Quick answer: A filler slab is a reinforced concrete slab in which part of the concrete in the lower zone is replaced with light filler material such as clay tiles, clay pots or hollow blocks. It uses less concrete and steel than a solid slab, weighs less and keeps rooms cooler, but it needs careful design and skilled workmanship.
Filler slab construction is a way of casting a roof or floor slab with less concrete and steel. It has drawn attention as a lower-cost and more sustainable alternative to the solid slab, and it makes use of locally available materials. This guide explains how it works, what it is made of, how it is built, and where it does and does not suit.
What is a filler slab?
A filler slab is a reinforced concrete slab that uses less concrete but still performs its structural function. Filler material is placed between the reinforcement bars in the lower part of the slab, which gives a ribbed structure. Because the fillers are light and are often low-cost construction materials, the slab ends up lighter and more economical.
How does a filler slab work?
When a slab bends under load, the top of the slab is squeezed and the bottom is stretched. Concrete is strong when squeezed but weak when stretched, so in the lower zone it is the steel bars that do the work. Much of the concrete there carries very little load.
A filler slab replaces that non-structural concrete with light material: tiles, clay pots, lightweight concrete blocks, or waste such as broken bricks and coconut shells. The fillers sit between the reinforcement bars, which are arranged in a grid. The concrete between the fillers forms ribs around the steel, and a continuous layer of concrete covers the top. The slab stays sound, with less weight and less material.
What is a filler slab made of?
- Concrete: a mix of Portland cement, sand, coarse aggregate and water in the designed proportions. It gives the slab its strength and stability.
- Filler material: light, non-structural pieces such as tiles, clay pots or hollow blocks, placed in the bottom zone between the bars.
- Main and distribution reinforcement: steel bars running in both directions in the ribs between the fillers. They carry the tension and spread the load across the slab.
- Mesh reinforcement: where the design calls for it, a mesh above the fillers supports the top layer and controls cracking.
Where is a filler slab suitable?
- Roof and floor slabs of houses and other low-rise buildings with ordinary room spans.
- Top-floor roofs, where the insulation from the fillers is most useful.
- Projects where the ceiling will be left exposed as a design feature.
It is not the right choice for slabs that carry heavy loads, and it is not suitable for every type of structure. The decision belongs to the structural engineer.
Design and planning
A filler slab has to be designed, not improvised on site. The design covers:
- Load analysis: what the slab has to carry over its span.
- Filler selection: the size and thickness of the filler decide the rib spacing and the slab depth.
- Reinforcement design: the bar spacing is set to match the filler size, so one filler fits neatly in each bay of the grid.
- Formwork design: the formwork carries the fillers as well as the wet concrete.
- Concrete mix design: the mix must flow into the narrow ribs and compact fully.
How to choose and prepare the filler
- Light: the filler must weigh less than the concrete it replaces.
- Inert: it must not react with cement or rot inside the slab.
- Consistent size: uneven fillers give uneven ribs.
- Sound: cracked tiles or pots break during concreting and fill with concrete, which defeats the purpose.
- Soaked before use: clay fillers are soaked in water before placing, so they do not draw water out of the fresh concrete.
Step-by-step construction of a filler slab
- Formwork: the formwork is the temporary mould for the concrete. In a filler slab it must also support the filler materials, so it has to be level and firm.
- Steel reinforcement: the bars are laid in a grid and tied with binding wire at the intersections. Extra reinforcement may be needed at edges and other high-stress areas.
- Placing the fillers: the fillers are set between the bars, secured so they cannot shift or float, and checked for alignment.
- Concreting and curing: concrete of the specified mix is poured over and between the fillers, and vibrated to remove air. The surface is levelled, and the slab is then cured for 7-14 days so the concrete gains strength.
Advantages of filler slab construction
- Less weight: lighter than a solid slab, so the load on beams, columns and foundations is lower.
- Lower cost: uses less steel and concrete.
- Thermal insulation: helps keep indoor temperatures steady.
- Lower environmental impact: less cement and steel, and fillers that are local or recycled.
- Appearance: the pattern of fillers can be left exposed as the ceiling.
Disadvantages of filler slab construction
- Complex design and execution: needs an engineer familiar with the method and skilled labour on site.
- Limited use: suitable only for certain types of structures and loads.
- Quality control: poor fillers or poor concreting can compromise the slab.
What to check on site
- Fillers are kept clear of the supports. The slab is left solid near beams and walls, as shown in the drawing.
- Every rib has its bar, on cover blocks, with concrete space all round it.
- Fillers are soaked, whole and fixed in position before the pour.
- Electrical conduits run through the ribs or the top layer, not across the fillers.
- The concrete is vibrated carefully so the ribs fill without displacing the fillers.
- The underside is inspected for gaps or honeycombing after the formwork is removed.
Common mistakes
- Building a filler slab from a mason’s experience, without a structural design.
- Placing dry clay fillers, which suck water from the mix and leave weak concrete around them.
- Walking on the fillers during concreting and cracking them.
- Making the ribs too narrow for the concrete to flow around the steel.
- Cutting the curing short.
A filler slab saves material only when it is planned, designed and supervised properly.
Related guides
To compare with a conventional slab, see the cost of an RCC slab per square foot and the difference between one-way and two-way slabs. Another slab system is covered in our guide to the flat slab. The steel in the ribs is explained in reinforcement in construction, and the last step in the types of concrete curing. To discuss the right slab for your house, see our construction service.
FAQs
Can filler slabs provide thermal insulation?
Yes. Fillers such as clay pots and hollow blocks hold air pockets inside the slab, and still air slows the flow of heat. A filler slab roof therefore keeps the room below cooler than a solid concrete slab does.
What are the primary components of a filler slab?
Concrete, steel reinforcement and filler materials. The concrete and steel carry the load. The fillers, such as tiles, clay pots or hollow blocks, only take up space in the lower zone between the reinforcement bars.
Can filler slabs be aesthetically customized?
Yes. The fillers are visible from below, so they can be laid in a regular pattern and left exposed. Many owners leave a filler slab ceiling unplastered so that the tiles or pots form the finished ceiling.
Is a filler slab as strong as a normal RCC slab?
When it is designed by a structural engineer for the actual span and load, a filler slab performs its structural function like a solid slab. The filler replaces only concrete that carries little load. It is not suitable for every structure or for heavy loads.
Which materials are used as fillers?
Common fillers are clay roofing tiles, clay pots, lightweight concrete blocks and hollow blocks. Waste materials such as broken bricks and coconut shells are also used. The filler should be light, inert and of a consistent size.
