Cover is the cheapest durability feature in a house and one of the most often wrong. A few millimetres of missing concrete over a bar is invisible on pour day and shows up years later as rust stains, cracks and spalling plaster on the ceiling.
What Does Cover Do?
- Corrosion protection. Concrete is alkaline, which keeps steel from rusting. Moisture and carbon dioxide slowly neutralise concrete from the surface inward; cover is the distance they must travel before reaching steel.
- Fire protection. Cover insulates the steel from heat.
- Bond. Steel needs concrete around it to transfer force.
How Much Cover Does IS 456 Require?
IS 456 sets nominal cover by exposure condition:
| Exposure | Nominal cover |
|---|---|
| Mild | 20 mm |
| Moderate | 30 mm |
| Severe | 45 mm |
| Very severe | 50 mm |
| Extreme | 75 mm |
For main reinforcement up to 12 mm in mild exposure, the nominal cover may be reduced by 5 mm.
Member-specific minimums also apply:
| Member | Typical cover in a house |
|---|---|
| Slab | 20 mm (mild), 30 mm (moderate, e.g. roof exposed to rain) |
| Beam | 25 – 30 mm |
| Column | At least 40 mm to longitudinal bars, and not less than the bar diameter |
| Footing | At least 50 mm |
Cover is measured to the outermost steel — usually the stirrups or ties, not the main bars.
Cover Blocks, Chairs and Spacers
| Spacer | Purpose |
|---|---|
| Cover blocks | Hold steel off the formwork at the specified cover — under slabs, beside beams and columns |
| Chairs | Support the top mat of steel in slabs and footings at the right height |
| Side spacers | Keep column and beam cages centred in the formwork |
Cover blocks should be:
- The correct thickness for the member, marked or sized consistently.
- At least as strong and dense as the concrete around them — cement mortar blocks of a rich mix, or approved plastic spacers.
- Tied to the steel with binding wire so they do not move during the pour.
- Closely spaced — roughly every 1 m or less under slabs and at regular intervals along beams.
Stones, brick bats and wood pieces are not substitutes: they are irregular, weak or porous, and they can rot or let water through to the steel.
What Goes Wrong Without Proper Cover
| Problem | Long-term result |
|---|---|
| Steel touching formwork | Bar visible on the soffit; rusts quickly |
| Cover too thin | Rust stains, then cracking and spalling |
| Top steel trodden down in slabs | Top bars lose effectiveness over supports |
| Cages off-centre in columns | One face has little or no cover |
Check Before Every Pour
- Cover blocks under the slab and beams at close intervals, tied in place.
- Chairs holding the top steel at the right level.
- Column cages centred, with side spacers.
- No steel touching formwork anywhere.
- Walkway planks so workers do not stand on the top steel during the pour.
The Short Version
Use 20 mm cover in protected slabs, 30 mm where exposed, 40 mm in columns and 50 mm in footings, measured to the outermost bar. Hold it with proper cover blocks and chairs, and check it before every pour — once the concrete is in, cover cannot be added.
