Why our 112.5 m² raft slab took over 20 m³ of concrete

When the concrete trucks started arriving for our build-under slab, the concreters kept commenting on the volume. They had seen the engineering drawings. They had poured plenty of slabs. They still couldn’t believe how much concrete was going in.

I was happy. The volume was exactly what I wanted to see.

For the macro cost picture this slab sits inside, see the full build-under cost breakdown.

The site we were building on

The Coorparoo block sits on shale rock with active ground. The geotech report came back clearly: this site needs a slab engineered for variable subsurface conditions, not a generic slab type.

The structural engineer specified a raft slab: and specified it at the heavier end of the range. A raft slab is a thick reinforced concrete slab that effectively “floats” on the ground. The load spreads across the whole slab rather than transferring through point footings. Deep edge beams extend the slab perimeter down into more stable material so the structure has something solid to sit on. On active ground and shale, the slab is thicker, the edge beams go deeper, the reinforcing is heavier and the slab uses more concrete than a raft built for a benign site would.

How a 112.5 m² footprint took over 20 m³ of concrete

Our enclosed build-under footprint is 12.5 x 9 m: 112.5 m². The slab pour came in at over 20 m³ of concrete. That ratio is what makes people pause.

A raft slab footprint is broadly the same as the building footprint above it. What varies with site conditions is not the slab area: it is the slab thickness, edge beam depth and reinforcing density. All three scale with what the engineer has specified, and all three scale the cubic metres of concrete poured per square metre of footprint. On shale with active ground, the engineer specified the slab thickness and edge beams at the higher end of the range so the slab had the mass and depth to resist ground movement.

The result was a slab that took materially more concrete per square metre than a slab built for a benign site would. That is exactly the point of a raft on this kind of ground.

What the concreters were reacting to

Watching trade reactions on a build tells you a lot. When a concreter says “wow, that’s a lot of pour” while their truck is still emptying, you are either over-engineered or you are getting the right answer for the site. The drawings told us we were getting the right answer.

The volume of concrete per square metre of footprint was what made them react: not the area of the slab. Deeper edge beams, a thicker slab and heavier reinforcing meant a slab that was not going to move with ground variation. Active ground does not forgive a thin slab.

Why I was happy with the $37,900

The slab line came in at $37,900 including earthworks, termite barrier, formwork, steel and the pour itself. That sits at the higher end of slab costs for a build-under of this size: because that is what active ground and shale rock require.

It is also the cost of a slab that is not going to crack the house above it five years from now. A slab failure on a raised Queenslander build-under does not just damage the slab. It compromises everything sitting on top: framing, services, fit-out, the lot. Over-spec’ing the slab is cheap compared to any of those failing because of subsurface movement.

The lesson

Don’t price-shop a slab on volume. A concreter quoting “we would usually pour X m³ on a job this size” is not quoting your site. They are quoting an average. Your slab depends on what is underneath you and what the engineer specifies. Trust the engineering, not the average.

When you get a slab quote that looks high, ask what soil class the quote is for. Ask if the engineer has specified edge beam dimensions and slab thickness. Ask whether there are deep footings or thickening sections. If the answers come back vague, you are not getting a slab priced for your site. You are getting an average.