Reduce the concrete in filler slabs
Filler slabs do not demand greater technical skills or supervision than the normal slabs, and hence can be employed widely.
We are talking common sense. When we need to buy something, if it costs high, either we shop for a different item or reduce the quantity of purchase. In the case of roof, concrete is not replaceable on most occasions. So, the least we can do is to reduce the RCC quantity, which can be done by partly replacing concrete by any other economical, beneficial or attractive material. Since the roof thickness gets filled so, technically such roofs are termed as filler slabs.
Incidentally, even though all RCC roofs are a single monolith cast of concrete along with steel, the area between the steel reinforcements play no role in load transfer. Yet, we cast the roof in full because it cannot be cast with innumerable voids in between, considering the form work or the temporary supports that are required for casting the roof.
One solution popularised by Lauri Baker, the pioneer architect of cost effective architecture in India, was to place two numbers of low grade Mangalore roof tiles in between the steel rods. Once the centering is done, the filler material is placed and then the steel rods tied as per the required spacing. Since then, architects and engineers have placed everything possible under the sun, including bottles, computer key boards, cement blocks, mud pots and such others to achieve filler slabs. The criteria in selecting the material has largely been the cost and its looks.
Since the heat gain from the roof is the highest, if the filler slab could be done using hollow materials, we get multiple advantages – passive solar cooling where the voids reduce the heat transfer, lesser roof weight thanks to the hollowness, different looks when seen from the room below, reduced sound transmission between the floors thanks to the voids in between and reduction of steel consumption considering greater distance between them. Accordingly, nowadays a special block called hollow clay roof block, also called as maruthi block or filler block, has gained popularity. These are manufactured by clay brick and tile manufacturers, mainly in Kerala and Karnataka.
Once the roof centering is complete, it is levelled by a layer of stabilised mud or very weak cement mortar. First the roof blocks are placed end to end as per the structural engineers design, placing them closely and compactly. The minor gaps in between are sealed with the same lean mortar to reduce curing water flowing down after the roof casting. These blocks come with specific spaces to run the steel rods, in both the directions as required by room spans. A layer of thin steel rods is placed on top, electric conduits are tied and the roof is concreted as usual.
Following the stipulated 21 days of curing and de-shuttering, the construction proceeds just like any other roof. Filler slabs do not demand greater technical skills or supervision than the normal slabs, hence can be employed widely