| Parts | Topping (flange) + ribs (stems) + voids between ribs |
|---|---|
| One-way ribbed | Ribs run in one direction, supported on beams or walls |
| Two-way ribbed | Ribs in both directions — called a waffle slab |
| Best for | Longer spans and lighter floors where a solid slab would be too heavy |
| IS 456:2000 | Clause 30.5 — ribbed, hollow block or voided slab |
When a solid slab spans more than about 5–6 m, its self-weight starts to dominate the design. Most of that concrete sits in the tension zone, where it only holds the steel in place. A ribbed slab removes that concrete and keeps a deep, efficient section. If you have read our guide to types of slabs, this is the type that trades a little formwork complexity for a lighter, stiffer floor.
How a ribbed slab works
The thin topping acts as the compression flange. Each rib acts like a small T-beam: it carries the tension steel at the bottom and the shear near the supports. Loads go from the topping to the ribs, then to the supporting beams, walls or columns. Because the section is deep, the slab is stiff for its weight and deflects less than a solid slab using the same amount of concrete.
Types of ribbed slab
| Type | Voids formed by | Notes |
|---|---|---|
| One-way ribbed (joist) slab | Removable moulds (steel or plastic) or permanent blocks (clay, hollow concrete, EPS) | Ribs span one way; simplest to design and detail |
| Two-way ribbed (waffle) slab | Square dome or box moulds | Ribs in both directions; good for near-square panels and exposed ceilings |
| Hollow block slab | Permanent hollow blocks left in place | Flat soffit; block can also give sound and heat insulation |
Typical dimensions (IS 456 Cl. 30.5)
IS 456:2000 gives detailing limits for ribbed slabs. The values most often quoted are below.
| Item | Common limit |
|---|---|
| Minimum rib width | 65 mm |
| Maximum rib spacing | 1.5 m centre to centre |
| Rib depth (excluding topping) | Not more than 4 times the rib width |
| Topping thickness | At least 1/10 of the clear distance between ribs, and not less than 50 mm |
Always check the clause numbers and limits against the latest IS code and your structural designer before using them on a project.
Ribbed slab vs solid slab
| Point | Ribbed slab | Solid slab |
|---|---|---|
| Self-weight | Lower | Higher |
| Concrete and steel | Less for the same span | More |
| Economical span | Longer spans | Short to medium spans |
| Formwork | More complex (moulds) | Simple flat shuttering |
| Fire and cover | Ribs are thin, so cover and fire rating need care | Easier to achieve |
| Services | Small holes between ribs; large openings need trimmed ribs | Easier to add openings |
Site checklist
• Fix the void moulds firmly so they do not float or move during concreting.
• Keep the specified clear cover to rib bars, as ribs are thin and exposed on three sides.
• Use a small-aggregate mix that flows into narrow ribs, and vibrate with care.
• Check the topping steel (mesh) and the thickness before pouring.
• Follow the removal time for shuttering in our guide on shuttering removal time as per IS 456.
For reinforcement drawings of normal slabs, see slab reinforcement details and the one-way vs two-way slab guide.
What is a ribbed slab?
What is the difference between a ribbed slab and a waffle slab?
Is a ribbed slab cheaper than a solid slab?
Which IS code covers ribbed slabs?
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