House slab calculator: raft and waffle pod

What a stiffened raft slab is

A stiffened raft is a floor slab with deeper beams cast into it: an edge beam all the way round and, on most sites, internal beams across the middle. The slab, the beams and any pads under heavy points are poured in one go.

So the concrete is the slab plus the parts of the beams that go below it. For the 12x10m example that is 12 m³ of slab and 3.85 m³ of edge beam.

How the edge beam corners are counted

Adding up the four sides of the edge beam counts each corner twice. The calculator instead takes the band inside the outline, so every corner is counted once. For the example that is 0.11 m³ less than adding the sides up.

The same holds for L and U shaped houses with square corners. Where internal beams cross, enter the number of crossings and the overlap is taken off once each. Sloped beam sides use the width halfway down.

Waffle pod slab volume

A waffle pod slab sits on polystyrene blocks laid on the ground, with ribs between them and a topping over them. The concrete is the whole depth minus the pods. On the same house with 225mm pods and an 85mm topping, 63 full pods leave 20.36 m³, against 37.2 m³ if the slab were solid to that depth.

Ordered as ready-mix with 10% extra, that waffle slab is 22.4 m³.

Why the pod count comes from your plans

Pod layouts depend on the rib width, the edge setback and where beams and set-downs fall, so two houses of the same area can take different numbers of pods. The calculator does not guess the count: type the full pods shown on your plans, and the area of any cut pods.

Open or moulded pods hold less than a solid block of the same size. For those, choose Other pod and type the void volume from the supplier, or the answer will be short.

What the calculator leaves to your engineer

Wall thickenings, garage step-down beams and deeper edges from your drawing go in as extra runs, each as length × width halfway down × depth below the slab, and piers under the beams go in as a count. Beyond that the tool stops and the drawing takes over.

• Beam depths and slab thickness by site class: your engineer sizes them to AS 2870, and the calculator only adds up what the drawing shows.
• Set-down recesses for wet areas: a few hundredths of a cubic metre a metre, left out.
• Steel is not deducted: mesh and bar take well under 1% of the volume.
• Over-excavation and an uneven base: measured on site, not from a plan, so order with the CCAA 10% allowance and the concreter's judgement.
• Classes E and P and filled sites, edge beams over 1.5m deep, slabs over 30m long: engineering advice territory.

Ordering ready-mix for a house slab

Cement Concrete and Aggregates Australia (CCAA) advises ordering about 10% more than the exact volume and rounding up, because premix is batched in 0.2 m³ steps. Suppliers often charge a small load as if it were a bigger one, so a top-up truck is poor value: order enough the first time.

Questions

Goes with this job

• Concrete slab calculator
• Concrete footing, pier and sonotube calculator
• Ready mix concrete calculator