The concrete floor slab of a house, often called the surface bed, sits on compacted fill inside the foundation walls. SANS 10400-J gives simple rules for it on an ordinary house, and following them avoids the two classic problems: damp coming up through the floor and cracks running across it. This guide covers those rules and how to take off the materials.
What the regulations allow
SANS 10400-J 4.4.1 covers a floor on the ground or on fill that is not part of a foundation system and does not pass over or rest on the foundation walls. For such a floor, in a building that isn’t used for storage or industry, it allows a plain concrete slab:
- at least 75 mm thick;
- of grade 10 concrete where the slab will be covered (tiles, carpet, vinyl), or grade 15 where the slab itself is the finished floor;
- laid on a polyolefin underfloor membrane;
- with no panel side longer than 3.5 m under carpet or other flexible floor coverings, or 2.5 m under tiles.
A plain slab bigger than those panel sizes falls outside the simple rules. It must be split into panels with joints, reinforced with mesh in line with the slab-on-ground rules in SANS 10400-H, or designed by a competent person (civil engineering). The finished floor must be level, or slope no more than 4 mm per metre.
When the simple rules don’t apply
SANS 10400-J 4.4.2 says a floor on the ground must meet SANS 10400-B or SANS 10400-H if it forms part of a foundation system, is plain concrete with any panel side over 3.5 m, or passes over or rests on the foundation walls. The common surface bed, cast with its edges resting on the foundation brickwork and the membrane carried over the wall, is in that last group. For it, the 75 mm thickness and panel sizes above are not the deemed-to-satisfy answer: use the slab detail on your approved plans, and where the plans don’t give one, have a competent person (civil engineering) design it. The membrane, fill and level rules below apply either way.
The damp-proof membrane
SANS 10400-J 4.4.4 asks for a polyolefin membrane at least 250 µm thick (or one with an Agrément certificate). It must be turned up around the edge of the floor by at least the slab thickness and overlapped 200 mm at every joint, and any pipes or punctures must be taped. AfriSam’s guidance adds that the membrane should reach the edges of the slab both ways and run halfway over the top of the foundation brickwork, usually the inside skin, with a few bricks on top to hold it in place while the slab is cast.
Lay it with as few joints as possible, and walk on it as little as you can. A torn membrane is one of the main ways damp gets into a new floor. See damp-proofing: DPC, DPM and rising damp for how the membrane works with the damp-proof course in the walls.
Fill under the slab
SANS 10400-J 4.4.5 to 4.4.7 set out the fill:
- little or no organic material, no stones larger than 75 mm, and no large clay lumps;
- moistened before compaction, so a handful squeezed in your hand is firm but not wet;
- placed in layers no thicker than 100 mm before compaction when compacting by hand, or 150 mm with a mechanical compactor, each layer well compacted before the next;
- fill deeper than 400 mm at any point must be designed and inspected by a competent person (civil engineering).
Mesh
South African welded mesh is sold in 6.0 × 2.4 m sheets with wires at 200 × 200 mm, in references such as Ref 100, Ref 193 and Ref 245. The number is roughly the mass in kilograms per square metre × 100: a Ref 193 sheet weighs about 27.8 kg. Suppliers recommend overlapping sheets by at least 200 to 300 mm. Mesh must sit inside the slab, so lift it on spacers before the pour; mesh lying on the membrane does very little. Use the reference on your plans.
Taking off a floor slab
Worked example: an 8 × 6 m floor slab cast free inside the walls (not resting on them), 75 mm thick, covered with carpet, no mesh.
- Concrete = 8 × 6 × 0.075 = 3.6 m³, or 3.96 m³ with 10% waste. At the Concrete Society’s medium mix for house floors (385 kg of cement per m³), that is 31 bags of cement and about 2.5 m³ each of concrete sand and stone.
- Membrane = floor area + perimeter × slab thickness = 48 + 28 × 0.075 = 50.1 m², so about 51 m², plus 200 mm at every joint for your roll width.
- Panels: both sides are longer than 3.5 m, so a plain slab needs joints. ⌈8 ÷ 3.5⌉ × ⌈6 ÷ 3.5⌉ = 3 × 2 = 6 panels, or reinforce it with mesh.
- With Ref 193 mesh instead, overlapping 300 mm, it takes 3 sheets across and 2 along: 6 sheets.
The floor slab calculator runs these checks and gives the material list; the reinforcing mesh calculator counts sheets for any slab.
Before you pour
- Check the fill is compacted in thin layers and level with the underside of the slab.
- Lay and tape the membrane, turned up at the edges.
- Mark the panel joints so they line up with doors and planned tile layouts.
- Set screed rails or level pegs so the slab is the right thickness all over.
- Keep the slab damp while it cures, especially in hot or windy weather.
This guide is general information for estimating. It is not engineering, legal or tax advice. Check SANS 10400 (the National Building Regulations) and your local municipality’s building control officer for your project.