Almost every new house in South East Queensland sits on one of two slab types. Your engineer picks which one; this is what they are choosing between and why it changes the price.
Waffle pod slabs
A waffle pod slab sits on top of the ground rather than in it. Polystyrene pods are laid out in a grid on a prepared, level pad, with gaps between them that become the concrete ribs. Steel goes in the ribs and over the top, and the whole thing is poured in one go. The slab you see from the side is the edge beam; underneath, it looks like a waffle.
Because the pods do the work of the ground, there is very little excavation. On flat lots in the big estates that makes a waffle pod fast and economical, and the stiff grid of ribs handles the moderate movement of the clay soils common across the Gold Coast, Logan and Ipswich.
Raft (conventional) slabs
A raft slab has its edge beams and internal beams dug into the ground. The beams are trenched, steel is placed in the trenches and across the slab, and the pour fills trenches and slab together. The slab is tied into the ground rather than sitting on it.
Engineers tend to go to a raft where the site is sloping, has been cut and filled, or where the soil report calls for extra stiffness in particular spots. There is more excavation and more concrete in the beams, so the price is usually higher than a pod slab of the same footprint.
What decides it
Your soil report. A geotechnical engineer classifies the site under AS 2870 (A, S, M, H1, H2, E or P), and the structural engineer designs the slab to suit. Reactive clay, fill, slope and trees nearby all push the design one way or the other. OnlyCrete pours whichever is specified; we do not change the engineering, we build exactly what is drawn.
What it means for the quote
- Pod slabs: less excavation, more polystyrene, usually quicker on site.
- Raft slabs: more excavation and more concrete in the beams, more machine time.
- Both: piers add cost where the soil report asks for them, and so do step-downs, drop edge beams and extra internal beams.
Send us the slab plan and the soil report and the quote comes back as a fixed price with pump, steel, termite system and inspections included.
Reece Montague is a QBCC licensed concreter (QBCC licence 15593005) and the founder of OnlyCrete Concreting Pty Ltd. He has poured house slabs, footings and driveways across the Gold Coast, Brisbane, Logan and Ipswich for more than ten years. About Reece.