Kumar IronUpdated
In this guide
What brickbat coba is
Brickbat coba is a layer of broken brick bats laid on the slab in a bed of mortar, then grouted and finished. The bats are set to a slope so that water runs to the outlets. Lime mortar is used in traditional lime terracing, and cement mortar is used in most modern work. The mason sets the slope with the bats, which is why the layer is heavy and labour intensive.
Slope first, not a waterproof membrane
The most important point is this: brickbat coba forms a slope and a protective layer, but on its own it does not make a roof waterproof. One trade article we checked states that it gives the terrace a slope to drain water away, and that it should not be called waterproofing. The same article notes that coba is trusted in coastal and high rain areas, and that the trust is misplaced where the layer is treated as a waterproof coat.
Public works tender specifications for new roofs often mix a waterproofing compound into the bedding mortar and lay it over the RCC slab. That compound is meant to conform to IS 2645, the BIS standard for integral cement waterproofing compounds. This is the sensible way to use coba: as a sloped, protected layer above a waterproof base.
Pros and cons side by side
| Factor | Brickbat coba | Chemical coat or membrane |
|---|---|---|
| Creates slope | Yes, the bats are set to fall to the outlets | No, slope comes from the screed below |
| Waterproofs by itself | No, it needs a waterproof layer under it | Yes, when applied correctly |
| Weight on the slab | Heavy, adds dead load | Very light |
| Cost direction | Often cited as economical on large roofs, but labour intensive | Cost depends on area and product, check current quotes |
| Movement and cracks | Can crack over time as the bed moves | Flexible types bridge small cracks |
| Curing | Several days of water curing for the bed | Follows the product sheet, often about a week before a water test |
| Repair | Break out and redo the bed | Patch the coat with the same product |
| Heat | Trade sources dispute the insulation claim | Not a heat insulation layer |
Weight on the slab
A brickbat bed adds real load to the slab. A chemical coat adds almost none. Before you build a coba roof on a new slab, ask the engineer what finish load the slab was designed for, and whether a sloped bed of that thickness is allowed. Older slabs need evaluation before a heavy layer is added. Do not estimate the weight by feel.
Time to cure
Plan the sequence. The mortar bed and grout need time to set, then the coba is kept wet for several days. Only after it dries should you apply a chemical coat over it, as the product sheet says. A coat laid on wet mortar will not bond properly. Allow more time before heavy foot traffic, tiles or water tests than you would for a coat on bare concrete.
When each one is used
- Brickbat coba on its own: a large flat roof that needs slope and cheap protection from sun and rain, with a waterproof layer already in place below.
- Chemical coat or membrane on its own: a roof with little slope to build, a bathroom, a water tank, a balcony or a roof that will be left exposed.
- Combined: the waterproof coat goes on the structural slab first and cures. Then the sloped coba bed is laid over it, with care not to puncture the coat. This is the usual approach for a new terrace.
Before you decide
Ask the engineer about the slab load and the slope design. Ask the mason how the coba will be cured and whether a waterproof layer goes under it. Check the chemical product's data sheet for cure and recoat times. Our waterproofing range is listed in the products section, and the cement calculator can help you estimate mortar for the bed.
Frequently asked questions
- Is brickbat coba waterproof?
- Not by itself. It forms a sloped, protective layer. Water protection should come from a waterproof coat or membrane under it, or from a compound mixed into the bedding mortar in the way the specification says.
- Does brickbat coba keep the roof cool?
- The claim is disputed. Trade sources we checked say the coba surface can reach about the same temperature as a bare slab at peak summer, so do not rely on it for heat.
- Can I lay brickbat coba over an old roof?
- Only after the engineer confirms the slab can take the extra weight. The old coat and the slab condition also need to be checked first.
- How long before I can use a coba terrace?
- Allow the full curing time for the bed, then any chemical coat cure time, then the water test. Heavy loads or tile work should wait longer.