Art of laying brick bat coba requires specialized skill and non-availability of manpower having this skillset is also the reason for the decline of this conventional method of waterproofing
Brick bat coba is a method to provide insulation for thermal comfort and waterproofing treatment to prevent leakage of water in case of RCC roofs. This is one of the oldest methods of waterproofing which was adopted by many when we moved towards flat roof construction. Once laid, brick bat coba can last at least 15 years if done by skillful applicators. This method was also preferred because of its efficiency in keeping the interiors cool.
This method involves laying lightweight mortar on the roof and spreading it to give gentle slopes for draining away the rainwater immediately. Before laying brick bats having an average thickness of about 110 mm, about 70 mm near rain water pipe and 150 mm at ridge are soaked overnight in water. The gaps between the brick bats are generally kept between 15 and 20 mm. These gaps are filled with cement sand mortar, (in 1:4 ratio), admixed with waterproofing chemicals. As an alternative, some embed broken tile or ceramic pieces in the plastic mortar which is often called china mosaic.
However, nowadays you hardly find any roofs (especially in Metros, Tier 1 & 2 cities) which are waterproofed with brick bat coba method. Advent of more efficient and easy to use and install water proofing methods are the main reasons for brick bat coba lose its prominence in waterproofing industry. Also, art of laying brick bat coba requires specialized skill and non-availability of manpower having this skillset is also the reason for the decline of this conventional method of waterproofing. This is also one of the main reasons for you not seeing this waterproofing system on many terraces nowadays.
Stone age did not come to an end because of scarcity of stones. Similarly, brick bat coba too may disappear from roofs not because of scarcity of bricks but due to other reasons.