Subbase (base course)
A layer of compacted granular material placed between the subgrade and the slab to provide uniform support and a level, drainable working surface.
What does subbase (base course) mean?
A good subbase evens out variations in the natural soil, gives the crew a stable platform to work from, and can act as a drainage layer. Open-graded clean stone provides a capillary break as well, while fine-heavy material holds water and can create a bathtub under the slab. Compaction in controlled lifts is what makes the layer perform rather than just exist.
In practice
The crushed stone dumped and plate-compacted before a garage slab is the subbase, and skimping on it is the most consequential invisible shortcut in flatwork.