How to Pour a Concrete Slab, Start to Finish

1. Layout and permits

Set the outline with string lines and batter boards outside the excavation, so the lines survive the digging. Square it by measuring the diagonals: when both diagonals are equal, the rectangle is square, and check the diagonals gives you the exact figure and the correction when they disagree.

Check the setbacks and whether you need a permit before you dig. Many jurisdictions do not permit a slab at grade; some do, and the fee is $50 to $400. Also call your utility locate service — it is free, it is usually legally required, and cutting a gas or fibre line is a bad day that begins with a phone call you did not make.

Decide the finished level now, and the fall. Exterior slabs need about 1/8 to 1/4 inch per foot of slope away from the house. A dead-level patio holds water, and in a freezing climate it holds ice.

2. Excavate and prepare the subgrade

Dig to the depth of the slab plus the base — so a 4 inch slab on a 4 inch base needs 8 inches below finished level, plus a little for the fall. Dig past the topsoil and any organic material; roots and turf rot away and leave voids under a slab that will not tolerate them.

Compact the subgrade before the base goes on. A plate compactor rents for $60 to $100 a day and this is not the step to skip — the base can only be as uniform as what it sits on.

If you hit soft spots, dig them out and fill with compacted stone rather than trying to compact the soft material. A slab bridging a soft patch is the classic settlement crack, and it will find you eventually.

3. Base and vapour retarder

Four inches of compacted crushed stone is standard, and it does two jobs: uniform support and drainage away from the slab underside. Use angular crushed stone, not rounded gravel — angular material interlocks under compaction and stays put, rounded material behaves like ball bearings. crushed stone calculator sizes the order.

Compact it in lifts of no more than 3 to 4 inches. Compaction energy does not reach the bottom of a deep layer, so 8 inches in one pass leaves a soft lower half that consolidates later, which is exactly when the slab above cracks. Wet it lightly as you compact; bone-dry stone will not knit together.

Under an interior slab, a 10 to 15 mil vapour retarder goes over the base, lapped 6 inches and taped, turned up at the edges. This is not optional for anything that will take flooring — moisture rising through a slab destroys adhesives, cups timber and traps under vinyl. Outdoors it is unnecessary.

4. Forms

Straight 2x lumber for the sides, staked every 3 to 4 feet, with the stakes driven below the top of the form so the screed can pass over. Check the top edge with a level or a string line and set the fall now, because the forms decide the finished surface.

Brace corners properly. Wet concrete pushes outward with real force and a form that bows leaves a permanent belly in the edge of the slab.

Oil the inside faces or line them, so the forms strip cleanly. And place isolation joint material — a compressible strip — anywhere the slab meets a wall, a column or an existing slab, so the two can move independently rather than cracking at the junction.

5. Reinforcement

Mesh or bar on chairs, sitting in the middle third of the slab depth. This is the step where most residential slabs quietly fail, because reinforcement laid on the base does nothing at all — steel works in tension and needs concrete on both sides of it to do that.

"Hooking it up" as the concrete goes in is the traditional shortcut and it is unreliable: you cannot see where it ends up, and it usually ends up too low. Chairs cost very little. Use them.

lay the bars out on a grid gives the bar count for a grid, and remember that spacing counts the gaps — a 24 ft span at 18 inch spacing has 16 gaps and needs 17 bars. Lap any run longer than a 20 ft stick by about 40 bar diameters, so 20 inches for a #4.

6. Order the concrete

Work out the volume, add 10% for waste, and round up to what the plant sells — usually whole or half yards. how much concrete the pour needs does the arithmetic including the waste, and the step-by-step version shows it worked by hand. Ordering slightly long is standard practice: surplus costs a little, running short mid-pour gives you a cold joint through the slab, which is permanent.

Specify five things, not one: strength in PSI, quantity, slump, air entrainment if it will freeze, and aggregate size. The PSI guide covers what to ask for.

Confirm access before the day. Can the truck reach the forms and chute directly? If not you need a pump at $800 to $1,500, or a lot of wheelbarrows and people, and both need arranging in advance rather than discovering at 7 am.

Book an early start in summer and a mid-morning one in cold weather. Have more hands than you think — a 400 square foot slab wants three people minimum, and four is better.

7. Place, screed and float

Place from the far corner and work back toward the truck. Move concrete with a rake or a come-along, not a vibrator — vibrating it sideways segregates the mix, leaving stone in one place and paste in another.

SCREED immediately: a straight board rested on the forms, worked across in a sawing motion, filling low spots as you go. This sets the level and it is the last chance to do so.

BULL FLOAT right after screeding, while the surface is still wet. This knocks down high spots, closes the surface and pushes the coarse aggregate down. Then stop.

The waiting is the hard part. Bleed water rises to the surface and must evaporate completely before any further finishing. Working the surface while bleed water is on it drives water back into the top layer and produces a weak, dusty, scaling surface. Depending on weather this wait is 30 minutes to several hours, and there is no way to hurry it.

8. Finish, joint and edge

Once the bleed water has gone and the surface will take a footprint about a quarter inch deep, finishing resumes.

EDGE the perimeter with an edging tool — it compacts and rounds the edge, which is the most vulnerable part of a slab.

FINISH to the texture you want. A broom finish is one pass with a stiff broom after floating, and it is the right choice for anything outdoors: it grips when wet, hides minor imperfection and is easy to repair. A steel-trowelled finish belongs indoors — troweled exterior concrete is a skating rink in the rain. Worth knowing for an interior slab: even a well trowelled floor is rarely flat enough for finished flooring straight off the pour, and an underlayment skim is a normal second step rather than a sign anything went wrong.

CUT CONTROL JOINTS within 6 to 18 hours, at least a quarter of the slab depth deep, at no more than 24 to 30 times the slab thickness in inches read as feet — so 8 to 10 feet for a 4 inch slab. Keep the panels roughly square. A joint cut too late is decorative: the slab has already cracked where it chose to. Why concrete cracks covers what the joints are actually doing.

9. Cure it — the step everyone skips

Concrete does not dry; it hydrates. Cement needs water to keep reacting, and a slab that dries out early simply stops gaining strength. Curing properly can be worth 30 to 50% of the final strength, and it costs almost nothing.

Keep the surface damp for at least the first 3 days and ideally 7: cover with plastic sheeting, mist it periodically, lay wet burlap, or spray a curing compound. In hot or windy weather start immediately and be more diligent, because those are the conditions that take water out fastest.

This is the most skipped step in residential concrete for an obvious reason — it happens after everyone has gone home and there is nothing to see. It is also the cheapest quality improvement available on any pour.

Cold weather has its own rules: concrete must not freeze before it has gained strength. Blankets, heated enclosures or an accelerator, and do not pour onto frozen ground at all.

What to have ready before the truck arrives

Everything on this list, because none of it can be fetched once concrete is moving.

Screed board longer than the slab is wide. Bull float. Edger. Groover or a saw for the joints. Magnesium float and a steel trowel if finishing indoors. Broom. Two or more wheelbarrows. Rubber boots and gloves — wet concrete is caustic and will burn skin over a few hours. Plastic sheeting or curing compound. Water on site. And more people than you think you need.

A washout area matters too: the truck needs somewhere to rinse the chute, and it is not your lawn or the storm drain.