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Slab Design Types: Waffle, Raft, Conventional, Suspended

Your home's slab dictates the cost of extensions, the pattern of any movement, and what a buyer's inspector will look for. Four slab types cover most Brisbane homes.

Slab design is determined primarily by soil conditions and era of construction. Brisbane's mix of clay, alluvial and sandy soils mean slab choice has always been site-specific. Four slab types dominate.

Conventional slab (stiffened raft)

A concrete slab with edge beams and integral footings. Most common slab in post-war brick Brisbane homes and in mid-tier 1970s to 2000s project homes. Typical thickness 100mm slab with 400 to 600mm deep perimeter beams. Generally robust. Signs of distress: cracks wider than 2mm, lintel cracks above doorways (indicating slab edge movement), or diagonal cracks in brick external cladding.

Waffle pod slab

A slab with polystyrene "pods" forming a grid of voids under the concrete. The concrete sits on top of the ground with the pods below, creating a rib-and-beam structure. Widely used from the early 2000s on house-and-land builds in outer suburbs because it is quick and soil-preparation light. Performs well on stable ground; more sensitive to reactive soils.

Raft slab

A thick slab (typically 150 to 200mm) with heavy internal beams and perimeter edge beams, designed to sit on reactive or poor ground as a "raft" that moves as one. Used on challenging soils. More expensive than conventional but tolerates movement better.

Suspended slab

A concrete slab supported by walls, columns or piers rather than resting on the ground. Common on sloping Brisbane sites where the ground level changes significantly across the footprint. A suspended slab with habitable space below is effectively an elevated floor. Requires structural engineering and is materially more expensive than slab-on-ground.

Strip footings (no slab)

Older Brisbane brick homes (pre-1960) often sit on brick or concrete strip footings supporting load-bearing walls, with timber floors between. Not a slab at all. Typical for double-brick Brisbane homes and some mid-century brick veneer.

Pier and stump (elevated timber)

Queenslanders sit on stumps, not slabs. See the anatomy article for the full breakdown. There is often a slab under the laundry or bathroom area only, cast on ground.

What inspection reports say about slabs

"Minor shrinkage cracking to perimeter beam" is cosmetic. "Diagonal step-cracking visible in brick cladding" indicates likely slab-edge movement and warrants further investigation. "Differential movement between original slab and extension" means the two parts of the house are settling differently and have not been tied together correctly. Each finding has a different remediation cost, from trivial (sealing surface cracks) to significant (underpinning).

Extensions on existing slabs

Adding a new wing to an existing home requires either tying the new slab to the existing (via chemical rebar ties and matched edge beams) or deliberately separating them with an articulation joint. Cheap extensions often get this wrong, producing movement cracks at the junction of old and new. When buying, pay attention to how an existing extension transitions to the original slab.

What building inspectors look for by slab type

A qualified building inspector applies a different lens depending on the slab type beneath the property. For a conventional stiffened raft, the inspector looks for cracking patterns on perimeter beams, diagonal step-cracking in brick cladding (which indicates edge movement where the beam has moved relative to the adjacent section), and measurable floor slope across the slab. For waffle pod slabs, the inspector checks for voiding at slab edges where reactive soils have contracted away from the polystyrene pods, and for cracking at re-entrant corners or window heads, which are the stress concentration points in a rib-and-beam structure. For raft slabs, the question is whether movement has exceeded design parameters: cracks wider than 2mm, or a consistent diagonal crack pattern, suggest the raft is no longer behaving as a single unit. For suspended slabs, the key concerns are structural cracking at the bearing points where the slab meets supporting walls or columns, and any sagging mid-span. For pier-and-stump homes (Queenslanders and post-war timber), the inspector enters the subfloor and checks stump condition, timber decay in bearers and joists, termite activity, and level variation between stumps, since a high stump in one corner and a low stump in another produces the same internal symptoms as slab movement in a slab-on-ground home.

Common slab and footing issues in pre-sale building reports

Brisbane pre-sale building reports flag slab-related defects in three broad categories. The first is cosmetic cracking: hairline shrinkage cracks in concrete are a normal consequence of curing and Queensland's climate. A competent inspector notes them but classifies them as low risk requiring no immediate action. The second is active movement: step-cracking in brick cladding, diagonal cracks wider than 2mm, or cracks that taper (wider at one end than the other) indicate ongoing movement rather than historic settlement that has stabilised. These findings warrant a structural engineering assessment before you sell, not after the buyer commissions their own inspection. The third category is subfloor and stump defects in elevated timber homes. Termite damage to stumps, timber decay in subfloor bearers and joists, and heavily out-of-level floors are among the most frequent critical findings in inner-east Brisbane pre-sale reports, particularly in Queenslanders and post-war timber homes built before 1960. Sellers who commission a pre-sale building and pest inspection and address the significant items before marketing retain price control over those issues. Sellers who discover them during a buyer's due diligence period negotiate from a weaker position, because the buyer's inspector report becomes the buyer's price adjustment tool.