Waffle pod slabs, raft slabs, strip footings, pad footings, bored piers, and raised subfloors — with AS 2870 site classification and NCC compliance guidance
A complete 2026 guide to every residential foundation type used in Australia. Covers AS 2870 site classification (Classes A through P), reactive clay risks, foundation selection logic, waffle pod vs. stiffened raft comparisons, raised subfloor systems, and when bored piers or screw piles are required — for home owners, builders, and structural engineers.
Essential knowledge for home owners, owner-builders, project managers, structural engineers, and building certifiers choosing and designing residential foundations in Australia in 2026
Australia's unique combination of highly reactive clay soils, a semi-arid to tropical climate across most capital cities, and a predominantly lightweight residential construction tradition makes foundation selection more consequential here than in most other countries. The seasonal moisture cycle in clay-dominated suburbs drives ground movements that dwarf the structural loads from the building itself — a reactive clay site (H2 or E class under AS 2870) can move 60–100 mm vertically between wet and dry seasons. Choosing the wrong foundation type on reactive clay is the single most common cause of structural damage to residential buildings in Australia in 2026.
All residential foundation design in Australia is governed by AS 2870-2011: Residential Slabs and Footings, which is adopted by reference into the National Construction Code (NCC) Volume Two (Housing Provisions, Part 4.2). AS 2870 covers Class 1 and Class 10a buildings (detached houses, townhouses, garages, sheds) and similar light structures. It provides a site classification system, deemed-to-satisfy footing designs for each site class, and performance criteria for custom-engineered solutions. Every residential foundation in Australia — whether a $50,000 slab or a $200,000 pier-and-beam system — must comply with AS 2870 or an equivalent engineered alternative that meets the NCC performance requirements.
Foundation selection in Australia follows a mandatory sequence under the NCC: first, a site classification must be obtained from a geotechnical engineer or qualified building practitioner — this assigns the site to one of seven AS 2870 classes (A, S, M, H1, H2, E, P) based on soil type and reactivity. Second, the builder or structural engineer selects the foundation type from the deemed-to-satisfy designs in AS 2870 for that class. Third, for Class E, Class P, or any site with unusual conditions, a site-specific engineered design is required. Skipping the site classification — and building a waffle slab on a Class H2 site because the neighbour has one — is responsible for a substantial proportion of residential foundation failures in Australian suburbs each year.
Before any foundation type can be selected, the site must be classified under AS 2870 Section 2. Site classification determines the expected ground movement from soil moisture change (expressed as the characteristic surface movement, ys), which is the primary driver of foundation design in most Australian locations. The classification is performed by a geotechnical engineer based on soil testing (Atterberg limits, linear shrinkage, soil profile depth) and is reported in a site classification certificate that must accompany the building permit application in most states and territories.
Foundation Types — Typical Suitability by Site Class
The site classification must always precede foundation design. A structural engineer must certify the design for Class H1 and above, and for all Class P sites.
Australia uses a wider variety of residential foundation systems than most comparable countries — driven by the diversity of soil conditions across its capital cities and regional areas, from the reactive black clay of Brisbane and Melbourne's outer suburbs to the stable granite and sandstone of Sydney's inner ring and the collapsing sands of Perth coastal sites. Understanding each foundation type — its construction, performance, and limitations — is essential for anyone making decisions about residential building in 2026.
The waffle pod slab (also called a waffle raft) is the most widely used residential foundation system in new project home developments across Australia. It consists of a perimeter edge beam, a series of internal ribs (beams) running in both directions, and a 85 mm structural topping slab. The void between the ribs is formed using expanded polystyrene (EPS) pods — typically 1090 mm square — that remain in place after the concrete is poured, acting as permanent void formers. The resulting underside, when viewed from below, resembles a waffle grid — hence the name. The slab sits on top of the prepared ground on the EPS pods, rather than bearing directly on the soil across the full base area.
Waffle pod slabs are fast to construct (1–2 days for a 200 m² house slab), require minimal excavation (no trenching for internal beams as the pods sit on a prepared surface), use less concrete than a conventional raft, are suitable for Class A, S, and M sites, and are preferred by volume builders because of their cost-efficiency and speed. The elevated concrete perimeter and rib structure provides good stiffness-to-weight ratio for lightly loaded residential structures. In 2026, waffle pod slabs remain the default choice on flat, prepared lots in new developments on reactive Class M sites in most Australian capital cities.
Waffle pod slabs have shallower beam depths than stiffened raft slabs — typically 350–500 mm edge beams compared to 600–900+ mm for raft slabs on reactive clay. This makes them less suitable for highly reactive soils (H1/H2/E) where deep moisture-change zones require deep foundations. The pods sit on grade, which means the slab has no separation from ground moisture — on sites with poor drainage or ponding water risk, moisture entering the subfloor void (if not sealed) can cause differential swelling beneath the slab. Waffle slabs also provide no access to services beneath the floor, unlike raised systems, making future plumbing and electrical modifications more disruptive.
A stiffened raft slab (also called a conventional raft or deepened edge beam slab) is a monolithically poured reinforced concrete slab where the edge beams and internal stiffening beams are formed by trenching into the ground — unlike a waffle pod slab where the internal beams sit on pods above the ground surface. The deep trenched beams extend into the soil below the active moisture-change zone, anchoring the foundation against differential heave from reactive clay. This is the preferred foundation system for Class H1, H2, and E sites in Australia where reactive clay poses a significant movement risk.
Stiffened raft design is more complex than waffle pod design because the beam depths, widths, reinforcement sizes, and spacing must all be calculated by a structural engineer based on the specific site class, building size, wall loading, and expected differential ground movement. AS 2870 provides reference designs, but for H2 and E class sites the design is invariably a site-specific engineered solution. The engineer calculates the bending moments and shear forces the raft must resist as the soil beneath parts of the slab heaves or settles, then designs the beams as two-way spanning members to bridge those movements without the slab cracking. For context on how the loads from the building above flow into and through the slab system, see our guide on understanding concrete load paths.
Internal beam formation: Raft — trenched into ground. Waffle — pods sit on surface.
Beam depth on reactive clay (H1): Raft — 600–900 mm. Waffle — 400–500 mm (typically insufficient for H1).
Excavation required: Raft — yes, trenching for all beams. Waffle — minimal, perimeter only.
Concrete volume: Raft — higher (filled trenches). Waffle — lower (pods displace concrete).
Construction time: Raft — 2–4 days typically. Waffle — 1–2 days.
Cost (indicative): Raft — $15,000–$35,000+ for 200 m² depending on site class. Waffle — $12,000–$22,000.
Preferred for: Raft — reactive clay (M, H1, H2, E). Waffle — stable, prepared lots (A, S, M).
AS 2870 compliance: Both comply when correctly designed for the site class.
Strip footings (continuous wall footings) are reinforced concrete beams poured in a continuous trench beneath load-bearing walls. They are the traditional foundation for masonry-walled homes in Australia and remain in widespread use under brick veneer, full brick, and concrete block residential construction. A strip footing distributes the wall load along its length to the soil, with the minimum width sized to keep the bearing pressure below the soil's allowable bearing capacity. Under AS 2870, strip footing dimensions are specified for each site class, with deeper embedment required for reactive clay sites to reach the zone of negligible moisture variation. Strip footings may be used as the sole foundation system on Class A to M sites, or in combination with an elevated floor frame on Class H1 and above.
Pad footings (also called isolated footings or column footings) are square or rectangular reinforced concrete pads that support individual column or post loads. In residential construction, pad footings are used under steel columns, timber posts, verandah posts, and as the base of stumps in raised subfloor systems. They are appropriate on Class A and S sites where the soil has adequate bearing capacity to support the concentrated column load within the pad's plan area. Pad footings require adequate embedment depth (minimum 300 mm below finished ground level, or to frost depth) and must be sized such that the bearing pressure on the soil does not exceed the allowable value. On reactive clay, isolated pad footings are not suitable as the primary foundation — the differential soil movement between adjacent pads will distort the floor frame.
Bored concrete piers (also called bored piles, drilled piers, or caissons) are cylindrical concrete elements cast in drilled holes that transfer building loads to a depth below the active moisture-change zone — or to a competent bearing stratum on weak soil sites. They are the standard solution for Class H2, E, and P sites where the active zone extends so deep that slab beams cannot reach the inactive zone economically, and for sites with soft or loose upper soils that cannot support shallow foundations. Bored piers are typically 300–450 mm in diameter and 1.5–5 m deep for residential applications, with a reinforced concrete ground beam spanning between pier heads at the surface level to support the floor and wall structure above.
On highly reactive clay sites, bored piers must be designed for both end bearing (compressive load from the building) and uplift resistance (tensile force generated by swelling clay gripping the pier shaft). This requires full-length tension reinforcement in each pier, and in severe cases a slip membrane around the upper portion of the pier shaft to reduce the upward friction from the swelling active zone. Pier-and-beam systems provide excellent performance on highly reactive sites but cost significantly more than slab-on-ground systems — a pier-and-beam foundation for a 200 m² home on a Class H2 site typically costs $30,000–$70,000 depending on pier depth and number.
Screw piles (helical piles) are tubular steel piles with helical blades welded to the shaft that are screwed into the ground using a hydraulic torque head — without excavation, concrete, or curing time. They are an increasingly popular alternative to bored concrete piers for residential applications in Australia, particularly on steep or difficult-access sites, Class P sites with soft upper soils, and sites with contaminated material that cannot be legally excavated and removed. Screw piles are installed in hours (compared to days for bored piers), begin carrying load immediately with no concrete cure waiting period, and can be installed close to existing structures without vibration or spoil removal. Structural design must comply with AS 2159 (Piling — design and installation). Pile capacity is confirmed by torque monitoring during installation per the pile manufacturer's correlation charts, with load testing required for critical applications.
Raised subfloor construction — where the floor of the home is elevated above the ground on vertical supports (stumps, piers, or screw piles) with a bearer-and-joist framing system carrying the floor deck — is the traditional residential foundation approach in Queensland, Victoria (pre-1960s), and areas with steep topography or flood risk. The subfloor space provides ventilation (reducing timber decay), easy access to plumbing and electrical services for the life of the building, and natural accommodation of uneven ground without expensive earthworks. Stumps can be timber (treated pine or hardwood), concrete, steel, or masonry, each sitting on an individual pad footing or embedded in the ground to the required depth. Raised subfloor construction must comply with AS 1684 (Timber Framing) and AS 3660 (Termite Management), with a minimum 400 mm ground clearance for adequate subfloor ventilation.
The table below provides a structured comparison of every residential foundation type used in Australia in 2026, with typical dimensions, site class suitability, relative cost, and key design considerations.
| Foundation Type | Suitable Site Classes | Typical Depth / Size | Relative Cost (200 m²) | Key Advantage | Key Limitation |
|---|---|---|---|---|---|
| Waffle Pod Slab | A, S, M (with care) | Edge beam 300–450 mm deep; 85 mm topping | $12,000–$22,000 | Fast, low-excavation, cost-efficient | Not suited to H1/H2/E reactive clay |
| Stiffened Raft Slab | A, S, M, H1, H2 (engineered) | Edge beams 450–1,200 mm deep; 100–150 mm slab | $15,000–$40,000+ | Best performance on reactive clay | More excavation, higher cost than waffle |
| Strip Footing | A, S, M | 300–600 mm wide; 300–600 mm deep | $10,000–$20,000 | Traditional, suits masonry walls | Not suited to highly reactive clay |
| Pad Footing | A, S | 400–800 mm sq.; 300–500 mm deep | $500–$2,000 per pad | Simple, low cost for isolated columns | Differential movement risk on clay |
| Bored Pier + Ground Beam | H1, H2, E, P | 300–450 mm dia.; 1.5–5 m deep | $30,000–$70,000+ | Best for extreme reactive clay & soft soil | High cost; requires engineer design |
| Screw Pile + Beam | P, steep sites, soft soil | 65–114 mm shaft; 2–6 m deep | $25,000–$55,000+ | No excavation, immediate loading | Specialist install; not all soils suitable |
| Raised Subfloor (Stumps) | All classes; steep/flood sites | Stumps on pad footings; 400 mm+ clearance | $20,000–$50,000+ | Access to services; steep sites; flood | Termite risk; higher thermal loss |
| Suspended Slab (Piled) | P, very soft soil, steep | Slab spanning between piers or walls | $40,000–$100,000+ | Spans poor soil completely | Highest cost; complex design |
Reactive (expansive) clay soils underlie the majority of residential land in Australia's major capital cities — particularly in Brisbane's western suburbs, Melbourne's outer ring, Adelaide's plains, and widespread areas of Sydney. These soils contain clay minerals (primarily smectite) that absorb water and swell when wet, then shrink and crack when dry. The seasonal moisture cycle — summer drought followed by winter rainfall — drives this volume change cyclically for the life of the building. The critical metric under AS 2870 is the characteristic surface movement (y_s) — the expected vertical movement at the ground surface from the full seasonal moisture cycle — which can exceed 75 mm on extreme (Class E) sites.
AS 2870-2011 is the governing Australian Standard for all residential foundation design. It provides the site classification system (Classes A through P), deemed-to-satisfy designs for waffle pod slabs, stiffened raft slabs, strip footings, pad footings, and piled systems for each site class, and performance criteria for engineered alternatives. It is adopted by reference into the NCC and is the mandatory design basis for every residential foundation in Australia in 2026. Obtain a copy through Standards Australia for your engineering and construction reference library.
Standards Australia →Understanding the soil type beneath your site is the foundation of every good foundation decision in Australia. From reactive black clay in Queensland to calcareous sands in Perth and filled estuarine deposits in Melbourne's Docklands, Australia has an extraordinarily diverse range of foundation conditions that all require different engineering approaches. Our comprehensive guide to soil types and their impact on concrete foundations covers all major Australian soil types, their bearing capacity, settlement behaviour, and appropriate foundation systems in detail.
Read the Guide →The performance of every Australian residential foundation — whether a waffle pod slab, stiffened raft, or pier-and-beam system — depends not only on the design but on the quality of sub-base preparation beneath the slab. Correct fill placement, compaction to 98% Standard Proctor density, termite management barriers, moisture-barrier membranes, and drainage provision are all critical preparatory steps that must be completed before any concrete is poured. Our guide to sub-base preparation covers all these requirements for residential slab construction in Australia in 2026.
Read the Guide →