Sunken Slab Filling Material: Brickbat, Sand, Cinder or Perlite Concrete?
A sunken slab is dropped below floor level in toilets, bathrooms and sometimes kitchens, so the drainage pipes can run inside the floor. Once the pipes are in, the depth between the slab and the finished floor has to be filled. What you fill it with decides how much dead load every one of those slabs carries for the life of the building.
The usual options
Filling material | Weight | Settlement risk | Surface for the finish |
Brickbat in mortar | Heavy, about 1,970 kg/m³ | Low if well packed | Needs a mortar bed |
Sand | Heavy, and loose | High, can settle and crack tiles | Needs a mortar bed |
Cinder / broken blocks | Lighter, but variable | Medium, voids can settle | Needs a mortar bed |
Foam concrete | Light | Low, but shrinkage cracking | Often soft at low density |
EPS bead concrete | Light | Low | Beads can show at the top |
Perlite concrete | Light, density chosen per project | Zero settlement | Hard, monolithic, takes the bed directly |
Why weight matters in sunk areas
Take a sunken toilet filled 300 mm deep:
Brickbat fill: about 590 kg per m²
Perlite concrete at the 1,200 grade: 360 kg per m²
That is about 230 kg less on every square metre. Across three toilets in a flat, it is close to 2.8 tonnes; across a 20-storey tower with four flats a floor, about 220 tonnes. The full working is in our article on brickbat coba weight and dead load.
How perlite concrete is used in sunk areas
After the pipes are laid and tested, the sunk area is filled with perlite concrete around and over them.
It sets to a hard, level, monolithic surface, ready for the floor bed and tiles.
It can be supplied in pre-mixed bags, which suits floor-by-floor work in a tower.
What perlite concrete is, and is not, in a sunk area
Perlite concrete is a lighter filler that replaces brickbat or other fills. It is not a waterproofing system. The sunk slab should be waterproofed as per your specification before filling, and the floor finish detailed as usual.
Frequently asked questions
What is the best filling material for a sunken slab?
One that is light, does not settle, and gives a hard base for the floor. Perlite concrete meets all three; sand and loose cinder can settle, and brickbat is heavy.
Can sand be used for sunken slab filling?
It is common, but sand is heavy and loose, and loose fills can settle over time and crack the tiles above.
Will perlite concrete damage the pipes?
No. It is placed like any concrete fill around tested pipes, and its lighter weight puts less load on them than brickbat.
Related reading: EPS concrete vs perlite concrete · Brickbat coba alternative
About Perlite India. Perlite India designs and lays perlite lightweight insulating concrete for roofs, floors, sunk fills and podiums, with 3M+ sq.ft laid across India. Supported by the Perlite Construction Division of Keltech Energies Ltd, a member of the Perlite Institute (USA).
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