How to check the thickness of a slab you are paying for
Thickness is the one thing about a slab you cannot see once it is poured, and it is the easiest place for a job to come up short. You have one good chance to check it.
What I found on mine
My project record
My contract with L&G Building & Concrete specifies 6 inches under the building. When I was told the pad was ready for concrete, I stood a tape measure on the vapor barrier and read it where the string line crossed: about 4½ inches. The photo is below. It shows one spot, and it does not prove what the finished slab measures. It is the reason I intend to have the slab cored.
Before the pour
- Find the top-of-slab reference. It is the top edge of the form boards, or a string line pulled between them. Ask the crew lead to show you, and ask what thickness they are set for.
- Measure down to the prepared base with the tape standing on the vapor barrier or the compacted stone. Do it in the middle of the pad as well as the edges. The middle is where a high subgrade hides.
- Take at least ten readings on a grid and write them on a sketch. A slab is only as thick as its thinnest area.
- Photograph every reading with the tape face and the string both in the shot, and something in the background that shows where on the pad you are.
- Send the photos to the company in writing before the truck is loaded. Say what you measured and that the contract calls for more. A text or email with a time stamp is what you will want later.
- Do not count on the inspector for this. Ask your county what its slab or under-slab inspection covers. It may be the vapor barrier, reinforcement and plumbing, and nobody may measure depth across the pad.
Also check that the reinforcement is held up off the base on chairs or blocks so it ends up inside the concrete, and that the vapor barrier matches the contract. Mine specified 6 mil.
After the pour
The accepted way to measure an existing slab is to drill cores and measure their length. A concrete testing lab or a geotechnical engineering firm can take a handful of small-diameter cores, measure them, patch the holes and give you a signed report. The same cores can be tested for compressive strength, which tells you whether you got the 3,500 psi you paid for.
There are non-destructive methods too, such as ground-penetrating radar, which are useful for mapping where a slab thins out between core locations. If the numbers are going to matter in a dispute, have an independent firm choose the core locations and do the work, and do not rely on measurements taken by either party.
The edge of a slab tells you very little. Edges are often thickened, and the form board sets that dimension. The question is the field of the slab, where the base may have been left high.
Put it in the contract
Ask for one sentence: minimum slab thickness of X inches measured anywhere in the field, to be verified by the customer before placement, with the contractor to correct the base at its own cost if it is short. My contract had the thickness. It had nothing about verifying it, no warranty on concrete, and full payment due before the pour.
Where these numbers come from
From my own signed contract with L&G Building & Concrete of Asheboro, NC, and what happened on my lot afterward. I am a customer, not a contractor or an engineer, and this is not professional advice. Prices are from February 2026 in western North Carolina.