Quick answer
What is a slab-on-grade foundation?
A slab-on-grade foundation is a single reinforced concrete slab poured directly onto prepared ground, with thickened edges — footings — around the perimeter and beneath load-bearing walls. It supports the structure and forms the ground floor at the same time. Under it sit a compacted aggregate base and a plastic vapour barrier that stops ground moisture rising through the concrete.
- Typical slab thickness
- 4–6"
- Perimeter footing depth
- 12–24"
- Vapour barrier
- 10–15 mil minimum
- Base course
- 4–6" compacted
A slab-on-grade foundation is deceptively simple to describe and unforgiving to build. It's one pour that has to be a structural foundation, a moisture barrier, a level floor, and a base for every finish above it — with essentially no opportunity to correct anything afterwards, because the house goes on top of it.
Almost everything that determines whether a slab foundation performs well happens before the concrete arrives. Understanding the layers makes it far easier to tell whether a job is being done properly, because the sequence is visible right up until the moment it isn't.
The layers of a slab foundation
How a slab foundation gets built
The sequence, and what to look for at each stage. Everything after step six is buried.
Site preparation and excavation
Topsoil, organic material, and any soft ground removed. The site is cut to level and the footing trenches excavated to the depth on the drawings.
- Organic material left under a slab decomposes and leaves voids. It has to come out, not be compacted over.
Subgrade preparation
Native soil graded and compacted to the specified density. On expansive soils this may involve moisture conditioning, lime treatment, or removal and replacement, depending on the geotechnical report.
Utilities roughed in
Under-slab plumbing, electrical conduit, and any radiant heating pipework installed, inspected, and pressure-tested before anything covers them.
- This is a hold point. Once concrete is poured, reaching a failed under-slab line means cutting the floor of the house.
Base course placed and compacted
Typically 4 to 6 inches of clean crushed aggregate, placed in lifts and compacted between each. It provides uniform support and a capillary break against rising moisture.
Vapour barrier installed
A polyethylene membrane, 10 to 15 mil minimum, laid over the base with lapped and taped seams and sealed around every penetration.
- Punctures and unsealed penetrations are the most common defect here, and they're the reason a house ends up with moisture coming through the floor years later.
Reinforcement placed
Rebar tied into a grid to the engineer's specification and chaired so it sits at the correct depth. Footing steel tied and lapped correctly. Post-tension cables installed and supported if the design calls for them.
Inspection
The building department inspects reinforcement, vapour barrier, and under-slab utilities before the pour. This is a required hold point and it exists precisely because none of it can be checked afterwards.
Pour, screed, and finish
Concrete placed, consolidated, screeded to level, then floated and finished. Interior slabs are usually power-trowelled; the flatness tolerance matters for the flooring that follows.
Cure
The slab is kept damp or covered while it gains strength. Post-tension cables are stressed once the concrete reaches the specified strength, typically within the first week.
Footings — the part that carries the house
The slab itself carries relatively little. The building's weight goes into thickened sections — footings — that run around the perimeter and under load-bearing internal walls and columns, spreading load into soil at a depth below seasonal moisture variation.
Footing depth and width come from the structural drawings, informed by the soil report and the loads above. A typical residential perimeter footing is 12 to 24 inches deep and 12 to 18 inches wide, with continuous reinforcement top and bottom. Deeper is common where expansive soil means the active zone extends further down.
Where footings are cast integrally with the slab — a monolithic pour — the whole foundation goes in as one piece, which is faster and avoids cold joints. Where they're poured separately, the connection between footing and slab has to be detailed properly with dowels or a keyway.
Slabs on expansive soil
On expansive clay, a conventional slab is a rigid plate sitting on ground that rises and falls seasonally. Left unaddressed, that produces the classic pattern of edge lift in winter, centre lift or edge drop in summer, and cracking in the structure above.
Three engineering approaches are common locally. A post-tensioned slab contains tensioned steel cables that put the concrete into compression, letting it resist bending far better than reinforcement alone. A stiffened or waffle slab uses a grid of deep integral beams to make the slab rigid enough to bridge soil movement. And drilled pier foundations carry the loads down through the active zone to stable material below, with the slab either structurally suspended or designed to move independently.
Which is appropriate is a geotechnical and structural engineering decision. It's genuinely site-specific, and a contractor offering a foundation specification without a soils report on a known expansive site is guessing with something expensive.
Slab foundation questions
Related reading
Key takeaways
- A slab-on-grade is one pour doing four jobs: foundation, floor, moisture barrier, and finish substrate.
- The layers — subgrade, compacted base, vapour barrier, reinforcement, slab — all matter and all get buried.
- The vapour barrier prevents moisture rising into the house. Punctures are common and consequential.
- Footings, not the slab, carry the building's weight into the soil.
- Expansive soil requires engineering: post-tensioned, stiffened, or piered designs.
- The pre-pour inspection is the last chance to see any of it. Be there.
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