Flat is fine on the roof
It was as if the century-long search for a perfect roof ended when flat RCC was discovered.
With both architecture and civil engineering stretching into newer frontiers, a dozen types of roof forms could be possible today, yet the flat RCC roof has won over India. From Shillong to Srinagar, Dwaraka to Rameshwaram, majority of roofs being cast are such concrete slabs. It is as if the century-long search for a perfect roof ended when flat RCC was discovered, which is partly true!
The traditional sloping roofs did not permit new additions, unless one removed it completely and then added upper floors. There was no option of terrace use anyway. Most houses of the past were not adequately fire proof, considering the easy combustibility of natural materials.
Geometrically controlled roofs such as sloping and curved ones could not cover any shape of a room, but dictated a proportion between length and width; or between roof height and wall heights. Sloping roofs needed to be well fixed to avoid their slippage which is among the common cause of failure. Against many such imperatives, flat roofs were always an attraction. It is not that there were no flat roofs in history, but that they were far less prevalent. They were traditionally done in low rainfall areas and hot dry climate, but their popularity was restricted to such vernacular practices only.
Being built with locally available materials, the room sizes were restricted, and mostly suffered from water seepage and caving of roofs after decades of existence. Gradually, as the advantages of flat roofs were discovered, people living in heavy rainfall areas also attempted flat roofs, Madras terrace being a good example.
In many ways, flat RCC roof has revolutionised Indian construction, hitherto dominated by regional variations and local practices. Structural issues such as deflection or the sagging of the slab could be arrested by designing the steel reinforcement accordingly. As such, much larger spaces are today covered than ever.
It is the flat roof that made rainwater collection possible and effective, even though people living in sloping roofed houses also had tried to collect rainwater through gutters, but with much less collection. Flat roofs lend themselves for a variety of utilitarian activities, so they double up as a small gathering space; roof garden; material dumping place; place to sleep at night or play area for children. Comparatively, they are far easier to cast, being just a horizontal surface, which could be placed on any irregular plan form.
From an ecological perspective, though heat gain is an associated problem, it could be reduced with many alternative methods such as mud phaska layer, brick bat coba, flat burnt brick tiles or roof mortar painted with white paint or lime coat. RCC roofs have high embodied energy and excessive use of RCC damages nature more than many other options. However, considering the ease of maintenance and affordable costs, they are the most popular roof form today.