What is a Raft Slab

A raft slab is a reinforced concrete slab-on-ground that is stiffened by a grid of concrete beams cast underneath it, running in both directions. The beams around the perimeter are called edge beams, and those across the interior are internal or stiffening beams. Together, the slab and beams act as a single “raft” that spreads the house load over the ground and resists movement of the soil beneath it.

In Australia, raft slabs (more fully, stiffened raft slabs) are one of the most common foundations for houses and extensions. Residential raft slabs are designed in accordance with AS 2870, the Australian Standard for residential slabs and footings.

Raft Slab Cross-Section

How Thick Is a Raft Slab?

The slab part of a residential raft is commonly 100 mm thick. It may be thickened, to around 150 mm or more, where it needs extra strength or must span further, such as under heavy point loads or over poor ground.

Most of the raft’s stiffness comes from the beams, not the slab. Edge beams are commonly 300 mm wide. Their depth, and the depth and spacing of the internal beams, depend mainly on how reactive the site’s clay soil is. That is set out in the site classification from a soil test. Highly reactive sites need deeper and more closely spaced beams.

Waffle pod slabs, a different form of raft, typically have a thinner top slab, often around 85 mm. The exact minimums for each slab type are set by AS 2870 and the project’s engineering design.

Raft Slab vs Other Slab Types

Raft slab vs waffle slab

Both are stiffened rafts that sit on the ground (see our article on waffle slabs). A conventional raft has beams dug into the ground. A waffle slab is built on top of the ground, using polystyrene pods to form a closely spaced grid of shallower beams. Waffle slabs suit flat sites and need little excavation. Conventional rafts suit sloping or variable sites and situations where beams need to reach further down.

Raft slab vs strip footings

With strip footings, separate concrete footings support the walls and the floor is a separate element. In a raft, the beams and slab are cast together and act as one stiff structure, which is why rafts handle reactive clay movement better.

Residential raft slab vs “raft foundation”

Internationally, “raft foundation” or “mat foundation” often means a thick, heavily reinforced concrete mat under a large building, used to spread very large loads. An Australian house raft slab is a much lighter structure designed mainly to cope with reactive soil movement. The name is shared, but the design approach is different.

How a Raft Slab Is Built

The sequence below is a common one for house raft slabs, particularly in Queensland where slab edges are often formed with blockwork. Some builders pour the edge beams and slab together in a single pour instead.

1. Set-out

The house footprint is set out on site. Using a surveyor for this avoids expensive mistakes with boundary setbacks and levels.

2. Edge beams excavated and reinforced

The perimeter edge beams are excavated. Reinforcement is supported on bar chairs so it has the required concrete cover and stays off the ground. If the specified founding material is deeper than expected, the excavation may be taken deeper, with the reinforcement kept at the position shown on the drawings.

3. Edge beams poured to ground level

In a two-stage pour, the edge beams are filled up to about ground level. Starter bars are cast in at the specified spacing so the later slab pour bonds to the beam and the two act together as one section. If the edge beams and slab are poured together, starter bars at this joint are not needed.

4. Fill placed and compacted

Sand or a similar granular fill is placed inside the edge beams to bring the base up to level, then compacted. Good compaction matters: poorly compacted fill can settle later, leaving the slab unsupported.

On sloping sites the edge blockwork is sometimes laid before the fill, which makes the fill much harder to place and compact properly.

5. Internal beams excavated

Trenches for the internal stiffening beams are dug through the fill and, where needed, into the natural ground to reach the required depth.

6. Vapour barrier

A polyethylene vapour barrier is laid over the whole area, including into the beam trenches, with joints lapped and taped and penetrations sealed. It limits ground moisture rising through the slab and stops the ground drawing water out of the fresh concrete.

7. Reinforcement

Trench mesh or bars go into the beams, and sheets of mesh are laid over the slab area, all supported on bar chairs at the specified height. Slab mesh is important for controlling the width of shrinkage cracks in the slab surface.

Around the perimeter, above the edge beams, formwork or blockwork forms the sides of the slab.

8. Pour, finish and cure

The beams and slab are poured, usually by pump. A boom pump is less likely than a line pump to knock reinforcement off its chairs. The surface is screeded and finished, then cured.

Curing matters. If the surface dries too fast it can crack, much like drying mud. Curing compounds or other curing methods slow moisture loss so the concrete gains strength properly.

Concrete is specified by its strength at 28 days, but the slab can be walked on and worked on well before then. Edge formwork is commonly stripped within a day or two.

Problems With Existing Raft Slabs

A well-designed and well-built raft slab performs well on most residential sites. When problems occur, they usually trace back to one of a few causes:

• Moisture changes in reactive clay, from poor drainage, leaking plumbing, garden watering or trees. These can cause slab heave or edge movement and lead to cracking in walls and floors.
• Settlement of poorly compacted fill under the slab or beams.
• Construction issues, such as beams shallower than designed, misplaced reinforcement, or poor curing.
• Shrinkage cracking in the slab surface. It is often cosmetic but sometimes confused with structural movement. See our article on slab cracking.

Telling these apart takes investigation, not just a look at the cracks. The right repair, whether drainage correction, monitoring, crack repair or, in some cases, underpinning, depends on the cause.

Cracking or Movement in a Slab-on-Ground House?

Cornell Engineers investigates cracking and movement in houses on raft and waffle slabs, works out the likely cause and advises on what should happen next. See our forensic structural engineering page for more.

Send us the property address, photographs, a short description of what you have noticed and any soil reports, drawings or previous reports. We will review the information and let you know whether the matter is a good fit for Cornell Engineers before preparing a fee proposal.

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