Slabs · Johannesburg & Gauteng
Rib & Block or Concrete Slab? Understand the Difference Before You Build.
Building a double-storey home, extending your property or planning a suspended concrete floor? Understand your slab options, explore how they work and send us your plans for an accurate quotation.
Connect with specialist suppliers and installers for lintels, rib-and-block slabs and other concrete floor solutions.
Explore both systems layer by layer
Switch layers on and off, tap a label to learn what it does, or play the construction sequence. These are simplified illustrations, not certified engineering drawings.
Rib-and-block system
Press Play to walk through the construction sequence, or switch layers below.
Conventional cast-in-situ slab
Press Play to walk through the construction sequence, or switch layers below.
What's the Difference Between Propped and Propless?
A propped system needs temporary props under the ribs or formwork while the concrete is placed and gains strength. The props come out only when the responsible professional authorises it.
Some proprietary rib-and-block systems are engineered to be built without intermediate temporary props, but only under defined conditions set by the manufacturer and engineer. Not all rib-and-block systems are propless.
We can't recommend a system from simple dimensions. Your engineer and the supplier decide what is suitable for your project.
Factors that affect the choice
- Span
- Loads
- Rib design
- Supporting conditions
- Manufacturer specifications
- Construction method
- Engineering approval
Side-by-side comparison
Neither system is always cheaper, faster or stronger — the result depends on your project.
| Topic | Rib-and-block | Conventional cast-in-situ |
|---|---|---|
| Basic method | Precast ribs and filler blocks placed, then a concrete topping is cast over them | Concrete cast in place into temporary formwork around fixed reinforcement |
| Components | Ribs, filler blocks, reinforcement, topping concrete | Formwork, props, reinforcement, concrete |
| Temporary support | Some systems need propping, others are designed to be propless under defined conditions — depends on design and specification | Formwork and props are normally required until the concrete may carry itself |
| Formwork | Blocks act as permanent formwork; little or no shuttering under the slab | Full shuttering under the slab |
| Typical sequence | See the 8-step sequence above | See the 7-step sequence above |
| Engineering | Rib layout, spans and topping from the engineer or manufacturer | Slab thickness and steel from the engineer |
| Services coordination | Plan sleeves and conduits before the topping is cast | Plan sleeves and conduits before the pour |
| Site access | Space to off-load and place ribs and blocks | Space for formwork, steel and concrete delivery or pumping |
| Delivery | Ribs and blocks delivered by the supplier; confirm off-loading | Formwork, steel and ready-mix deliveries |
| Labour | Depends on design and specification | Depends on design and specification |
| Suitable applications | Suspended floors in homes, extensions and other buildings — subject to design | Suspended floors and roofs of many shapes — subject to design |
| What affects cost | Area, spans, loads, propping, reinforcement, topping, delivery and access | Area, thickness, steel, formwork hire, concrete, pumping and access |
Slab basics
What is a rib-and-block slab?
A suspended floor made from precast concrete ribs (also called beams) with filler blocks laid between them and a structural concrete topping cast over the top. It is also called a beam-and-block slab. It's often searched for as "rib and block" in South Africa.
What is a conventional cast-in-situ slab?
A slab where reinforcement is fixed and concrete is poured on site into temporary formwork, which is removed once authorised.
What affects slab pricing?
Area, spans, loads, the system chosen, propping, reinforcement, concrete, delivery, access and labour. Pricing comes from the supplier's quotation for your specific project. We don't publish rates: suppliers quote from your plans.
Need lintels too? See lintels. Planning the whole build? See construction or use our quote form.
Tell Us About Your Slab
This planner doesn't choose a system, calculate spans or loads, or give prices. It helps you send a clear enquiry to suitable suppliers.
Approx. area: —
Length × width, a geometric estimate only — not an engineering design or material quantity. Irregular shapes or drawings need detailed measurements.
Complete the choices above to see your project summary.
Frequently asked questions
Important
Slab design, specifications and suitability must be confirmed by your engineer or project professional and the manufacturer. BuildItS.A does not provide structural advice.