It is an irony to realise that the problem of modern roofing is often the problem of erecting the centering too. The more complex the roof with curves and fluid shapes, equally complex will be the temporary support. Incidentally, even with the ordinary flat RCC roofs, the centering cost could be anywhere around 25% of the total cost of roof, excluding the finishing cost.
This happens partly because the concrete roof has to be cast by pouring the mix on a flat pre-set surface; hence the surface has to be created before hand. Traditional buildings most often had stone slabs, wooden beams, clay tiles, thatch and such individual components to be lifted up and placed, hence required part support only. Even today, by reverting to the old practice, we can save on the centering cost.
ResearchIn this direction, pioneering research has been done by Jagadish, Reddy and Yogananda, civil engineers of the Indian Institute of Science, who formed a group called ASTRA. They suggested alternative roofs using masonry and found curved roofs called vaults to be strong and economical. Instead of placing a full surface centering, they suggested we can provide part support to erect the roof and keep moving this support to complete the roofing.
Once the walls of the room, preferably square or rectangular, reach the roof level, a wall beam is placed in L shape along the lower ends of the vault. This shape houses the two ends of the specially fabricated steel truss, two of them placed adjacent with just enough gaps where masonry blocks can be kept on top and the roof built using rich mortar. The two pieces are tied to avoid them falling apart. Once the blocks are placed, the formwork is moved the next day to repeat the process.
Though stabilised blocks have been popularly used in Bangalore, clay or cement blocks can also be used, as long as it is all done under supervision.
Where the upper floor has to be useable, vaults can be done fairly flat with the sides filled up to get another floor. Normally these are used for the last roof and top surface finished with joint water proofing.
This system does not immediately suit barrel vaults, but works well with segmental vaults with catenary curve. Profiling the catenary curve is important to achieve best results. The wall beams are necessary to balance the side thrusts.
In traditional buildings, thick walls or side buttressing were used to get the structural stability, which we can not afford to do today. Up to 12 ft. span, this system makes ecological and economical sense. For wider spans, thicker base and thinner top can be provided. The thickness of vault itself can be altered based on the room sizes.
Building vaults with moving supports has been around Bangalore for over 20 years now, the early models still going strong. Unfortunately, such radical ideas are not known to too many owners and builders, as such continue to be ignored.
(The writer is an architect working for eco-friendly designs and can be contacted at varanashi@gmail.com)
