Filler roofs are designed using the same principles as RCC, except for replacing part of the concrete with some alternative material. A look by Sathya Prakash Varanashi
Last week belonged to queries about filler roofs, following the preceding essay introducing the idea. No wonder there were so many questions about it, considering it is an exciting solution, though less heard of. While the regular RCC is widespread even in remote villages, it is surprising that filler roofs are hardly known around and have minimal visibility.
By the early 1980s, many roofs were cast with filler material in Kerala and later in Bangalore too. With nearly 30 years behind them, their durability has been repeatedly proven. Incidentally, when a commoner wonders if the roof is strong enough, what actually he enquires about is whether the roof will structurally perform well with load transfer, tension, compression, deflection and such technical matters. Since filler roofs are designed using the same principles as normal RCC, except for replacing part of the concrete by some alternative material, worries about strength can go unquestioned. It is the designing for tension load, hence the spacing of steel reinforcement that would differ as per the sizes of filler material.
No new problem
Behaviourally, the slab acts like any other slab, and as such does not pose any new problem not seen in regular RCC roof — be it with possibilities of water seepage or cracks! In either case, normal RCC or filler, such issues need to be attended to, should they appear unfortunately. Quality of construction at site is a pre-condition which can affect both the mainstream and the alternative practices, and as such should not be used only to degrade the alternative!
Availability of the hollow clay filler blocks could be an issue, to be explored by the potential user, but any local clay roof tile supplier could be of help. Alternatively, a variety of locally available materials could be inserted as filler, including stabilised sun dried mud bricks. When we place glass bottles or perforated jali blocks, we get skylight effect.
Cement blocks have already been experimented with, which then could be plastered and concealed. There are many filler roofs done with mud pots or bowls as filler material, where we get the looks of an artistic ceiling, at a lower cost.
While concreting, the filler blocks may move due to labour movement, which demands careful on site handling. Depending upon the room size, there could be savings in the steel cost; however the cost of block and increased labour would ensure our savings are spent! In large span roofs, where it is desired to avoid the beams, one may design a thicker slab, then introduce filler concept to save on concrete.
When the whole roof gets cast as clay filler slab, it's possible that some family member may not like the red look at the bottom of the roof. Such people have the choice of getting the ceiling painted white, giving a designer false ceiling appearance, thanks to the fluted profile of the block!
(The writer is an architect working for eco-friendly designs and can be contacted at email@example.com)