Slab On Ground Versus Suspended Slab

A slab on ground transfers its load directly into the soil beneath it. A suspended slab spans between footings, columns or walls, and the ground underneath carries nothing.

They look similar when finished and they behave completely differently.

Slab on ground

The soil under it is part of the structure. The slab is designed to work with the ground, and its stiffness is what controls the movement the ground imposes on the building.

Which means the site classification governs the design. A stable sand site and a highly reactive clay site produce very different slabs, and that relationship is the substance of [site classification and soil investigation](/blog/site-classification-soil-investigation-basics).

The common forms are a stiffened raft, usually a [waffle pod or a conventional raft](/blog/waffle-pod-vs-conventional-raft-slabs), and both work by being stiff enough to ride out the ground movement without distorting the building above.

It is the cheaper option where the ground allows it, and on ordinary residential sites it is the default.

Suspended slab

The slab spans between supports. The ground below is irrelevant to the structure, and the load goes into footings or piles that reach competent material.

Used where the ground cannot be relied on, where there is a void below, or where the levels demand it.

More expensive, and completely independent of what the soil does.

What drives the choice

Ground conditions. Deep uncontrolled fill, soft soils, or a highly reactive profile can all make a slab on ground impractical. At some point the raft required to cope becomes more expensive than piling and suspending.

A basement or a void below. If there is space underneath, the slab is suspended. There is no decision to make.

Steep sites. On a sloping site, a slab on ground needs cut and fill. Fill under a slab has to be engineered and compacted, and deep fill under one end and natural ground under the other is a settlement problem waiting to happen. Beyond a certain fall, a suspended slab on piers is both cheaper and safer, and where deep fill is unavoidable it becomes a [ground improvement](/blog/ground-improvement-techniques-explained) question instead.

Trees and their zone of influence. Where a significant tree sits close enough to affect the soil moisture, a suspended slab on piles founded below the influence zone removes the problem entirely.

Flooding. Where a floor level has to sit above a flood planning level, suspending it is frequently the only practical way to get there, as covered in [engineering for flood prone sites](/blog/engineering-for-flood-prone-sites).

Services and maintenance. A suspended floor with a subfloor gives access to services. A slab on ground buries them.

The mistakes that get made

Deep fill under a slab on ground. Fill compressing under a slab designed to sit on natural ground is one of the more common causes of residential settlement, and it is entirely avoidable with engineered fill and compaction testing.

Assuming the classification. The site classification comes from investigation. Assuming it from the neighbouring property is how the wrong slab gets designed, and Sydney ground varies sharply over short distances.

Ignoring the edge. Moisture change is largest at the perimeter of a slab on ground, which is why edge beams are deeper and why drainage and paving around the building matter as much as the slab itself.

Mixing systems in one building. Part slab on ground and part suspended, without an articulation joint between them. They move differently, and the junction cracks. Where both are genuinely needed, they are separated deliberately.

The subfloor question

A suspended timber or concrete floor over a subfloor needs that space ventilated, or moisture builds up and the structure above suffers.

That is a straightforward requirement and it is frequently compromised by later landscaping blocking the vents, which is covered in [subfloor ventilation and structural timber](/blog/subfloor-ventilation-and-structural-timber).

Settling it early

This is a decision that comes out of the geotechnical report and the levels, and it should be made before the design is far advanced, because it changes the cost, the floor levels and the site works.

ACSES provides structural engineering and footing design for residential and commercial projects across Sydney. Get in touch.