A shallow foundation consisting of a concrete slab poured directly over a prepared subgrade or base layer of gravel. Because it rests on the ground, it relies entirely on the soil's bearing capacity rather than spanning between beams or piers. This type of slab is common for residential floors, garages, and commercial warehouses, usually requiring a vapor retarder and reinforcement to handle ground movement and moisture.
The engineers want a six-inch slab on grade with a ten-mil vapor barrier and double mat rebar for the warehouse floor.
The biggest risk to a slab on grade is poor subgrade preparation. If the dirt settles, the concrete will crack and follow it down.
Also heard as
- SOG
- ground slab
Do not mix it up with
- Rat slab: A mud slab is a thin, unreinforced layer used only to provide a clean working surface, while a slab on grade is a finished, load-bearing floor.
- Rat slab: A rat slab is a very thin, low-quality pour meant just to keep out pests in crawlspaces, whereas a slab on grade is engineered to support structural loads.
Related terms
Flatwork
Any concrete work poured along a horizontal plane to form a surface.
Rat slab
A thin layer of lean concrete over dirt that gives a clean working surface or seals a crawlspace floor.
Vapor retarder
A plastic sheet under a slab on grade that slows ground moisture from rising into the concrete.
WWR
Welded wire reinforcement, a grid of steel wires welded at every crossing and sold in sheets or rolls.