Concrete slab vs timber stumps: which foundation?
Slab-on-ground or a stumped subfloor? How your soil, slope and flood risk really make this call, well before personal taste gets a say.
The foundation goes in before anything else, and honestly your soil report has more say in it than you do. Most new homes across SA and Victoria sit on a concrete slab engineered to suit the ground. A raised floor on stumps, timber or steel, is less common in modern volume builds, but it still earns its place on sloping blocks, flood-prone sites and some period-style homes.
| Concrete slab | Timber or steel stumps | |
|---|---|---|
| How common | Standard on most new builds | Less common, used where the site calls for it |
| Best for | Flat to gently sloping blocks | Sloping, flood-prone or period-style sites |
| Reactive clay | Waffle pod or stiffened raft rides the movement | Stumps can be re-levelled down the track |
| Underfoot | Solid, with handy thermal mass | A little give, and easy access below |
| Services | Plumbing is set in the slab, harder to move later | Easy to reach pipes and wiring under the floor |
| Sloping ground | Needs cut and fill | Follows the fall with less earthwork |
Your soil report calls the shots
Before you get attached to either option, your geotechnical soil report classifies the block, from stable ground through to highly reactive clay. On reactive clay a slab is engineered as a waffle pod or stiffened raft so it can ride ground movement. The single best thing you can do is finalise the soil testing and engineering before you sign a fixed-price contract, so this is settled up front rather than sprung on you later.
Where stumps still make sense
A raised floor comes into its own on a block with a decent fall, where it follows the slope instead of demanding a mountain of cut and fill, and on flood-prone land where you want the living level lifted clear. It also hands you easy access to plumbing and wiring under the floor, which some owners quietly love for future changes.
Concrete slab
- Standard, and every builder knows it inside out
- Solid floor with useful thermal mass
- Engineered systems cope with reactive clay
- Very little to maintain
- Moving plumbing later is a real job
- Wants cut and fill on a slope
- Poor soil can mean piering
Timber stumps
- Follows a sloping block with less earthwork
- Easy access to services under the floor
- Can be re-levelled if the ground shifts
- Lifts living areas above flood risk
- Less common, so fewer builders offer it
- Floor can feel a touch less solid
- Stumps and bearers want the odd check
- Often the harder sell on a flat block
For most flat to gently sloping blocks a concrete slab is the standard, sensible choice. Reach for a stumped subfloor on sloping or flood-prone sites, or where under-floor access matters to you. Let the soil report and the slope lead, and the rest tends to fall into place.
Still weighing it up?
Tell us about your block and what you want, and we will match you with three builders across SA or Victoria who do this sort of work well.
Match me with 3 buildersCommon questions
Are concrete slabs better than stumps?
For most flat blocks a slab is the standard choice, with a solid floor and almost nothing to maintain. Stumps suit sloping or flood-prone sites and give under-floor access. The better option really comes down to your soil report and slope.
What is a waffle pod slab?
It is a concrete slab poured over a grid of foam pods, which creates a stiff raft that sits on the ground and rides reactive clay movement. It is one of the most common slab systems on Australian new builds.
Why do I need a soil test before choosing a foundation?
The report classifies how reactive and stable your block is, which drives the slab design or the stump depth. Getting it done before a fixed-price contract means the foundation is locked in properly rather than becoming a surprise later.
Can you build on stumps on a flat block?
You can, but it is less common on flat ground and is usually the harder sell. Stumps earn their keep on slope, difficult access or flood risk, or where you really want services access under the floor.