Slab & Deck Engineering
The foundation that everything else depends on
Concrete slabs and decks are among the most critical structural elements in any building. They carry every load above them, they transfer forces to the ground below, and they need to perform without cracking, settling, or deflecting for the entire life of the structure. Getting slab design right is not optional, it is the starting point of every successful project.
In Australian conditions, slab design is heavily influenced by soil reactivity. Expansive clay soils across Sydney and NSW can swell and shrink dramatically with changes in moisture content, placing enormous stress on slabs that are not properly designed for the conditions. That is why AS 2870 exists, and why the geotechnical report is the first document we ask for on every residential slab project.
Our team designs every type of slab system, from simple ground floor slabs for single dwellings through to complex suspended decks for multi-storey commercial buildings. Every design is tailored to the specific site conditions, structural loads, and construction methodology, and every set of drawings leaves our office council-ready.
Reinforcement mesh and edge beams ready for concrete placement on a residential waffle pod slab.
Choosing the right slab system
There is no one-size-fits-all slab. The right system depends on your soil classification, the loads the slab will carry, whether you need a ground-bearing or suspended solution, and what the construction programme and budget allow. Here is how we approach it:
Waffle Pod Slab
The most common slab system for residential construction in Australia. Expanded polystyrene (EPS) void formers create a grid of reinforced concrete ribs beneath the slab, reducing the volume of concrete and excavation required. Waffle pods work well on Class A (stable) through to Class H1 (highly reactive) sites, though the rib depth and reinforcement increase significantly with soil reactivity. They are fast to construct and cost-effective for most house builds.
Raft Slab
A thickened-edge slab that distributes the building load across the entire footprint. Raft slabs are the go-to for moderately to highly reactive soils where the ground movement is too aggressive for waffle pods. The continuous concrete mass provides stiffness and resistance to differential settlement. They require more concrete and excavation than waffle pods, but on difficult sites they are the more reliable choice.
Suspended Slab
An elevated concrete slab that spans between beams, walls, or columns rather than sitting on the ground. Suspended slabs are used for upper floors in multi-storey buildings, basement roofs, structures built over car parks, and sites with steep terrain where a ground-bearing slab is not practical. The design involves detailed analysis of bending moments, shear forces, deflection limits, and crack control.
Composite Steel Deck
Profiled steel decking acts as permanent formwork and tensile reinforcement for the concrete topping, creating a high-performance composite floor system. This is the standard approach for commercial and multi-storey construction because it eliminates the need for temporary formwork, speeds up construction, and provides a working platform for trades during the build. We design to manufacturers' load-span tables and verify with AS 2327 composite slab provisions.
"The soil report tells you what the ground wants to do. The slab design tells it what it is allowed to do."
IA Engineering Design Team
Residential and commercial: different standards, different thinking
Residential slab design in Australia is governed primarily by AS 2870, which provides a deemed-to-comply framework based on soil classification and building configuration. For most houses, the geotechnical report determines the site class (A, S, M, H1, H2, or E), and the slab type and reinforcement follow from there. It is a well-established system, but it still requires careful attention to edge beams, internal stiffening beams, moisture management, and articulation joints.
When you move to commercial
The design shifts to AS 3600 (Concrete Structures), and the complexity increases considerably. Commercial slabs carry heavier loads, from forklift traffic and pallet racking through to plant equipment and vehicle access. Suspended slabs require detailed structural analysis for bending, shear, punching shear at columns, deflection under sustained loads, and crack width control. The reinforcement detailing is more complex, and the inspection and hold-point requirements during construction are more rigorous.
What both have in common
Regardless of scale, every slab design starts with understanding the ground conditions and the loads. We work from the geotechnical report, the architectural plans, and the structural loads to develop a slab system that performs reliably over the building's design life. We specify concrete grade, reinforcement layout, construction joints, control joints, and any special provisions such as fibre reinforcement or post-tensioning.
The details that make a slab work
A slab is more than just concrete and steel. The details in the design are what separate a slab that performs well from one that cracks, settles, or fails to meet serviceability requirements over time.
Moisture management is critical on reactive sites. We specify moisture barriers, drainage provisions, and edge rebates to control moisture variation around the slab perimeter. On highly reactive sites, the difference between a well-drained and a poorly-drained slab can mean the difference between acceptable and unacceptable ground movement.
Control joints and construction joints are placed strategically to manage shrinkage cracking. Concrete shrinks as it cures, and if the slab is too large or too constrained, that shrinkage will result in random cracking. Our joint layouts are designed to control where cracks occur so they happen at planned locations rather than through the middle of a living room floor.
Reinforcement detailing specifies bar sizes, spacings, lap lengths, cover requirements, and chair heights for every zone of the slab. For suspended slabs, the top and bottom reinforcement is detailed separately, with particular attention to continuity over supports and anchorage at free edges. Every bar needs to be in the right place and at the right level for the slab to perform as designed.
Standards & Compliance
Every slab design is certified to Australian Standards
All slab and deck designs are prepared in accordance with the relevant Australian Standards and certified by our Registered Professional Engineers (RPE). Documentation is council-ready and suitable for building certification.
How we work with you
We start with your geotechnical report and architectural plans. The soil classification drives the slab type selection, while the architectural layout determines beam locations, step-downs, penetrations, and any special requirements like heated slabs or in-slab drainage.
From there, we develop the full slab design, including edge beam and internal beam sizing, reinforcement layouts, concrete specification, joint details, and any special provisions. For suspended slabs, we provide full structural analysis including bending moment and shear force diagrams, deflection checks, and detailed reinforcement schedules.
The deliverable is a complete set of structural drawings and specifications, ready for council submission and construction. We stay available throughout the build to answer questions from your builder, respond to site queries, and carry out any required inspections at key hold points.