Every slab quote starts with a letter from the soil report. It describes how much the ground under your house is expected to move, and it drives how much steel, concrete and digging your slab needs.
The classes
- A: sand and rock, little or no movement. Rare in the growth corridors.
- S: slightly reactive clay, small movement.
- M: moderately reactive. Common across the Gold Coast and Logan estates.
- H1 and H2: highly reactive clay. The ground swells when wet and shrinks when dry by enough to crack a slab that is not designed for it.
- E: extremely reactive. Deep beams, heavy steel and often piers.
- P: problem sites. Fill, soft soil, landslip risk or trees. These need an engineer's specific design rather than a standard one.
Why it costs more as the letter goes up
A more reactive site needs a stiffer slab. Stiffness comes from deeper edge and internal beams, more and heavier reinforcing steel (SL92 mesh instead of SL72, more bars in the beams), and sometimes bored piers that carry the slab down to stable ground. Each of those is more concrete, more steel and more time on site, which is why two slabs with the same floor plan can have very different prices on different lots.
What we do with the classification
Nothing creative. The structural engineer turns the class into a drawing, and OnlyCrete builds the drawing exactly: beam depths, mesh grade, bar sizes, pier depths and spacing. The certifier inspects the piers and the slab before we order concrete, so what is poured is what was designed.
Tips for a cheaper slab
- Get the soil test done before you fall in love with a block. The class can swing the slab price by thousands.
- Keep the site cut neat and the pad level. Fill that has to be fixed costs more than fill that was never there.
- Decide on step-downs, bathroom recesses and alfresco levels before the slab is drawn. Changes after set-out are expensive.
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.