Typical pricing below

Raft Slabs

A raft slab is a type of house slab designed as one continuous, heavily reinforced structure that spans the entire building footprint, rather than relying on separate footings. It works like a raft floating on the soil, spreading the building's weight evenly so the slab moves as a single rigid unit if the ground shifts. It's typically specified on reactive clay, filled or otherwise poor-bearing sites where standard footings aren't reliable enough.

Get free quotes for raft slabs Read the FAQs

Pricing guide: How much does a raft slab cost?

Raft slabs typically cost more than a standard slab because of the extra concrete and steel in the edge beams, usually in the range of $180 to $280 per square metre. The exact cost depends on the soil report and the engineer's design.

Why homeowners choose it

Spreads structural load evenly across the whole footprint
Well suited to reactive clay, filled or uneven sites
Moves as a single rigid unit rather than cracking at weak points
Reduces reliance on deep, individual footings
Specified by an engineer to match the site's soil report

How it works

1

Soil classification

A geotechnical soil report identifies the site as reactive, filled or otherwise requiring a raft design.
2

Engineering design

An engineer sizes the raft's thickened edge beams, internal ribs and reinforcement for the specific site.
3

Excavation and edge beams

The pad is excavated, and deeper trenches are formed around the perimeter and load-bearing lines for the thickened edge.
4

Reinforcement placement

Heavy reinforcing steel is tied throughout the edge beams and slab area to make the raft act as one rigid structure.
5

Single continuous pour

The entire raft is poured in one continuous operation so there are no cold joints weakening the structure.

Hiring guide

Questions worth asking before you commit to a contractor.

Are they licensed and insured for this work?

Ask to see it, not just take their word for it. Insurance protects your property if something goes wrong on site.

What exactly is included in the price?

Site preparation, disposal of old material, and any council permits can be included or charged separately — get it in writing before you compare quotes.

Who will actually be on site?

Some contractors do the work themselves; others subcontract parts of it. Neither is wrong, but you should know who is doing your job.

What happens if something unexpected turns up?

Old concrete, tree roots or drainage issues are common surprises. Ask how a change of scope gets costed before it happens, not after.

What is the warranty on the work?

Materials usually carry a manufacturer's warranty. Ask specifically about the contractor's own workmanship warranty too.

More guides and advice

Frequently asked questions

How much does a raft slab cost?

When is a raft slab needed instead of a standard slab?

Raft slabs are specified when a soil report shows highly reactive clay, filled ground, or other conditions where standard strip footings won't adequately support the building.

How is a raft slab different from a waffle slab?

Both spread load across reactive soil, but a raft slab is typically a solid, uniformly thick slab with deep edge beams, while a waffle slab uses a grid of ribs and void formers to achieve a similar result with less concrete.

Does a raft slab need special reinforcement?

Yes. Raft slabs use heavier and more closely spaced reinforcement than a standard slab, particularly in the edge beams, as specified by the structural engineer.