Planning a concrete patio, slab or driveway

Concrete is unforgiving. Once the truck arrives, you have a short window to place, level and finish it, and a mistake in the ground below the slab shows up as a crack years later. Most of the work happens before the pour: the base, the forms, the steel and the joints. Most of the quantity sums come from the same few numbers, the area and the thickness.

This planner follows a patio, a garage or shed slab and a driveway through the same sequence. They differ mainly in thickness, in the base, and in whether a truck will drive on the result.

Step 1: Choose the size, the thickness and the slope

Patios and walks are usually 4 inches thick. A slab that carries cars is 4 to 5 inches, and a driveway or anything for heavy trucks is 5 to 6 inches, subject to your local code. Plan a slope of at least 1/4 inch per foot so that water runs off, away from the house and the garage door.

Step 2: Dig, compact and build the base

Dig out the base and the slab depth together, remove topsoil and soft spots, and compact what is left. Add crushed stone in layers of 4 inches or less and compact each layer. A slab under a floor, a coating or stored goods also needs a vapor barrier over the base. If you are hauling soil away, work out how much there is and how many loads.

Step 3: Set the forms and the reinforcement

Forms are boards staked at the height and slope of the finished surface, and they must be square and strong enough to hold wet concrete. Reinforcing steel or wire mesh sits mid-depth on chairs, not on the ground. Steel does not stop concrete cracking, but it holds the cracks tight.

Step 4: Work out how much concrete to order

The volume is the area times the thickness, with 5 to 10% extra for uneven ground and spillage. Small jobs use bags of mix; more than about a cubic yard is too much to mix by hand before it starts to set, so order ready-mix, which is sold by the cubic yard and often has a minimum load.

Step 5: If you mix it yourself

For small amounts or remote sites, you can mix cement, sand and gravel in a ratio such as 1:2:4 by volume. Use clean aggregates and as little water as will work, because extra water makes weaker concrete. Know the weight of what you are moving: concrete is heavy.

Step 6: Pour, finish, joint and cure

Have helpers, tools and the weather sorted before the truck arrives. Screed the surface level, float it, edge it and finish it with a broom for grip. Cut or tool control joints a quarter of the slab depth so that cracks follow them, and cure the concrete by keeping it damp or covered for about a week. Keep traffic off for the time your mix needs.

Before you start

• Permit and setback rules, and the rules for the apron if the drive meets a public road.
• Call 811 before you dig.
• A pour day with dry weather, no frost forecast and enough helpers.
• Forms, steel, vapor barrier and tools ready before the truck is booked.
• A plan for where the wash-out water and any leftover concrete will go.

Mistakes that cost people money

• Skipping the compacted base, so the slab settles and cracks.
• Adding water to the truck to make the concrete easier to place. It weakens the slab.
• No control joints, or cutting them too late.
• Ordering too little. A second delivery leaves a cold joint, so always allow extra.
• Using de-icing salts on the new slab in its first winter.

Frequently asked questions