Raft footings: construction and site classification guide for residential builds
Stiffened raft footings in Australia: AS 2870 edge beam depths by site class, internal beams, N20 concrete, SL72 mesh, DTS vs engineering paths.
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A stiffened raft footing is a flat concrete panel with deepened edge and internal beams cast monolithically into the ground. It is the most common residential foundation in Australia and suits Class A, S, and M sites under the ABCB Housing Provisions 2022 DTS tables (clause 4.2.14): N20 minimum concrete, SL72 mesh, 100 mm panel, edge beams from 300 mm deep on Class A/S up to 850 mm on Class M. For Class H1, H2, E, and P sites, the Housing Provisions explicitly defer to AS 2870 engineering design: no DTS path exists. The two things that kill raft jobs are pouring into ground that hasn’t been inspected for the hold point, and failing to trench to consistent beam depth so the engineer’s design is compromised at pour.
When you do this
A stiffened raft footing is the foundation system for a slab-on-ground build where continuous edge and internal beams resist differential ground movement. You use it when:
- The site report classifies the site as Class A, S, or M (DTS path) or H1/H2 (engineering required)
- The building is a Class 1 residential building (house, townhouse, duplex) on flat to gently sloping ground
- The design doesn’t call for a waffle pod slab (which uses polystyrene pods rather than trenched beams under the panel)
- A strip footing or pad footing system isn’t appropriate for the building load distribution
Raft footings are NOT the right choice for:
- Class H1, H2, E, or P sites without engineering. The Housing Provisions explicitly refer these to AS 2870 for full engineering design (verified 2026-05-10, ABCB Housing Provisions 2022 Part 4.2).
- Steeply sloping sites where the level differences between edge beams and internal beams become unworkable without engineered solutions
- Isolated post or column loads: those go to pad footings
- Continuous loadbearing wall lines without a slab: those go to strip footings
Who’s involved
| Role | Responsibility |
|---|---|
| Builder | Sequence, compliance, hold point management, documentation |
| Concretor | Set-out, excavation, formwork, reinforcement placement, pour and finish |
| Engineer | Design on Class H/E/P sites or any non-DTS case; sign-off on footing drawings |
| Certifier / Building Inspector | Pre-pour inspection hold point: verifies beam depth, reinforcement, bearing, DPC |
Steps
1. Confirm site classification and design path
Before any excavation, confirm the site class from the soil report (geotech). The class determines the entire design:
- Class A and S: ABCB Housing Provisions 2022 Part 4.2 DTS tables apply. Edge beam depths and reinforcement are taken from Table 4.2.14a (Class A) and Table 4.2.14b (Class S). No engineer required for standard DTS cases.
- Class M: Housing Provisions 2022 Part 4.2 Table 4.2.14c applies for standard residential construction. Edge beams go deeper. For unusual site features (trees, drainage issues, large variation across the site), get an engineer.
- Class H1, H2, E, P: Engineering under AS 2870:2011 is mandatory. The Housing Provisions DTS path does not cover these classes (verified 2026-05-10, ABCB Housing Provisions 2022 Part 4.2).
If the site is Class H1 or worse, the engineer’s drawings are the design document. The Housing Provisions tables are irrelevant on those sites.
2. Understand the structural geometry
A stiffened raft has three beam elements plus the panel:
- Edge beams (perimeter): run continuously around the building perimeter. Depth is set by site class and construction type. Standard width is 300 mm. On reactive sites, edge beams are deeper to anchor the slab against differential movement.
- Internal beams: run through the slab in two directions at regular spacing to form a grid. They are trenched into the ground below the membrane, cast as part of the same concrete pour as the slab. Maximum spacing is 6 m between beam centres on standard designs, though the engineer or DTS table may require tighter spacing.
- Slab panel: the flat topping slab, typically 100 mm thick for standard residential construction. The panel ties the beam grid together and distributes floor loads.
Edge beams connect to internal beams at T-intersections. All elements are cast monolithically: one pour, one structural unit.
3. Set out and excavate
Set out the building footprint from the survey pegs. The concretor:
- Marks the edge beam lines and excavates around the perimeter to the design depth
- Excavates internal beam trenches to the design depth, running in both plan directions to form the grid
- Checks that beam trench bases are in natural soil at the bearing capacity specified (minimum 50 kPa for internal beams and edge beams per Housing Provisions clause 4.2.5, verified 2026-05-10, ABCB Housing Provisions 2022 Part 4.2)
- Cleans beam trenches of loose material
Beam depth is measured from the underside of the slab panel, not from finished ground level. This distinction matters on sloping sites where the slab may sit above grade on one side.
4. Confirm beam depth by site class
Edge beam depths under the DTS tables vary significantly by class and construction type. These are the AS 2870 and Housing Provisions design ranges (verified 2026-05-10, ABCB Housing Provisions 2022 Part 4.2):
| Site class | Reactivity | Edge beam depth (range) | DTS path |
|---|---|---|---|
| Class A | Non-reactive | 300 mm (min) | Housing Provisions Table 4.2.14a |
| Class S | Slightly reactive | 300 mm (min) | Housing Provisions Table 4.2.14b |
| Class M | Moderately reactive | 300 to 850 mm (by construction type) | Housing Provisions Table 4.2.14c |
| Class H1 | Highly reactive | 600 to 900+ mm (engineer specifies) | AS 2870 engineering required |
| Class H2 | Highly reactive (deep) | 900 to 1,200+ mm (engineer specifies) | AS 2870 engineering required |
| Class E | Extremely reactive | Engineer-designed | AS 2870 engineering required |
| Class P | Problem site | Engineer-designed | AS 2870 engineering required |
The practical implication: on a Class M site you may be excavating 850 mm deep beams around the perimeter before you ever start the slab panel. Concrete volume and cost scale with beam depth. Get the soil report before budgeting.
5. Install subfloor services and damp proof membrane
Once excavation is complete:
- Run plumbing drainage and conduits for electrical in their final positions under the slab. These cannot be relocated after the pour.
- Compact the slab panel substrate and bring it to the correct level using sand or controlled fill per the engineer’s specification or Housing Provisions clause 4.2.4.
- Install the damp proof membrane (DPC): minimum 0.2 mm polyethylene sheeting, joints lapped 200 mm minimum, edges turned up and taped at beam penetrations. The membrane sits under the slab panel and separates it from the ground.
The membrane does NOT run under the beam trenches. The beam concrete is cast directly against the natural soil in the trench.
6. Set reinforcement
Reinforcement for the DTS path (Class A, S, M) is specified in Housing Provisions Part 4.2 tables. The standard DTS reinforcement schedule:
Slab panel:
- SL72 mesh is the minimum mesh for Class A and S sites (verified 2026-05-10, ABCB Housing Provisions 2022 Part 4.2)
- Class M sites may require SL82 or SL92 depending on construction type and beam spacing
Edge and internal beams:
- Minimum for Class A: 3-L8TM (trench mesh) bottom bars per beam
- Class S and M: 3-L8TM to 3-N16 bars (varies by table; construction type governs)
- Where edge beams are wider than 300 mm, additional bottom bars are required at the rate of one equivalent bar per 100 mm additional width (Housing Provisions clause 4.2.14)
Concrete cover requirements (Housing Provisions clause 4.2.11):
| Surface condition | Minimum cover |
|---|---|
| Unprotected ground (beam against soil) | 40 mm |
| On membrane (under slab panel) | 30 mm |
| Internal surfaces | 20 mm |
Use bar chairs or spacers. Reo sitting on soil before the pour is a pre-pour inspection fail.
7. Clear the pre-pour inspection hold point
Pre-pour inspection is a mandatory hold point. The building inspector must sight and sign off:
- Beam excavation depth and bearing material quality at the trench base
- Slab panel substrate level and compaction
- DPC installation: coverage, laps, turn-ups
- Reinforcement placement and cover in beams and panel
- Service penetrations correctly sleeved
- Re-entrant corners reinforced (Housing Provisions clause 4.2.11(4) requires additional reinforcement at all re-entrant corners in the slab)
Do not pour before this hold point is cleared. Concrete placed before inspection is a locked-in defect with no remedy short of demolition.
8. Pour concrete
Minimum concrete grade: N20 (20 MPa) per Housing Provisions clause 4.2.10 (verified 2026-05-10, ABCB Housing Provisions 2022 Part 4.2). In practice, most residential raft pours use N25 or N32, particularly on reactive or chemically aggressive sites. N20 is the DTS floor.
Pour sequence for a monolithic raft:
- Verify beam trenches are free of water, loose material, and debris
- Place concrete into beams first where possible, vibrating to consolidate around all reinforcement
- Bring the pour up to panel level continuously, avoiding cold joints between beam and panel concrete
- Screed and float the panel surface
- Power trowel or broom finish to the specification
- Cure: plastic sheeting or curing compound applied immediately after finishing. Minimum 7 days moist cure per Housing Provisions clause 4.2.10
9. Post-pour: survey and sign-off
After the pour and cure:
- Survey the slab: level tolerance must meet the engineer’s or DTS specification
- Contractor to produce a record of the pour: concrete docket (confirming grade), batch plant, pour date, ambient temperature, cure method
- Certifier re-inspects and signs off the completed footing before framing commences
Tolerances and acceptance
Slab-level workmanship tolerances are pending HIA member access. [HIA-077]
Verifiable AS 2870 and Housing Provisions requirements:
| Check | Requirement | Source |
|---|---|---|
| Concrete grade | N20 minimum | HP clause 4.2.10 |
| Slab panel thickness | 100 mm (standard residential) | HP / AS 2870 design intent |
| Bearing capacity | 50 kPa minimum at beam trench base | HP clause 4.2.5 |
| Cover to reo (to ground) | 40 mm minimum | HP clause 4.2.11 |
| Cover to reo (on membrane) | 30 mm minimum | HP clause 4.2.11 |
| Cover to reo (internal) | 20 mm minimum | HP clause 4.2.11 |
| Edge beam width | 300 mm minimum (or per engineer) | HP clause 4.2.14 / AS 2870 |
| Re-entrant corner reo | Additional reinforcement required | HP clause 4.2.11(4) |
| Lap splice length | 500 to 700 mm minimum | HP clause 4.2.11 |
Stiffened raft vs waffle pod: when to use which
| Factor | Stiffened raft | Waffle pod |
|---|---|---|
| Beam formation | Trenched into ground | Polystyrene pods, surface-formed beams |
| Site classes (DTS) | A, S, M | A, S (limited M) |
| Class H1/H2 | Engineered design available | Generally not suited (limited DTS for H1-D) |
| Panel thickness | 100 mm min | 85 mm min |
| Excavation volume | Full trench to beam depth | Perimeter trench only |
| Internal beams | Grid through whole slab | Between pod rows |
| Cost driver | Concrete in deep beams | Pod supply and placement |
| Best application | Reactive sites, engineered H-class builds | Class A/S cost-sensitive jobs |
The key difference is beam depth into the ground. On reactive sites, trenched beams penetrate below the active moisture change zone. Waffle pod beams sit on the soil surface and rely on the pod grid to achieve stiffness. That’s why Class H1 and above nearly always go to stiffened rafts under engineering.
Documents needed
- Soil classification report confirming site class
- Engineer’s drawings (Class H/E/P sites, or any non-DTS case)
- Housing Provisions Part 4.2 tables (DTS path, Class A/S/M)
- Pre-pour inspection sign-off from certifier
- Concrete docket confirming grade, slump, and batch plant
- As-poured slab survey (level certification)
- Service penetration as-built drawing
Common holds
| Hold | Cause | Resolution |
|---|---|---|
| Pre-pour not cleared | Inspector not notified or site not ready | Notify certifier early; stage inspection at correct readiness |
| Beam not to depth | Excavation shallow; soft zone at design depth | Re-excavate to firm natural strata; notify engineer if depth change is significant |
| DPC not lapped at intersections | Membrane pulled apart at beam junctions | Tape all laps and intersections before steel goes in |
| Cover insufficient in beams | Bar chairs not used; reo on ground | Install correct chairs; re-inspect |
| Re-entrant corner not reinforced | Corners missed during reo placement | Check plans for every internal corner; place diagonal bars per HP clause 4.2.11(4) |
| Cold joint between beam and panel | Beam pour stopped and panel poured separately next day | Pour monolithically in one continuous operation; plan pump reach and crew before pour day |
| Wrong site class used | Soil report class differs from builder’s assumption | Always read the actual report; do not assume class from visual inspection |
References
- Australian Building Codes Board, ABCB Housing Provisions 2022, Part 4.2 Footings, slabs and associated elements. https://ncc.abcb.gov.au/editions/ncc-2022/adopted/housing-provisions/4-footings-and-slabs/part-42-footings-slabs-and-associated-elements (verified 2026-05-10).
- Standards Australia, AS 2870:2011 Residential slabs and footings (product page). https://store.standards.org.au/product/as-2870-2011 (verified 2026-05-10).
- Australian Building Codes Board, NCC 2022 Volume Two, Part H1 Structure. https://ncc.abcb.gov.au/editions/ncc-2022/adopted/volume-two/h-class-1-and-10-buildings/part-h1-structure (verified 2026-05-10).
- Cornell Engineers, What is a Raft Slab. https://cornellengineers.com.au/what-is-a-raft-slab/ (verified 2026-05-10).
Related
- AS 2870:2011 residential slabs and footings, the standard covering site classification and all residential footing types
- Slab on ground construction, the full slab build sequence including pre-pour hold points and tolerances
- Stiffened raft slab, plain-English definition of the stiffened raft slab type
- Soil report (geotech), what a soil classification report covers and how to read the site class output
- Site classification, the classification process and AS 2870 site classes
- Reactive soil, plain-English explanation of reactive clay and its effect on residential foundations
- Reinforcement (reo), trench mesh, slab mesh, and bar specifications used in residential footings
- Strip footings, continuous footing under loadbearing walls
See also
- NCC 2022 Volume Two, the residential building code framework
- Slab on ground, plain-English definition of a slab on ground
- Engineer’s details, the structural drawings issued for non-DTS footing design
- Footing, plain-English definition of residential footings
- Waffle pod slab, the alternative residential slab type using polystyrene pods
- Concrete grade, N20, N25, N32 and what the grade means on site
- Waffle pod slab construction, the full waffle pod build sequence for comparison
- Pad footings, isolated pad footings for point loads
Last updated: 2026-05-10. Verified: 2026-05-10. Quarterly review for currency: confirm current Housing Provisions Part 4.2 table values and AS 2870:2011 edition status at Standards Australia.