About this calculator
Everything for a concrete floor slab laid on the ground: the concrete, the 250 µm damp-proof membrane, optional mesh, and the fill underneath, checked against SANS 10400-J.
How it’s calculated
Concrete = length × width × thickness, plus waste. The membrane covers the slab area and turns up around the edge by the slab thickness. With mesh, sheets of 6.0 × 2.4 m are counted with a 300 mm overlap.
SANS 10400-J 4.4.1 covers only a floor that is not part of a foundation system and does not pass over or rest on the foundation walls. A floor that does, like the usual surface bed with its edges on the foundation brickwork, must meet SANS 10400-B or -H instead (4.4.2), so build it to your approved plans or an engineer’s design. For a slab cast free inside the walls, 4.4.1 allows a plain concrete slab at least 75 mm thick on a polyolefin membrane for buildings not used for storage or industry: grade 10 concrete where the floor is covered, grade 15 where the slab is the finished floor. No panel side may be longer than 3.5 m under carpet or flexible floor coverings, or 2.5 m under tiles; bigger slabs need joints, mesh to SANS 10400-H, or an engineer’s design. The membrane is at least 250 µm, turned up by the slab thickness and overlapped 200 mm at joints (4.4.4).
Fill must be free of organic matter, stones over 75 mm and clay lumps, moistened, and placed in layers of no more than 100 mm by hand or 150 mm by machine before compaction, so the compacted depth takes at least that many layers. Fill deeper than 400 mm needs an engineer (4.4.5 to 4.4.7). The floor must be level or slope no more than 4 mm per metre (4.4.3).
Worked example
With these inputs:
- Slab length
- 8 m
- Slab width
- 6 m
- Slab thickness
- 75 mm
- Slab rests on or passes over the foundation walls
- Yes
- Floor covering
- Carpet or vinyl (flexible)
- Reinforce with mesh
- No
- Fill under the slab (compacted depth)
- 0 mm
- Supply as
- Mix on site (cement, sand and stone)
- Concrete mix
- Medium strength (house floors, footpaths, driveways)
- Waste allowance
- 10%
the result is:
- Concrete needed
- 3.96 m³
- Slab area
- 48 m²
- Concrete (incl. waste)
- 3.96 m³
- Cement (50 kg bags)
- 31 bags
- Concrete sand
- 2.46 m³
Tips from the trade
- Lay the membrane with as few joints as possible and tape every pipe that passes through it.
- Mark the panel joints before the pour so they line up with doorways and tile layouts.
- Keep the slab damp while it cures, especially in hot, windy weather.
Common questions
How thick should a floor slab be in South Africa?
For a slab that doesn’t rest on the foundation walls, SANS 10400-J allows plain concrete at least 75 mm thick for houses, laid on a 250 µm damp-proof membrane, with panels no longer than 3.5 m (2.5 m under tiles). A surface bed that rests on or passes over the foundation walls must meet SANS 10400-B or -H instead, so follow your plans or an engineer.
Results are estimates for planning and pricing. Check quantities against your drawings, the manufacturer’s data and SANS 10400 (the National Building Regulations) and your local municipality before ordering.