Rib and Block Slab Calculator, South Africa
Work out ribs, infill blocks and topping concrete for a suspended rib and block floor slab, using Rhino Slabs' published specification.
Quick answer: A rib and block slab uses pre-stressed concrete ribs at 590mm centres, hollow infill blocks between them, and a 50mm concrete topping (Rhino Slabs' published spec). Block heights run S120 to S290, giving slab depths of 170mm to 340mm. Supply-only material cost is roughly R140–R510/m² plus topping, based on 2023/24 pricing not yet re-confirmed for 2026, always get a current quote.
⚠️ For planning purposes only. Slab depth, span and reinforcement must be confirmed by a structural engineer before ordering.
How to Use This Calculator
Enter your slab's length (the direction the ribs span) and width (the direction across the ribs), both in metres.
Choose your block type from S120 to S290, based on the slab depth your structural engineer has specified. Results show ribs, blocks, topping concrete volume and an estimated supply cost.
What a rib and block slab actually is
A rib and block slab is a suspended-floor system built from pre-stressed concrete ribs laid across a span, with hollow concrete infill blocks placed between them, then finished with a thin reinforced concrete topping. It's used for basement, first-floor and roof slabs as an alternative to a fully poured concrete slab, and needs no cranes or formwork to install, the props go in, the ribs and blocks are packed by hand, and levelling happens as the weight settles onto the props.
The figures on this page are Rhino Slabs' own published specification, not a nationwide South African standard. Other SA precast suppliers (Royal Concrete Slabs, Amalgamated Slab Company, Mobicast) use different block series and dimensions for the same product category, always confirm the exact spec with your chosen supplier before ordering.
Rhino Slabs block types and dimensions
| Block Type | Block Height | Dimensions (L×W×H) | Finished Slab Depth |
|---|---|---|---|
| S120 | 120mm | 450×220×120mm | 170mm |
| S150 | 150mm | 450×220×150mm | 200mm |
| S205 | 205mm | 450×220×205mm | 255mm |
| S290 | 290mm | 450×220×290mm | 340mm |
Ribs are pre-stressed, 140mm × 60mm in cross-section, weigh 17kg per metre, and are rated to span up to 7.8m. Rib centres are fixed at 590mm regardless of block type, and the concrete topping is consistently 50mm across all four block heights (each slab depth is exactly the block height plus 50mm).
Rib and block span limits
There is no published span-per-block-type lookup table, slab depth and span are determined by a structural engineer per SANS 1879 and SANS 10100 based on your building's actual load and layout, not a fixed rule this calculator can give you. As general context only: Rhino Slabs' own ribs are rated to span up to 7.8m, and rib and block systems as a category generally achieve clear spans up to around 10m. Always get an engineer's design before ordering, this calculator estimates quantities for a design you already have, it does not replace one.
Rib and block cost per m² in South Africa
Based on the most detailed published breakdown found, dated 2023/24 and not yet re-verified for 2026, supply-only material for ribs and blocks runs roughly R140–R510/m² depending on block depth, plus around R210/m² for topping concrete if you're managing that yourself. A fully supplied and installed slab runs approximately R825–R1,080/m². Treat this as a starting guideline, not a current market rate, and get a fresh quote from your supplier before budgeting, building material pricing moves and this specific figure is the oldest sourced on this page. If you're pricing a full build rather than just the slab, our Building Cost Calculator covers the wider construction budget this fits into.
A worked example: 6m × 4m slab, S150 blocks
Take a 6m × 4m floor (24m²) using S150 blocks (200mm slab depth). Ribs run the 6m length and are spaced at 590mm across the 4m width, needing 8 ribs (CEIL(4 ÷ 0.59) + 1), each 6m long, for 48m of total rib length. Between the 8 ribs sit 7 bays, each needing blocks laid end to end along the 6m run, at 450mm per block that's 14 blocks per bay (CEIL(6 ÷ 0.45)), for 98 blocks total. Topping concrete at 50mm across 24m² comes to 1.2m³. At S150's finished 200mm slab depth, supply cost for this floor lands in the R3,360–R12,240 range for ribs and blocks (R140–R510/m² × 24m²), plus roughly R5,040 for topping concrete (R210/m² × 24m²) if self-managed, all 2023/24 pricing, confirm current rates with your supplier.