If we pull out any textbook on flat roofing written during the last century, it's possible that the most prominent chapter would be on Madras terrace roofing. Those were the days when flat roofs were becoming popular, where vernacular flat roof forms had one or the other problems. The Madras terrace method of construction, in many ways, incorporated the best of all known roof forms, avoiding their pitfalls. Thatch roofs were not fireproof, roofs with twigs and branches had termite problems while mud floor roofs were thick and heavy.
Also, there was a desire to move from the kutchha or seemingly short-life approach to pucca , long-term methods. The role played by the British engineers in the evolution of Madras terrace needs to be mentioned here.
Wooden beams, normally teak wood in those days, would be first placed upon opposite walls across the width of the room, 18 to 24 inches apart.
In case room spans are wider, steel sections would be first placed dividing the room into shorter spans, along which teak beams run.
High density and high strength clay bricks, made to special thin size measuring 1”x3”x6”, were used in Madras terracing. Properly mixed and matured lime mortar was used for bonding the flat tiles that are placed at an angle of 45 degrees to the wall, or diagonally across the room width.
These terrace tiles, placed on the edge, ensured tensile strength.
The roof was cured for a minimum of one week to achieve early setting.
Thereafter, a three-inch thick layer of broken bricks or brick bats would be laid where nearly half the volume would be made up of lime mortar, three parts brick, one part gravel and one part sand. This layer provided the compressive strength and load bearing capacity to the roof.
This layer needs to be well compacted, cured and levelled.
The final layer would depend upon the slab being an intermediate one or the final roof.
If intermediate, a floor finish like red oxide or lime mortar would be applied and if final, there would be courses of flat weather-proof tiles topped by thick mortar to slope.
In case we believe Madras terrace roofs have already been written off, we need to rethink.
There have been attempts in a few cases, including at Auroville, to revive this technology.
We may not get the same old terrace tiles, but get thin perforated WPC tiles, cladding tiles and such others that can be used to build up the roof, supported by steel sections.
The roof would be thicker than our RCC slabs that should be considered in ensuring required floor heights.
Incidentally, the roof type was termed as Madras roof due to its widespread use in Tamil Nadu, though it became popular all over south India.
Even today, we occasionally meet people who can recollect having got their houses done with Madras roof.