Everything above the foundation depends on it being right. A step-by-step look at how a slab foundation is built, and the details that get skipped.
If you are building an ADU, an addition, a garage, or a shop, the foundation is the one part of the project that cannot be corrected later without enormous cost. It is worth understanding what should be happening, so you can tell whether it is.
1. Site preparation and grading
Vegetation and topsoil are stripped, because organic material decomposes and leaves voids. The pad is then cut or filled to the correct elevation and compacted in lifts rather than all at once — compaction only reaches so deep per pass.
This stage is where corners get cut most often, and where the consequences appear years later as differential settlement.
2. Excavation for footings
Footings are excavated to the bearing depth specified by the engineer, which is determined by soil conditions rather than by convention. The trench bottom must be firm and undisturbed — loose spoil in the bottom of a footing trench defeats its purpose entirely.
3. Under-slab utilities and vapor barrier
Plumbing and any conduit that will pass under the slab go in now, get inspected, and are protected before concrete arrives. Anything missed at this stage means saw-cutting a finished slab later.
A vapor barrier is then laid over the compacted base, with seams lapped and taped. This is what stops ground moisture from wicking up into the slab and, eventually, into your flooring.
4. Reinforcement
Rebar is placed at the spacing the drawings call for and — this is the part that matters — supported on chairs so it sits within the slab rather than on the ground. Steel lying on the vapor barrier provides almost none of the tensile strength it was specified for.
Anchor bolts and hold-downs are set to the plan at this stage, because their position determines whether the framing above can be attached correctly.
5. Inspection before the pour
The city or county inspector reviews forms, steel, and embedded items before any concrete is placed. We schedule this hold deliberately. Pouring ahead of inspection risks being told to remove it.
6. Placement, finishing, and curing
Concrete is placed continuously to avoid cold joints, consolidated to remove voids, screeded flat, and floated. Finish depends on what goes on top — a slab receiving tile is finished differently from one that will be the final surface.
Curing is the stage most people underestimate. Concrete gains the large majority of its strength in the first week, and it needs moisture to do it. A slab allowed to dry too fast in Inland Empire summer heat ends up weaker and more prone to surface cracking than one that was kept damp or covered.
None of this is exotic. It is a sequence of unglamorous steps done properly, in order, with an inspection hold in the middle. That is what a foundation is.