Brickbat Coba Alternative: A Lighter, Insulating Option for Terraces and Sunk Fills
Brickbat coba has been the default terrace and sunk-area filler in India for decades. Broken bricks are laid in cement mortar, rammed, and finished with a mortar top. It works, but it is heavy, slow to lay, and its quality depends on two things you rarely control on site: the quality of the bricks and the skill of the gang laying them.
This article explains what brickbat coba actually does, where it falls short, and how perlite lightweight concrete replaces it, with the numbers worked out.
What brickbat coba is supposed to do
On a terrace, coba does three jobs:
Gives the slope that carries rainwater to the outlets
Provides a base for the waterproofing and the finish on top
Adds some protection from the sun's heat
In sunken toilets and bathrooms, the brickbat fill simply fills the depth between the slab and the finished floor, around the pipes.
Where brickbat coba falls short
Weight. Using IS 875 (Part 1) unit weights, brick masonry is about 1,920 kg/m³ and cement mortar about 2,080 kg/m³, so coba works out to roughly 1,970 kg/m³. At 115 mm average thickness that is about 226 kg on every square metre of terrace.
Heat. Brick and mortar are poor insulators. A coba terrace soaks up heat all day and releases it into the top floor through the evening.
Speed and labour. Brickbat has to be broken, carried up, laid and rammed by hand. It needs a skilled gang and many days on a large terrace.
Consistency. Bat size, packing and mortar quality vary from gang to gang and floor to floor.
The alternative: perlite lightweight concrete
Perlite lightweight concrete is made from Kelite, a construction-grade expanded perlite manufactured by Keltech Energies Ltd, mixed with cement (or cement and sand) and an admixture. Depending on the mix, its dry density ranges from about 300 to 2,000 kg/m³, so the same material can be tuned for insulation or for a harder, heavier fill.
For a straight replacement of brickbat coba at the same average thickness, we use the 1,200 or 1,400 kg/m³ grades. Where heat insulation is the main goal, a lighter insulating grade goes on the roof instead (see our guide to terrace heat insulation).
Weight saving, worked out
Average thickness | Brickbat coba (kg/m²) | Perlite concrete 1,200 grade (kg/m²) | Saving |
75 mm | 148 | 90 | 58 kg/m² (39%) |
115 mm | 226 | 138 | 88 kg/m² (39%) |
150 mm | 295 | 180 | 115 kg/m² (39%) |
On a 600 m² terrace at 115 mm average thickness, the 1,200 grade takes about 53 tonnes off the roof compared with brickbat coba. At the 1,400 grade the saving is about 29%.
What you gain on site
Pumpable or bagged. It can be site-mixed, supplied in pre-mixed bags, or delivered by RMC and pumped to the terrace.
Laid to slope directly. The slope is formed in the concrete itself, levelled with an aluminium straight edge.
Monolithic, hard, smooth finish. A table-top surface that takes waterproofing, tiles or any other finish directly.
Zero settlement once cured.
No dependence on brick quality or on a specialised coba gang.
Non-combustible. Perlite is an inorganic volcanic mineral that does not burn.
Where it is used
Terrace slope screeds and roof insulation
Sunken toilet, bath and kitchen fills (as a lighter filler replacing brickbat; see our sunken slab filling guide)
Floor screeds over RCC and deck slabs
Podium fills
Frequently asked questions
Is perlite concrete cheaper than brickbat coba?
Compare it per square metre of finished terrace at the thickness you need, not per bag or per cubic metre, and add what you save in dead load, labour time and heat. Send us your drawings and we will work out the comparison for your project.
Can it be laid on an existing terrace?
Yes. On an existing building the weight saving matters even more, because the structure was never designed for extra load. The old coba is removed and perlite concrete is laid to slope in its place.
Does it replace waterproofing?
No. Perlite concrete forms the slope and the base. Waterproofing is a separate layer and should be specified as usual.
What thickness can be laid?
The minimum practical section is about 25 mm. There is no practical upper limit for fills.
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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