Filler Slab Roofs – A NG (New Generation) roofing technique

A roof is a covering component on uppermost part of the building which provides protection from weather primarily against rain. As roofs are just covering components, live loads will not act upon it. Still the common practise is to cast solid RCC slabs like floor slabs to keep the aesthetic view of the structure or building. Now innovative ideas are brought in by engineers and constructors to cast roof slabs in an economical way without hindering the aesthetic side. One such new generation idea is filler slab.

Concrete which is the principal element in construction is strong in compression, but weak to withstand tension.  In a simply supported slab, lower part is subjected to tension(tension zone) and upper part is subjected to compression(compression zone). To avoid the failure of slab, steel reinforcement is provided in tension zone as steel can withstand tension. The concrete in tension zone has no structural purpose except to hold the reinforcement. These unnecessary concrete portions can be filled with some light weight materials and such slabs where concrete in tension zone is reduced by the use of light weight fillers are known as filler slabs. Though coconut shells, mud bricks, earthen pots, thermocol etc can be used as filling materials, clay roofing tiles are commonly used as fillers in slabs. Clay roofing tiles can be installed in two layers (2 tiles by one over the other) and thus an air pocket is made in between which is an extra advantage for reducing the quantity of concrete.

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Things to be considered for casting RCC filler slabs

  • The important thing is that the filler material should be of low cost when compared to the quantity of concrete to be replaced as this method is mainly adopted for economical advantage
  • Filler material should not react with the concrete and steel
  • Filler material should have low water absorption rate and if not, the water-cement ratio of the concrete may be affected. If clay roofing tiles are used, the same may be soaked in water to reduce its water absorption capacity.
  • Filler materials using in a simply supported slab should be of same size and cross-section without affecting the spacing of reinforcement.
  • Filler material should be of good finish as the material will be in visible condition on bottom part of the slab i.e. inside the building.
  • Considering the above points, it is better to use clay roofing tiles as filler material in simply supported slabs.
  • If rectangular clay roofing tiles are used in a one way slab, the tiles should be placed in such a way that the shorter side of the tile is parallel to the longer span of the slab so that the spacing between the main bars can be kept to minimum. In one way slab main bars are bars along the shorter span.(See the image shown above)
  • Slab thickness should be minimum 110 mm.

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Advantages of Filler Slabs

  • There is considerable saving in concrete and steel by adopting RCC filler slabs instead of conventional RCC solid slabs and approximate cost saving is Rs.200/sqm.
  • If clay tile roof of a building is removed for constructing RCC slab roof, the clay tiles can be reclaimed and can be used as filler material for constructing RCC filler slabs
  • If done with proper workmanship, the filler material can be exposed inside the building to create aesthetically good ceiling.
  • Reduction in concrete and steel reduces the generation of carbon which in turn makes the slab construction eco-friendly.

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