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Slab Placement

Written for Canada. Plans are metric, but many crews and suppliers work in feet and inches, so this site gives both: millimetres and metres, with feet and inches beside them. Codes are the National Building Code of Canada 2020 (NBC 2020) Part 9 as your province adopts it. A number marked Confirm has not been checked against every province, so check it with your own authority before you act on it.

Reviewed by a working tradesman with 17+ years in the field.

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A slab is the biggest visible surface in the building, and it shows every mistake. The work is a sequence, and the timing between the steps matters as much as the technique.

I am a:

Tap your level and the boxes below open to the advice for you. Remembered on this device.

What it isPlacing, screeding, finishing, jointing and curing a slab on ground.
Your jobRun the sequence in order, and keep the timing between steps.
The hard ruleNever finish while bleed water is on the surface, and never add water to speed it up.
The checkLevel and slope as drawn, joints cut in time, cured right away.

By the end you can

  • Run the slab sequence: place, screed, bull float, wait, edge and joint, finish, cure.
  • Tell when the bleed water is gone and finishing can start.
  • Lay out control joints and know how deep to cut them.
  • Protect a slab from heat, wind, cold and rain.
  • Cut concrete without breathing silica dust.
Stop and read

Cutting or grinding concrete releases dust that contains silica, which can cause silicosis, a serious and sometimes fatal lung disease. Cut wet or use dust extraction, and do not rely on a dust mask. Wet concrete also burns skin, so wear gloves, long sleeves, tall waterproof boots and eye protection, and use kneeboards so you are not kneeling in it. Keep clear of trucks, pump booms and hoses.

The big idea

A slab is the biggest visible surface in the building, and it shows every mistake. The work is a sequence, and the timing between the steps matters as much as the technique.

Under NBC Part 9, a concrete slab is at least 75 mm (3 in) thick, not counting any topping (9.16.4.3). Concrete is at least 20 MPa for floors and 32 MPa with 5 to 8 percent air for garage and carport floors and exterior steps (9.3.1.6). The drawings can ask for more.

Before you pour

Run the checklist on the Concrete Delivery & Preparation page, plus these slab checks.

  • The base is compacted and fine-graded, and the vapour barrier and insulation are placed where required (see Slab and Pad Forms).
  • Mesh or rebar is on chairs, thickened edges are ready, and the slope is set as drawn.
  • Forms are straight, tight and staked to the correct elevation.

Placing and consolidating

  • Start at the farthest point and work back toward the truck.
  • Use rakes to pull concrete, and do not drag mesh or rebar with it.
  • Fill thickened edges and beams first, and use an internal vibrator there.
  • Rod or tamp standard slabs. Do not dump a pile in one spot and push it across.

Example quantity. A slab 6 m × 4 m (19 ft 8 in × 13 ft 1 in) at 100 mm (4 in) holds 6 × 4 × 0.1 = 2.4 m³ (3.1 yd³) before any allowance. The sizes are made up.

The finishing sequence

Six panels in order: place and screed, bull float, wait until the bleed water is gone, edge and joint, float then trowel or broom, and cure under a cover.
Finishing sequence for a slab. Place and screed, bull float, wait until the bleed water is gone, edge and joint, float and then trowel (inside) or broom (outside), and cure. Timing depends on the weather and the mix. Not to scale.
  • Bleed water. Water rises to the surface after the concrete is placed. Do not finish while it is there. One federal highways brief suggests laying an 18 in (457 mm) square of clear plastic on the surface to see whether the bleed water has stopped.
  • Each step has its own page: Screeding, Floating, Troweling and Broom Finishing.
Hot, dry or windy

When water leaves the surface faster than it rises, the top dries and cracks while the concrete underneath is still soft. A concrete-products maker citing ACI 305 says plastic shrinkage cracking becomes a strong possibility above about 1.0 kg/m² per hour (0.2 lb/ft² per hour), and that controls should be considered from about half that. Temperature, humidity and wind all push the rate up. Plan for windbreaks, shade, an evaporation retarder or an early cover.

Joints

  • Control joints are a weakened line that makes cracking happen where you want it. The Portland Cement Association guidance is a spacing of 24 to 36 times the slab thickness, panels square or nearly so (length no more than 1.5 times width), and a groove at least one quarter of the thickness and not less than 25 mm (1 in). The drawings come first.
  • Isolation and expansion joints go where the drawings put them, around columns, walls and fixtures.
  • Saw cutting is done as soon as the concrete can take the saw without ravelling. An NRMCA joint guide for paving says that with a conventional saw this is generally 4 to 12 hours, depending on the weather. Too early and the edges ravel. Too late and the slab cracks first. Ask your supplier about your mix.

Example. A 6 m × 4 m (19 ft 8 in × 13 ft 1 in) slab at 100 mm (4 in): spacing is 24 × 100 = 2.4 m to 36 × 100 = 3.6 m (7 ft 10 in to 11 ft 10 in). One joint across the 6 m length at 3 m (9 ft 10 in) makes two panels of 3 m × 4 m (9 ft 10 in × 13 ft 1 in). The ratio is 4 ÷ 3 = 1.33, under 1.5. The groove is at least 25 mm (1 in) deep. The NRMCA guide gives 3 m (10 ft) as its maximum for a slab 4 to under 5 in thick, which agrees.

Silica

Cut wet, or use a saw with dust collection. Dry cutting and dry grinding release silica dust. Exposure limits and rules differ by province, so confirm them with your regulator. A respirator is a backup, not the control.

Curing and protection

  • Start curing once finishing is done and bleeding has stopped, with curing compound, a wet cover or blankets.
  • Keep traffic off.
  • Below 5°C (41°F), Part 9 requires the concrete to be kept between 10°C and 25°C (50°F and 77°F) while mixed and placed, and at 10°C (50°F) or warmer for 72 hours after placing (9.3.1.9).
  • Do not cut joints too early or too late.

Common mistakes

  • Wrong elevation or slope
  • Mesh on the ground
  • Over-vibration
  • Poor screeding
  • Finishing while bleed water is present
  • Missing or late joints
  • Adding water to speed up finishing
Pro note

Never finish while there is water on the surface. Waiting feels slow, but it is the biggest thing that separates a good slab from a flaky one.

Weather note

Hot, windy days dry the surface early and cause plastic cracking. Cold days delay the set, and finishing too early on a cold day traps water. Rain on a fresh slab leaves a weak, dusty surface.

What the inspector looks at

Before the pour: base, barrier, insulation, steel and forms. After: slope, thickness and joints, and the strength at the time of loading.

✅ Check yourselfTap a question
A slab is 6 m × 4 m × 100 mm (19 ft 8 in × 13 ft 1 in × 4 in). How much concrete is that?
6 × 4 × 0.1 = 2.4 m³ (3.1 yd³), before any allowance. Order a little over, as your supplier advises.
Joints go at 24 to 36 times the slab thickness. What spacing is that for 100 mm (4 in), and how would you cut that slab?
24 × 100 = 2.4 m and 36 × 100 = 3.6 m (7 ft 10 in to 11 ft 10 in). One joint across the 6 m length at 3 m (9 ft 10 in) gives two panels of 3 m × 4 m (9 ft 10 in × 13 ft 1 in). The ratio is 4 ÷ 3 = 1.33, under the 1.5 limit.
How deep is the control joint groove for a 100 mm (4 in) slab?
At least one quarter of the thickness, 25 mm (1 in), and not less than 25 mm. The drawings can ask for more.
There is shiny water on the surface right after the bull float. Do you start troweling?
No. Wait until the bleed water is gone. Finishing while it is there traps water and gives a weak, flaky surface.
Wind and sun are drying the surface faster than you can finish. At what evaporation rate does plastic shrinkage cracking become a strong possibility?
Above about 1.0 kg/m² per hour (0.2 lb/ft² per hour), according to a concrete-products maker citing ACI 305. Control measures start around half that.
You cut the slab dry to save time. What is the hazard?
Concrete dust contains silica, which can cause silicosis. Cut wet or with dust extraction. A dust mask alone does not control it.
🧰 Gear for this stepMust-haveRent itNice to have
  • Must-haveLayout and checking:
  • Must-havePlacing by hand:
  • Rent itPlacing by machine: a pump or chute
  • Must-haveAccess: kneeboards or plywood to spread weight
  • Must-haveScreeding: a straight screed board
  • Rent itScreeding by machine: a vibrating screed
  • Must-haveBull floating: a bull float with extension handles
  • Must-haveFinishing by hand:
  • Rent itFinishing by machine: a power trowel, on large slabs
  • Rent itJoints by machine: a concrete saw with a wet-cut or dust-collection setup
  • Must-haveConsumables:
  • Must-havePPE: green-triangle boots (CSA Z195), tall waterproof boots for wet work, eye protection (CSA Z94.3), waterproof alkali-resistant gloves, long sleeves, knee pads, a hard hat (CSA Z94.1) near pump booms and trucks, hearing protection for saws, and a respirator (CSA Z94.4) only as a backup to wet cutting or dust extraction
👷 Who can do thisTap your level
Home DIY
A capable owner can pour a small basement or garage slab with help, if the base, barrier, steel and permit are right. Structural, heated or elevated slabs and any engineered work need the drawings and a pro. Do not dry-cut slabs. Cut wet or with dust extraction, because a dust mask does not control silica.
Laborer or helper
Move material, set chairs and clean tools under direction. Do not dry-cut. Cut wet or with dust extraction as directed. Do not decide when the slab is ready for finishing.
1st-year apprentice
You can screed, float and edge under supervision, and place chairs and barrier. Learn timing, and when the bleed water is gone, first.
Apprentice, 2nd to 4th year
Screed, float, edge and place chairs and barrier under supervision. Learn to read the bleed water first. Cut only with wet cutting or dust extraction. Tell your journeyperson at once if the surface is drying too fast or joints cannot be cut in time.
Journeyperson
You own the sequence, timing, joints and curing. Stop-work triggers: hot or windy weather drying the surface before it can be finished, joints that cannot be cut in time, or a base or weather that is not fit for the pour (turn it down).
Lead hand or foreman
Set the finishing sequence and crew positions before the truck arrives. Enforce wet cutting or extraction on every cut. Turn the pour down if the base or weather is not fit, and say so before the load is placed, not after.
Contractor or business owner
Price the finishing crew, power trowels, saw cutting and curing. Document inspections and tickets. Have a dust-control plan for silica (wet cutting or extraction), and remember a bad finish is a call-back. Ask your lawyer or insurer about warranty terms.
Starting a new business
Learn the sequence of a slab pour and subcontract finishing and saw cutting until your own crew is proven. Write a dust-control plan for silica. Confirm silica rules with your provincial regulator, and ask your lawyer and insurer about call-back and warranty terms.
By Province:

NBC Part 9, Section 9.16 sets a slab at least 75 mm (3 in) thick (9.16.4.3), and Section 9.3.1 sets 20 MPa for floors and 32 MPa for garage and carport floors. Provinces adopt the code with changes, so check your edition.

  • All provinces: silica exposure limits, dust-control rules, radon rough-in and under-slab insulation requirements differ by province and by municipality. I did not verify provincial figures on this pass, so confirm them with your provincial regulator (for silica) and your local building department (for radon and insulation).

See Canada by Province for the full comparison.

Sources:

Last reviewed October 2026. General information, not engineering or legal advice.

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