Roof it with shell

The human mind never ceases to think and explore, even when we know that all the innovations may not be beneficial, for the simple reason that one valid ideation among a hundred failed ones is still worth the effort. On a comparative note, the building industry does not indulge in continuous and steady research; hence a whole lot of established concepts continue to be employed. As such, occasionally when a great new idea springs up, we should not lose out on the opportunity to explore it or even employ it.

The last century has seen innovative roofing ideas such as hyperbolic structures, catenary curved vaults, tensile membranes, domes without formwork, prefabricated shelters, geodesic domes, ferro-cement roof panels, space frames, and pre-engineered structures. Mostly known to architects and engineers, these high-ended ideas demand a thorough knowledge of roof behaviour, technological expertise to manufacture the components and skilled labour force to execute them at site. However, these conditions do not mean they should be excluded from routine public buildings and normal house constructions.

Funicular shells also belong to this set of alternative solutions. Many architects have used this idea extensively in their projects. Shell roofs are most economical in short spans of 1.5 to 3 metres, so the roof area is divided into grids of judicious spans, preferably square, even though rectangular shapes are also workable. These grids are formed by RCC beams, virtually creating a framework of side supports with the central part to be filled by the shell roof. The central filler part is a two-way curved shallow dome-like profile generated by the required geometry. Upon the form work, bricks are laid in a chosen pattern, which becomes a self-supporting system, held by the beams in the edge, with minimal steel and concrete.

Column design

In case of large halls, the main columns can be in the designed spacing, but the space in between needs to be subdivided by cross beams to take the shells, while in areas like bedrooms, each room can get one roof. The edges of this shell roof shall be thick to balance the thrust effect of the roof weight, becoming thinner as it reaches the top centre. The upper part of the shell can be filled with brick bats or lean mortar to get the level floor finish as required. The weight of the flooring material pushes the shell down, stiffening it in the process and making it stable.

Achieving perfect geometry is very important to ensure arch action within the curved shell, transferring the loads to the side supports. Considering the detailed calculations required for roof form, making fibre glass formworks in smaller parts becomes even more economical than individual formworks, besides the possibility of them in different combinations to get roof forms.

Once these basics are followed, a wide variety of patterns can be worked out for the roof. What initially appears as complex, emerges as a do-able idea.

(The writer is an architect working for eco-friendly designs and can be contacted at

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Printable version | Apr 11, 2021 12:56:47 PM |

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