement cubes that crumble in the hand. SP 34 cl. 13.4.4.1 adds that a support on horizontal shuttering has to carry the weight of the steel, the load of placing the concrete and people walking over the mesh, without piercing the shuttering.
- Concrete blocks often come with a tie wire cast in (SP 34 cl. 13.4.4.1 a). Tie them to the bars so they don't get kicked aside.
- Plastic chair-type supports can carry heavy loads but touch the shuttering over a large area. SP 34 calls the clip-on circular type weaker, better suited to vertical bars than to slab mesh (cl. 13.4.4.1 c).
- In columns, SP 34 says it is better to clip circular spacers onto the horizontal ties than onto the vertical bars, where they get in the way of the concrete (cl. 13.4.4.2).
- In a water tank or sump, mortar blocks should not be used on the faces in contact with the water (SP 34 note to cl. 13.4.4.1), and supports left on the inner face are best avoided (cl. 13.4.5). Use PVC there.
Check a few blocks from each lot the day they arrive. Measure the thickness with a tape against the drawing, reject cracked or crumbling ones, and pull on the wire to see that it holds. Keep each size in its own sack with the thickness written on it.
Worked examples
1. Cover blocks for a 1,000 sq ft roof slab and its beams
- Roof slab 10.0 m × 9.3 m = 93.0 m² (about 1,000 sq ft); drawing: slab cover 20 mm, beam cover 25 mm
- Slab: 10.0 rounds up to 10, 9.3 rounds up to 10, so 10 × 10 = 100 blocks of 20 mm
- Beams: four edge beams (10.0, 10.0, 9.3 and 9.3 m) and one middle beam of 9.3 m = 10 + 10 + 10 + 10 + 10 = 50 positions
- 50 positions × 3 (under the cage and one each side) = 150 blocks of 25 mm
- Add a tenth for breakage: 110 of 20 mm and 165 of 25 mm
Result: Order about 275 blocks in two labelled lots: 110 at 20 mm for the slab and 165 at 25 mm for the beams.
2. How tall to bend the chairs, and how many
- Slab 125 mm, cover 20 mm top and bottom (drawing)
- Bottom steel: 10 mm main bars with 8 mm distribution bars on top of them
- Top steel over the middle beam: 8 mm bars both ways
- Chair height = 125 − 20 − 20 − (10 + 8) − (8 + 8) = 51 mm
- Top bars run 0.9 m each side of the middle beam: a band 1.8 m × 9.3 m, so 2 × 10 = 20 chairs
Result: Bend 20 chairs to 50 or 51 mm, from bar of about 12 mm as SP 34 suggests. A 53 mm chair would leave only 18 mm of top cover, less than the drawing's 20 mm.
3. Spacers for an underground sump
- Sump 3.0 m × 2.0 m inside, walls 200 mm thick and 1.5 m high, base 3.4 m × 2.4 m on blinding concrete; covers from the drawing
- Inner wall face (in contact with water): perimeter 2 × (3.0 + 2.0) = 10 m, so 10 positions × 2 rows = 20 PVC spacers
- Outer wall face, if it is shuttered: 2 × (3.4 + 2.4) = 11.6 m, so 12 positions × 2 rows = 24 spacers
- Base, bottom mesh on the blinding: 4 × 3 = 12 blocks
- No mortar blocks on the water side (SP 34 note to cl. 13.4.4.1)
Result: 20 PVC spacers for the inner face, plus 36 concrete or PVC blocks for the outer face and the base, before the tenth for breakage.
Tips and tricks
- Get the cover for every member in writing before you buy blocks: slab, beam, column and footing. A house often needs four sizes.
- Keep each size in its own sack with the thickness written on it. A 40 mm column block under a 20 mm slab lifts the bars, and once the concrete is in nobody will know.
- Tie every block to the bars with its cast-in wire. Loose blocks get kicked out of place when the crew walks the mesh.
- Ask the fixer to bend the chairs from offcuts left over after cutting the bars.
- Photograph the blocks with a tape held against one, the afternoon before the pour. It is the only record of the cover once the slab is cast.
What we would do
We'd ask the engineer for the cover of every member, then buy concrete blocks of the same strength (or PVC spacers) with tie wires in those sizes: about one per square metre of slab, three per metre of beam and a tenth spare, each size in a labelled sack. We'd have the chairs bent to the subtraction above, a millimetre short rather than long, and walk the mesh with the engineer the afternoon before the pour with a tape in hand. The slab casting day checklist puts this check into the rest of the day.
Frequently asked questions
What size of cover block is used for a slab?
How many cover blocks are needed per square metre?
What is the cover for columns and footings?
How do you calculate chair height in a slab?
Can sand-cement mortar cover blocks be used?
Sources and date checked
- IS 456:2000 Plain and Reinforced Concrete, Code of Practice: cl. 12.2, 12.3, 12.3.1 and 12.3.2 (cover blocks and spacers same size as the nominal cover, spacers or chairs at most 1 m apart, concrete of the same strength or PVC, cover tolerance +10/0 mm; printed p. 26); Table 3 exposure conditions (p. 18); cl. 26.4.1, 26.4.2, 26.4.2.1 and 26.4.2.2 and Table 16 with Notes 1 to 3 (pp. 46 to 47). Read 19 Oct 2026 from the page images. Later amendments exist; ask your engineer which edition the drawings follow.. Checked Oct 2026.
- SP 34(S&T):1987 Handbook on Concrete Reinforcement and Detailing (BIS): cl. 9.5 (chairs under cantilever top steel), 13.4.3.1 a) and Figs. 13.9 and 13.10 (chairs), 13.4.4.1 with its note (supports on horizontal shuttering; no mortar blocks on faces in contact with liquid), 13.4.4.2, 13.4.4.3 (top steel rests on chairs) and 13.4.5 (printed pp. 200 to 204). Read 19 Oct 2026. The block counts, the three-per-metre beam rule and the tenth for breakage are our worked examples, not figures from either document.. Checked Oct 2026.
This guide covers block sizes, counts, chair heights and checks on site. The cover, the bars, the slab thickness and any change to them are for your structural engineer to decide. Have the engineer inspect the steel and the cover before the pour, and keep everyone off the mesh except on walking planks.