Raft Slab :

Raft slabThe steel reinforced raft slab is also commnly called an engineered slab. It consists of thick steel reinforced concrete slab integrated with steel reinforced concrete beams founded into the bearing soil for strength and support.

The classic approach is:

  1. Trenches about 450mm wide x 450mm deep are dug around the perimeter of external walls and under load bearing walls
  2. Reinforcement cages are laid in the trenches and reinforcement bars are also laid at this time, and turned up to pass above the top of the strip footing to accommodate later slab structure
  3. Concrete mix is poured into the trench to ground level with vertical reinforcement bars protruding up to be formed with the floor slab
  4. Moisture protection, termite protection, electrical conduit, drainage and sewerage pipes for bathrooms and kitchens are positioned
  5. Formwork for the slab is then erected along the outer edges of the building footprint and on top of the cured concrete footings
  6. Steel reinforcement is laid for the slab
  7. Concrete is poured in one operation creating a slab that covers the entire floor area

Waffle Slabs

waffle slabWaffle slabs are a reinforced concrete footing and slab system constructed on ground. They consist of a perimeter footing (edge beam) and a series of narrow internal beams (strip footings) at one metre nominal centres running each way. The whole footing and slab system is constructed on top of the ground.

The sides of the slab are made by edged Formwork, and polystyrene blocks (pods) create the formed voids between the strip footings. When viewed from underneath, the system of internal strip footings looks like a waffle – hence its called the Waffle Slab.

Waffle slabs achieve their strength by varying their height above ground. The higher the slab above ground – the deeper the beams. The deeper the beams – the more stiffness the system has.

It is InnoHomes’ preference to build on Engineered Raft Slabs. While Waffle Slabs may be cheaper to build in some circumstances, they do not work very well in:

  • Soft ground conditions or where the soil can move.
  • Sloping sites. In simplistic terms – there is only their weight stopping them from sliding down a hill.
  • Cyclonic areas and high wind areas. High wind forces will engage enough of the waffle slab to resist the force but not without deflection of the slab. In very high Wind conditions it is possibe that the waffle slab will lift and the walls crack.

Also these are some considerations in our opinion, for preferring an engineered raft slab

  1. If the ground around your house gets eroded or washed away, the area under a waffle slab is exposed. Exposed areas under the Waffle Slab can attract reptiles and other wildlife seeking Shelter.
  2. Ground preparation must be immaculate and kept that way for the life of the slab, by the owner/resident of the House. Care must be taken not to overwater the ground near your house, build up the ground around your house, allow surface water to run towards your house (and under your house) or plant gardens next to your house, as this may all affect the slab.
  3. Raft slabs are cast against the ground whereas waffle slabs are cast onto polystyrene void formers and strips of concrete. An overloaded raft slab is less likely to crack because it is cast onto the ground.

Diagrams used are from the RMIT website.

Leave a Reply