Key takeaways
- The site classification is the engineer’s prediction of how the ground will move, and the slab is designed around that prediction rather than around the builder’s preference.
- The slab types open to a designer are engineered responses to different ground conditions, and they are not interchangeable products sold at different prices.
- A change to the slab type, the edge detail, the footing depth or the reinforcing is a structural change that needs the engineer’s written sign-off and the certifier’s acceptance.
- Drainage, plumbing and retained trees are the parts of the design the builder controls, and they decide whether the ground behaves the way the report assumed.
- A documented design change protects the builder for years, while an undocumented substitution becomes the defect the builder ends up paying to fix.
On a reactive site the slab is not a product you pick from a catalogue. It is a structural design, built on a classification that someone else measured in the ground, and the choice of slab type follows from that classification. When a builder swaps a waffle raft for a conventional slab because the subcontractor knows the other one better, or thins an edge beam to match a price, the house is no longer the one the engineer designed and the certifier approved.
This article covers how the site classification on the engineer’s documents drives the slab selection, what the classes mean in plain terms, and why a substitution needs the engineer’s sign-off before anyone touches the formwork. It is written for the licensed builder who already knows the trade and needs to hold the line on a job where someone on site wants to do it differently.
The commercial stake is substantial and it arrives late. Movement that shows up in the years after handover is the kind of defect that comes back to the builder, because the builder made the substitution, and the cost of underpinning or re-levelling a slab is out of all proportion to the saving that made the change look attractive at the time.
What the site classification actually tells you
The site classification is the engineer’s summary of how the ground under the house is expected to behave. It comes from a geotechnical investigation, which usually means boring test holes at set positions across the block, testing the soil, and interpreting what that soil is likely to do as its moisture content changes over time. The report then places the site in one of the classes set out in the residential slab and footing standard. The classes run from non-reactive ground through to highly and extremely reactive ground, and there are separate categories for filled sites, soft soils and ground affected by mining or other hazards.
The classification is a prediction about movement, not a description of the soil in a jar. A clay that is stable when dry can swell when it is wet, and a site that is stable under its existing vegetation can move once the trees come out and the garden is watered. That is why the report normally carries conditions alongside the classification. Read those conditions as part of the classification, because they set out what the design assumes will happen to the site after the house goes on it. Reading a geotechnical report well is a skill, and the PointCPD guide to NSW builder CPD explains how structured learning of that kind fits into the renewal cycle.
Why the ground moves and the house has to move with it
Reactive clays expand when they take up water and shrink when they lose it. Under a house the moisture is never even. The ground beneath the centre of a slab is shielded from rain and evaporation, while the ground at the edges is exposed to the weather, to the drainage from the roof and to the water that runs off paths and garden beds. The result is a dish or a mound under the slab, and the slab has to be stiff enough to ride over that movement without breaking up.
Trees and plumbing change the picture. A large tree close to the house draws water out of the ground in dry periods, and the soil shrinks around its roots. A leaking pipe does the reverse and keeps the ground wet for months before anyone notices. The engineer’s assumptions depend on those things staying as they were when the investigation was done, which is why the classification is only as good as the site conditions that existed at the time.
The classification also tells you what to watch for once the house is up. In the first years after the slab is poured, the builder and the owner should look for changes at the junctions between the slab and the walls, at door heads and at the corners of openings. Those are the places where movement tends to show first. A dated note of what was seen helps the engineer tell normal settling from a problem, and it is worth putting a plain-language summary of what the design expects into the handover pack.
The slab options and where each one sits
On a reactive site the designer has a limited set of options, and they are not interchangeable. A waffle raft, or a stiffened raft, puts a stiff grid of beams under a thinner slab and spreads the load so the whole slab moves as one unit. A slab on ground with thickened edges and footings is more common on the less reactive classes, and it works when the engineer has sized the edges for the movement expected. Where the ground is poor enough, the house is carried on piers taken down to a firmer layer, with a suspended floor above. Each option has its own reinforcing, its own footing depth and its own construction sequence, and each is chosen for a reason the engineer can explain.
How the common slab options differ
| Option | Where it usually sits |
|---|---|
| Waffle or stiffened raft | Reactive sites, where the whole slab has to move as one unit |
| Slab on ground with thickened edges | Less reactive classes, where the edges are sized for the movement the engineer expects |
| Piers with a suspended floor | Poor ground, where the load is carried down to a firmer layer |
| Any substitution | Only where the engineer has designed the change and the certifier has accepted it |
The practical point is that these options are not the same product sold in different sizes. A raft designed for a moderately reactive site uses a particular grid and reinforcing layout that does not work on a site with a different classification. A conventional slab with thickened edges has its strength in the edges, so reducing those edges removes the thing that was resisting the movement in the first place. A builder who thinks of the slab as a finish that can be swapped is working from the wrong model.
Why the engineer’s design cannot be swapped on the day
The slab design is a structural design, and the person responsible for it is the engineer who signed it. The certifier relied on that design when the construction certificate was issued, and the approved plans and the engineer’s drawings are the documents the house has to be built to. A builder who changes the slab type, reduces the edge beam, alters the footing depth or drops to a lighter reinforcing layout has changed the structure without the design being checked. That is not a builder’s decision, even when it is made in good faith and even when the new slab would probably perform.
The contract points the same way. A change to the works agreed with the owner is a variation, and it has to be documented and priced before it is carried out. The Contracts for home building work guidance from the NSW Government sets out the general principle that changes to the contract need to be agreed in writing. A slab change made without the engineer’s sign-off is not a variation the owner can rely on, and it is not a design change the certifier has seen. The builder carries both gaps.
Lead in. The mistake that costs the most is changing the slab type or the edge details on a reactive site to save time or money, without the engineer’s written sign-off.
Moisture is the part of the design the builder controls
The engineer designs the slab for the ground as it is expected to behave, and the builder controls much of what makes that behaviour happen. Drainage around the slab matters most. The ground at the edges should fall away from the house, stormwater should be connected as soon as it is practical to do so, and garden beds should not be built against the slab edge where they will hold water. The same moisture discipline is what keeps pests and decay at bay, which is covered in the termite management guide for new homes, and it matters just as much for the slab.
Plumbing under and around the slab should be pressure tested before the pour, because a leak found after the slab is down can wet the ground for a long time before anyone notices. Services in a raft are fixed to the reinforcing before the concrete goes in, and the electrician working around that grid needs to know where the beams are. Cut or displaced reinforcing is a common cause of later trouble, and the safety of the work around cable routes follows the guidance on electrical safety from Safe Work Australia.
Be careful with the things the engineer has asked you to leave alone. A tree that was to be retained should be protected at the distance the engineer or arborist specifies, not at the distance that suits the excavator. Excavation near a retained tree changes the moisture balance in ways that do not show up for a season. Keep the ground in the condition the report describes, and keep a note of what was done, so the record shows the design was built on the site it was designed for.
Flood-prone and filled sites add another layer
A site classification does not answer every question about the ground. A block on fill, on a low-lying lot or close to a waterway may also be affected by flood planning requirements that set the height the floor has to be above and the way the slab can be constructed. The NSW flood risk management manual is the reference councils work from, and the council’s flood planning information will say whether those requirements apply to the block. A slab designed to resist ground movement can still be the wrong slab if the floor level is set too low for the flood planning level.
Fill is the other concern. Placed fill that has not been compacted in layers, or that contains organic material, can settle unevenly and does not behave like natural ground. Where a site has been filled the classification will usually say so, and the engineer will design the slab for the fill as well as the natural ground under it. The builder’s job is to confirm which material is actually on the site, that it matches what the report describes, and to stop and call the engineer if the excavation tells a different story.
Worked example: a waffle raft that became a conventional slab
A builder in the Hunter was building a single-storey house on a block the engineer had classed as moderately reactive. The design was a waffle raft, with the grid and reinforcing laid out on the drawings and a footing pattern for the edges. Shortly before the pour, the slab subcontractor said his crew knew the conventional slab better, and that thickened edges would be cheaper to form and pour. The builder liked the idea of a faster programme and asked whether the change could be made.
The builder did not go straight to the subcontractor. He rang the engineer first, described the change and sent a sketch. The engineer said no, and explained that the grid was there because the classification assumed the slab would have to move as one unit. A conventional slab with the same edge detail would concentrate the movement at the edges, which is where the house would show it. The engineer offered a different route: a revised design with a changed edge detail, which would need to go back to the certifier before anything was poured.
The builder paid for the amended design, waited for the certifier to accept it, and then poured the slab the drawings described. The programme slipped, and the subcontractor was not pleased for the rest of the job. The alternative was a house with cracks appearing in the years after handover, a claim against the builder, and an underpinning job that cost far more than the saving. The sensible decision took a phone call and a sketch, and it spared the builder a defect he would have carried for years.
How to handle a request to change the slab
When someone on the job asks for a slab change, the answer should follow the same steps every time. Stop the work that depends on the change, get the classification and the design in front of you, and ask the engineer for a written response. If the engineer agrees to a different design, the amended drawings go to the certifier and the variation is priced and agreed with the owner before the work is done. If the engineer says no, the design stays, and the request is closed in writing.
How to handle a request to change the slab
- 1Stop the dependent workNothing that relies on the change goes ahead until the question is answered.
- 2Get the classification in front of youRead the report and its conditions, not only the classification line.
- 3Ask the engineer in writingSend the sketch and the reason, and ask for a written response.
- 4Amend and approveIf the engineer redesigns, the certifier accepts the amended drawings before anything is poured.
- 5Price the variationThe change is priced and agreed with the owner in writing before the work is done.
- 6Keep the fileThe engineer’s response, the drawings, the variation and the acceptance all go on the job file.
The record matters as much as the decision. Keep the engineer’s written response, the amended drawings if there are any, the variation and the certifier’s acceptance on the job file. That file is what protects the builder when a question comes up years later, and it is the difference between a documented design change and a substitution nobody can explain. A builder who keeps that file is the one who can answer the question on the day it is asked.
Expect the question to come back. A subcontractor who asked once for a change will often ask again on the next job, and a builder with a written process does not have to argue the point from scratch each time. The process also protects the trades. A crew that built to the drawings they were given is far less likely to be blamed for a structural problem than a crew that was told to change the design on the day.
For a practical companion on what goes wrong with raft slabs and what to check before the pour, read the waffle raft slab defects checklist for builders. Before any slab goes down, the two questions are whether the ground on the site matches the ground in the report, and whether the drawings on site are the drawings the engineer signed. If either answer is no, stop and call the engineer.
The PointCPD home page sets out the NSW builder CPD courses on offer, and the catalogue is the place to look for the wider topics that sit around footings, slabs and site preparation.
Questions builders ask
What does the site classification on the engineer’s report actually mean for my slab?
It is the engineer’s assessment of how the ground under the house is expected to move as its moisture changes. The slab design is built around that assessment, so the classification tells you what kind of slab the engineer has designed and why. Read the conditions attached to the classification as well, because the design assumes those conditions will be kept.
Can I change the slab type on a reactive site if the subcontractor says the new one is just as good?
Not without the engineer’s written approval. A subcontractor’s view of a slab is a view about construction, and the slab is a structural design the engineer owns. A substitution made without that approval is a change the owner cannot rely on and the certifier has not seen.
Who signs off a change to the slab design?
The engineer who designed the slab has to design the change and sign the amended drawings. The certifier then has to accept the amended documentation before the work is carried out. The builder’s role is to ask for the change in writing, not to make it.
Does a flood planning requirement affect the slab I can build?
It can. Flood planning requirements may set the height the floor has to be above and the way the slab can be constructed on the block. The local council’s flood planning information will say whether those requirements apply to the site, and the engineer needs to know about them.
What should I check if the block has been filled?
Confirm the material on the site matches what the report describes, and ask for the records of how the fill was placed and compacted. Fill that has not been properly placed can settle unevenly. If the excavation shows a different ground condition, stop and call the engineer before the slab is formed.
Can I save money by thinning the edge beams on a reactive site?
No. On a reactive site the edges are doing the work of resisting movement, so reducing them removes the thing the design depends on. Any change to the edge beams is a structural change, and it needs the engineer’s written sign-off before it is built.
Sources
- Continuing professional development for builders and swimming pool builders · NSW Government
- General building work licensing · NSW Government
- Home Building Act 1989 (NSW) · NSW Legislation
Published 23 September 2026. General information for builders, not legal, financial or engineering advice for a specific job. Check current requirements with the relevant regulator or standard before relying on them.
