Hiring a Concrete Contractor for a Slab: Quotes, Specs and Red Flags

Dimensions and thickness; concrete strength (3,000–4,000 psi) and whether it's air-entrained; the base material and compacted depth; reinforcement — wire mesh gauge or rebar size and spacing — and that it will be on chairs; a vapor barrier if the slab is enclosed; the joint plan; the finish; excavation, spoil removal and grading; forms and stripping; curing method; permit and inspections; payment schedule; and a written warranty. If any of those is missing, ask. If the contractor won't put it in writing, move on.

Look for a company that pours flatwork every week — slabs, driveways, patios — rather than a general contractor or landscaper who subs it out. Ask for two or three addresses of slabs at least a year old and go look at them for cracks, spalling and drainage. Confirm a contractor's license where your state requires one, general liability insurance, and workers' comp if they have employees. Three written quotes on the same spec will show you who has thought about your site and who has just multiplied square feet by a rate.

A 10×10 to 20×20 slab is usually two days: one for excavation, base, forms and steel, and a morning for the pour and finish. A 30×30 or larger is two or three days of prep. If a building will sit on it, add the inspection between forms and pour, which can be same-day or a week out depending on the department. The slab is walkable in two days, ready for a shed in three, and ready for vehicles in a week.

Spring through fall, on a day between about 50°F and 85°F with no rain forecast. Below 40°F the concrete needs accelerators and insulated blankets, and pouring on frozen ground is not allowed; above 90°F it sets too fast and needs retarders, shade or an early-morning pour. Contractors book up in late spring; late summer and fall often have shorter waits and the ground is dry.

The contractor, in most cases, and the permit fee should be a line in the quote. A permit is usually needed for a slab with a garage, carport or enclosed structure on it, and often for sheds over a local size limit — 120 or 200 square feet is common. At-grade patios usually don't need one. A contractor who suggests skipping a required permit is telling you how they handle the rest of the job.

A concrete contractor or finisher pours flatwork daily and prices it by the square foot. A general contractor manages the whole structure — the slab, the framing, the roof — and subs the slab to a finisher with a markup of 10–20%. If you're building a garage or shop, one contract with a GC is simpler and the slab spec gets coordinated with the building's footings. If all you want is the slab, hire the concrete contractor directly.

One to two years on workmanship is typical, covering settling, heaving, drainage problems and surface defects like scaling and spalling. No contractor warrants against all cracking — concrete cracks — but they should warrant that it cracks at the joints, not across the field, and that the pieces stay level. Get it in writing with the payment terms, and don't pay the final installment until the forms are stripped and you've walked it.

A deposit of 10–30% at signing is normal, mostly to cover the concrete order, with the balance due after the pour and your inspection. Some contractors take a materials draw the day before the truck comes. Paying in full before the pour, or in cash for a discount, removes the leverage you'll want if the slab comes out wrong.