Category: Structure & materials
Prepared by Eng. Ahmed Fathy, Projects Manager ·
Definition
A slab on grade is cast directly on compacted fill over an insulation layer or polythene sheet, usually with steel mesh, commonly 10 to 15 cm thick in villas and thicker in warehouses with heavy loads.
Why it matters on a project
It is the ground floor or warehouse floor, and any settlement in the fill below cracks the tiles above. Run compaction and density tests before pouring it.
Measurement and pricing
Concrete in cubic metres, mesh in square metres or tonnes, polythene in square metres, and saw-cut joints in linear metres.
Worked example
An 18 x 15 m slab on grade 10 cm thick: concrete 18 × 15 × 0.10 = 27 m³, and about 270 m² of polythene plus laps.
Common mistake
Casting on fill not compacted in layers, so it settles after months and floors crack.
Frequently asked questions
Why lay polythene under the slab?
To stop ground moisture rising and to keep the concrete's water from draining into the soil during the pour.
Does a slab on grade need joints?
Yes on large areas, with saw-cut joints at regular spacing to control cracking.
Related terms
- BackfillRefilling excavations around foundations with suitable soil in compacted layers.
- Compaction testA test proving backfill layers are compacted to the required density.
- Termite treatmentTreating the soil before pouring to protect the building from termites.
- Floor screedA thin mortar or concrete layer that levels the floor before tiles or parquet.
Read next
- Quantity Takeoff for Contractors, Step by Step
- Work out quantities with our free calculators: concrete, tiles, paint and plaster. Open the calculators
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