How to Pour a Concrete Slab

Overview of the job

Every slab follows the same sequence. The order matters because each step depends on the one before it. A rushed base, wobbly forms or a late truck cannot be fixed once the concrete is in. Plan for a helper or two, and choose a day with mild weather and no rain in the forecast.

Step 1: Plan, measure and check the rules

Decide the size, thickness and slope. Check with the building department about permits, setbacks and any requirements for the base, reinforcement and joints. Call utility locating services before you dig. Choose the position of the slab so water drains away from buildings, with a slope of about 1/8 to 1/4 in per foot, which is 1 to 2%. Work out the concrete quantity with the slab calculator.

Step 2: Excavate and prepare the sub-grade

Remove sod, roots and soft soil. Excavate deep enough for the base plus the slab thickness, so that the finished surface sits at the planned height. A 4 in slab on a 4 in base needs about 8 in of excavation below the finished grade. Compact the sub-grade with a plate compactor and fix any soft spots with stone. Slabs do not forgive soft spots.

Step 3: Build the base

Spread crushed stone in layers and compact each lift. A 4 in compacted base is common under residential slabs; heavier loads and poor soil call for more. The base should be uniform in thickness and slightly wider than the slab. Estimate the quantity with the gravel base calculator. Some designs add a vapor barrier over the base under interior slabs.

Step 4: Set the forms

Forms hold the concrete in place and define the finished edge and level. Use straight 2×4 or 2×6 boards, staked on the outside every 2 to 3 ft and braced at corners. Set the tops at the finished height with a string line and a level, and set the slope you planned. See formwork basics. Coat the inside faces with a release agent so the boards come off cleanly.

Step 5: Place reinforcement

If the design calls for it, lay rebar or welded wire on chairs so that it sits in the middle or upper third of the slab. Tie the intersections and lap the bars as required. Keep reinforcement a couple of inches clear of the form edges. See the rebar guide. Use the rebar calculator to buy the right quantity.

Step 6: Order the concrete

For a 12 × 16 ft slab at 4 in, the order is 2.61 yd³ with 10% waste. Call the plant a few days ahead, state the strength and air content you need, and confirm the delivery window. See how to order ready-mix. For a small slab of a yard or less you can use bags: 118 bags of 80 lb mix for this slab, which is a big day of mixing.

1. Convert to feet (inches ÷ 12, centimeters ÷ 30.48): Length 12 ft; Width 16 ft; Thickness 4 in = 0.3333 ft
2. Volume = 12 × 16 × 0.3333 × 1 = 64 ft³
3. Net volume = 64 ft³ ÷ 27 = 2.3704 yd³ (× 0.0283168 = 1.8123 m³)
4. Order quantity = 2.3704 yd³ × (1 + 10%) = 2.6074 yd³ = 70.4 ft³
5. 80 lb bags = 70.4 ft³ ÷ 0.6 ft³ per bag = 117.33 → 118 bags (always round up)
6. Weight = 70.4 ft³ × 150 lb/ft³ = 10,560 lb

Step 7: Pour and strike off

Wet the base lightly if it is dry and hot. Start at the far end and place the concrete in the forms with shovels and rakes, working it into corners and against the forms. Do not drop it from a great height or pile it and drag it. Use a screed board across the forms, moving it back and forth in a sawing motion to strike off the extra and fill the low spots. Work steadily; concrete begins to stiffen within an hour or so.

Step 8: Float, edge and joint

Right after screeding, pass a bull float over the surface to embed the stone and close any voids. Then stop. Let the bleed water evaporate from the surface without working it back in. When the sheen is gone and the surface can hold your weight with a slight indentation, run an edger around the perimeter, and groove control joints with a jointer where planned. Use a hand float for small areas. See control joints for spacing.

Step 9: Finish

For an exterior slab, a broom finish gives traction. Drag a clean, damp broom lightly across the surface in one direction after floating. For an interior slab, steel troweling gives a smooth, hard finish, but do not trowel exterior slabs which will be wet because they get slippery. Do not add water to the surface to make finishing easier, since it weakens the top layer.

Step 10: Cure and protect

Begin curing as soon as the finish allows. Cover the slab with plastic or spray with curing compound, or keep it damp for at least seven days. Keep foot traffic, pets and rain off for the first day, and cars off for about a week. Cut joints on time if you are saw cutting. Strip forms after a day or two, then backfill the edges. The curing guide has the timeline.

Tools and materials checklist

• Plate compactor, shovels, rakes and a wheelbarrow.
• String line, level and tape measure.
• Stakes, form lumber, screws or nails, release agent.
• Rebar, chairs, tie wire and cutters, or welded wire sheets.
• Screed board, bull float, hand float, edger, jointer, broom.
• Kneeboards for finishing without leaving marks.
• Curing compound or plastic sheeting.
• Gloves, rubber boots, eye protection and a long-sleeve shirt, since wet concrete irritates skin.

Common mistakes

• Skipping base compaction.
• Starting finishing while bleed water is on the surface.
• Adding water to the mix at the truck.
• Not having enough helpers when the truck arrives.
• Forgetting to plan joints.
• Letting the slab dry out in the first days.

Frequently asked questions