How to Pour a Concrete Slab

What a slab pour involves

Here are the seven stages at a glance, with the point of each.

Layout and base

Mark the slab with stakes and string, then check the corners are square by measuring the diagonals. They should match. Excavate the area to allow for both your slab thickness and the gravel base beneath it, so a 4-inch slab on 5 inches of gravel means digging out about 9 inches. Fill with crushed gravel, rake it level, and compact it firmly with a plate compactor. A solid base is the difference between a slab that lasts and one that sinks.

Forms and reinforcement

Build the form from straight lumber staked every couple of feet, set to the finished height of the slab. Use a level and slope it slightly, about a quarter inch per foot, if you want water to run off. Pitch the gravel rather than the top of the slab if you can, because sloping the top over a 16 foot run adds half again as much concrete; the slope calculator has the arithmetic and the conversion chart. Lay a vapor retarder for any slab that will be enclosed or heated, directly against the underside of the slab rather than buried under a blotter of sand, and in the thickness your adopted code edition requires, which has changed twice in three cycles. The vapor barrier page has the ASTM classes, the code editions and the exceptions. Then set wire mesh or a rebar grid on chairs so the steel sits in the upper third of the slab, not on the ground. ACI 302.1R puts it in the upper third or 2 inches below the surface, whichever is closer to the top, because a shrinkage crack is widest at the surface.

Pouring and finishing

Order ready-mix for anything over about half a yard, or mix bags for a small pad. Place the concrete starting at the far corner, working back toward your exit. Screed it level by dragging a straight board across the top of the forms in a sawing motion. Once the surface water disappears, float it smooth, run an edging tool around the perimeter, cut control joints, and drag a broom across it for grip. Work quickly, because you have less time than you think before it stiffens.

The dry-pour shortcut

You may have seen the dry-pour method, where you spread dry bagged mix, screed it level, and wet it with a hose. It is far easier and popular for small pads and stepping stones. It also produces a weaker, more porous slab than a proper wet pour, so it is fine for a light-duty shed floor or garden pad but not for a driveway, garage, or anything structural.

Pouring a floor inside, where four things change

The seven stages above are the same for an interior floor slab, but four details flip, and they are the ones that get a garage or basement floor wrong.

What goes under it. An exterior patio can sit straight on compacted base. A floor inside a conditioned building gets a vapor retarder, and current ACI guidance puts it in direct contact with the underside of the slab rather than under a layer of sand, which reverses what older editions said. Insulation goes under that where the climate or a heated slab calls for it. Under-slab insulation and the vapor barrier has the classes, the thicknesses and the reasoning.

Level instead of sloped. A patio or driveway is screeded to a fall so water runs off. An interior floor is screeded flat, and flatness is judged by a straightedge rather than by eye, so your screed rails and wet screeds matter more than they do outside.

Isolation joints at everything it touches. A floor slab inside a building meets walls, columns and footings that move on their own schedule. Those get a compressible isolation joint so the slab can shrink without cracking off the obstruction. Outside, you are mostly dealing with control joints in open field.

A harder finish. Exterior concrete gets a broom finish for grip and is air entrained, which rules out hard steel troweling. An interior floor is usually steel troweled, often power troweled, and that finish wants a mix with little or no entrained air. Thickness itself barely changes: concrete floor thickness covers the code minimum and what moves it.

Curing time

Concrete is walkable in 24 to 48 hours, reaches about 70 percent of its strength in 7 days, and hits full design strength around 28 days. Keep the surface damp for the first several days, since concrete hardens through a chemical reaction that needs moisture, not by drying out. You can usually park a car on a driveway slab after 7 days.

Before you start, work out exactly how much concrete to order with the concrete slab calculator so you are not caught short mid-pour. Thinking of skipping the mixing and wetting the bags in place? Dry pour concrete covers what that method does and does not do. Pouring an interior floor slab instead? The concrete floor thickness guide walks through how thick to pour and how to finish it. Not sure what strength to order? What psi concrete you need covers the code minimums and what to ask for.

Steps are the same job with a harder finishing problem, since every riser is a vertical form you have to work behind before the concrete sets: how to build concrete steps covers the forms, the early strip and the footing that is bigger than the steps.