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Monolithic slab: what it is and how it is built
A monolithic slab is a concrete foundation where the floor slab and its thickened edge footing are poured in one piece, at one time.
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Written by HyreFoundation Research Desk Primary-source research and fact checking
The short answer
A monolithic slab is a concrete foundation where the floor slab and its thickened edge footing are poured in one piece, at one time. It is fast and uses less formwork than a stem-wall foundation.
The IRC sets the slab at no less than 3 1/2 inches thick, over a 4 inch base course where below grade, with a 6 mil vapor retarder. Its edge must still reach the depth the code and frost line require.
Key takeaways
When a slab may need a closer look
How a monolithic slab goes in
Remove topsoil and organic material. R506 sets fill depth limits of 24 in for clean sand or gravel and 8 in for earth, with listed conditions.
A trench around the perimeter forms the thickened edge, which acts as the footing.
A 4 in base of clean graded sand, gravel or crushed stone where the slab is below grade, then 6 mil polyethylene with laps of at least 6 in.
Rebar or mesh as designed; plumbing and conduit are placed before the pour, so they end up in or under the concrete.
Edge and slab are placed together, finished and cured. Control joints guide where shrinkage cracks form.
Monolithic against stem-wall slab
| Option | What it means for you |
|---|---|
| Pours | One. A stem-wall system pours footings, then walls, then the slab. |
| Frost | Suited to mild climates, or frost-protected shallow designs. Deep frost lines usually favor stem walls. |
| Sloped lots | Best on flat ground. Stem walls handle grade changes more easily. |
What to ask before you sign
The full checklist is on choosing a foundation contractor.
Where this goes wrong
Shrinkage cracks are expected in concrete. Width and change over time matter. See normal foundation cracks.
Poor drainage at the perimeter is a common cause of slab edge movement.
Leaks under a slab can wet the soil and move it. Get them tested.
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Questions
What is a monolithic slab?
How thick is a monolithic slab?
Is a monolithic slab good in cold climates?
Does a monolithic slab need a vapor barrier?
Can a monolithic slab be repaired?
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HyreFoundation Research Desk
Primary-source research and fact checking
We read the model building code, the federal soil and moisture guidance and the insurer position ourselves, and we publish each figure with the document it came from and the date we retrieved it.
Where a number cannot be traced to a primary source, we say so instead of printing it. HyreFoundation does not inspect, engineer or repair foundations. Authorship is organizational: this desk, not a named persona.
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