How we supply it: Cover blocks are arranged through a partner supplier who quotes, supplies and bills them and pays us a referral commission. They are light, so we add them to a TMT load from our yards; if you prefer to cast your own, we deliver the 6 mm chips and M-Sand.
In one line: A cover block is a small spacer of mortar, concrete or plastic placed between reinforcement and formwork so the steel ends up the required distance inside the concrete surface.
Why a small block decides how long the slab lasts
A cover block is a small spacer of mortar, concrete or plastic placed between reinforcement and formwork so the steel ends up the required distance inside the concrete surface.
Cover is what keeps moisture and salt away from the steel. Lose 10 mm of it because a bar sagged onto the shuttering, and rust stains on the soffit follow within a few monsoons.
Nominal cover by exposure (IS 456)
Columns need at least 40 mm and footings at least 50 mm whatever the exposure. Sites close to the sea at Ennore or Pulicat are likely to be severe or worse, so ask your engineer which class the drawing assumes.
| Exposure | Nominal cover |
|---|---|
| Mild | 20 mm |
| Moderate | 30 mm |
| Severe | 45 mm |
| Very severe | 50 mm |
| Extreme | 75 mm |
Block sizes for a typical house
Mortar or concrete blocks with a tie wire cast in are the norm on house sites; plastic spacers clip onto bars and suit soffits that stay exposed. Top steel in slabs and footings sits on bent-bar chairs, not on cover blocks.
| Member | Inland site (mild or moderate) | Coastal site (severe) |
|---|---|---|
| Slab bottom | 20 mm | 45 mm |
| Beam bottom and sides | 25 mm | 45 mm |
| Column | 40 mm | 45 mm |
| Footing and raft bottom | 50 mm | 50 mm or more |
How many blocks to order
Typical estimate: one block every 0.6 to 1.0 m each way under slab and footing mesh, and about every metre along beams and columns on each face.
- 1,000 sq ft slab (93 m²): about 100 blocks at 1 m spacing, 260 at 0.6 m
- Beams: about 2 to 3 per metre of run, bottom and sides
- Columns: 4 per metre of height, one on each face
- Add 10% for breakage
Checks before the pour
- Measure a few blocks with a steel rule; a 25 mm block that is really 20 mm loses 5 mm of cover everywhere it sits.
- Knock two blocks together: well-cured ones ring and do not crumble at the edges.
- Pull the cast-in tie wire; it should not come loose.
- Walk the deck and look under the mesh: no bar should touch the shuttering anywhere.
- Check that chairs, not blocks, hold the top mesh at its level.
Keeping sizes apart on site
- Keep blocks in bags or crates by size and label them; mixed sizes end up in the wrong member.
- Store under cover and keep site-cast blocks wet for a week.
- Do not tip them from height onto hard ground; chipped edges cut the cover.
Spacer shortcuts that cost cover
- Using brick pieces or stones as spacers; they are porous and weaken the cover zone.
- One size for every member.
- Blocks under top bars instead of proper chairs.
- Too few blocks mid-slab, exactly where workers walk on the mesh.
- Site-cast blocks weaker than the concrete around them.