How to Pour a Concrete Slab

Pouring a concrete slab isn’t mysterious, but it does reward planning. You can prep the whole job by yourself—layout, excavation, compaction, forms, reinforcement—and then bring in one helper for pour day. This blog walks through the exact process - used for a shed slab, including squaring with the 3-4-5 method, sub-base prep for freeze–thaw climates, building forms with fall for drainage, wiring a rebar grid, when to screed in sections (huge time saver), and how to cut joints so tiny internal cracks don’t turn into spiderwebs.

Whether you’re pouring for a shed, garage, or patio, the steps are the same. The details—thickness, reinforcement, slope—scale to the use case and your soil/climate.

At a glance

• Skill level: Confident DIYer

• Crew: One person for all prep; 2–3 people on pour day

• Time: 1–3 days of prep + the pour

• Typical slab thickness: 4 in (100 mm) for sheds/patios; 5–6 in (125–150 mm) for vehicles

• Core steps: Plan & square → Excavate → Compact → Gravel base → Forms with slope → Reinforcement → Pour & screed in sections → Float/finish → Cure → Saw-cut joints

Materials & tools

Materials

• 57 stone / ¾ in (20 mm) clean, angular gravel for base (no sand fines)

• Ready-mix concrete (order by cubic yards/m³)

• Rebar #4 (½ in / 12 mm) or 6×6 WWM mesh (welded wire mesh)

• Tie wire (16–18 gauge) + rebar chairs (to lift steel ~2 in / 50 mm)

• Form boards (straight 2×4 / 2×6 depending on slab thickness)

• Screws (#8 or #9, 2–2½ in / 50–64 mm) for forms

• Marking paint, stakes, string

• Expansion joint filler (if abutting existing concrete)

• Curing method: water, curing compound, or plastic sheeting

Tools

• Tape measure, framing square

• Mason’s line (string line) + string-line level or laser

• Shovel, mattock, rake

• Plate compactor (rental) or hand tamper (workout)

• Circular saw (for forms); diamond blade for concrete saw cuts

• Bull float + hand floats/edgers/finishing trowels (as needed)

• 2×4 screed board (straight) or aluminum screed

• Wheelbarrow, come-along rake (for moving mud)

• PPE: safety glasses, hearing protection, alkali-resistant gloves, long pants, knee pads (concrete burns are real)

Step 1: Pick a reference line and square the layout

Choose a clean reference—the fence line—and sets the slab parallel to it. Stretch a string 1.5 m (≈ 5 ft) off the fence so the finished slab reads square in the yard.

To square the perpendicular edges, use the 3-4-5 method (a field version of Pythagoras):

1. From the corner peg on your reference string, mark 3 ft (0.9 m) along the first side.

2. From the same corner, mark 4 ft (1.2 m) along the second side.

3. Adjust that second side until the distance between the 3-ft and 4-ft marks measures exactly 5 ft (1.5 m). Now the angle is a true 90°.

4. Paint your lines on the ground. Extend to full length (e.g., 12 ft × 25 ft / 3.6 m × 7.3 m in Bart’s case).

Why it matters: If forms aren’t square, the slab won’t look right against fences, walls, or pavers. It also complicates shed placement later.

Step 2: Excavate and strip organics

Strip sod and organic material completely within the footprint. Organic matter rots and leaves voids. Excavate to allow for:

• Gravel base (typ. 4 in / 100 mm in freeze–thaw zones; 2–4 in / 50–100 mm in mild climates)

• Slab thickness (min 4 in / 100 mm for sheds/patios; thicker where posts/point loads land)

• Any desired slope (Bart builds in fall for drainage)

If you’re pouring post footings or a monolithic slab with thickened edges/columns (e.g., for a carport or shed roof posts), dig those piers/trenches now to plan the steel.

Step 3: Compact the subgrade (don’t skip)

Compaction matters more than almost anything. Loose soil settles; water moves; slabs crack.

• Best: Rent a plate compactor (~$60 a day) and make multiple passes.

• Water between passes to help fines bind.

• Target a firm, uniform base with no footprints deeper than a few millimeters.

Rule of thumb: Disturbed soil needs compaction to 95% of its original density. The plate compactor plus a little moisture gets you close.

Step 4: Place a clean gravel base

Use ¾ in (20 mm) angular “57 stone”—no sand fines. In Chicago’s freeze–thaw, a capillary-breaking layer keeps ground moisture from wicking up and expanding under the slab. Spread ~4 in (100 mm) and compact it.

• Why not sand here? Sand pulls moisture up; fines hold water. Clean, angular gravel drains and locks when compacted.

• Rake level; check thickness around the perimeter with a stick marked in inches/mm.

Gravel volume quick math - Area × depth = volume.Example 12×25 ft slab, 4 in base (0.333 ft):12 × 25 × 0.333 = 100 ft³ ≈ 3.7 yd³ (add 5–10% waste).Metric (3.6×7.3 m, 0.10 m): 3.6 × 7.3 × 0.10 = 2.63 m³.

Step 5: Build straight, level forms (with fall)

Use straight 2×4/2×6 boards for your perimeter. Set stakes every 3–4 ft (0.9–1.2 m). Screw forms to stakes so you can tweak them. If drainage matters, build fall into the form tops. Bart uses about 0.035 in/ft (~0.3%)—roughly ⅜ in drop over 12 ft (~10 mm over 3.6 m). For patios, 1/8–1/4 in per foot (1–2%) is more typical; choose what fits your site.

• Square again: Re-check all corners with 3-4-5.

• Straighten bows: If a form board bows, run a string from corner to corner, then push/pull the board to match the string and lock it with stakes.

• Set elevation: The top of form = top of slab. Mark a benchmark and use a level or laser.

Double and triple check here. Wet concrete takes the exact shape you built. Fixing crooked forms after the pour is not a thing.

Step 6: Reinforce the slab

Bart expects real loads (tractor), so he uses #4 (½ in / 12 mm) rebar in a grid at 24 in (600 mm) centers. For lighter slabs, WWM (6×6) placed mid-depth is common, but it must be supported—mesh lying on the base does nothing.

• Lay out the grid and tie intersections with tie wire every crossing and overlap.

• Set on chairs to lift the steel ~2 in (50 mm) so it sits near mid-slab after the pour.

• Add extra bars under posts or door openings.

Important: Steel belongs in the tension zone. In a slab-on-grade, mid-depth is a safe compromise as the slab rides the subgrade.

Step 7: Order the right amount of concrete

Concrete volume: Area × thickness.

• 12×25 ft at 4 in: 12 × 25 × 0.333 = 100 ft³ ≈ 3.7 yd³ → order 4.0 yd³ (waste, edge thickness, spillage).

• 3.6×7.3 m at 0.10 m: 3.6 × 7.3 × 0.10 = 2.63 m³ → order 2.9–3.0 m³.

Mix: Ask for 3,500–4,000 psi (25–28 MPa) with air entrainment in freeze–thaw regions. Let the plant dial slump based on weather and your placement method (pump vs chute). Lower slump (stiffer) holds shape but needs more muscle.

Step 8: Plan the pour—work in sections

Big lesson: don’t flood the entire form at once on a large slab. You lose control of highs/lows and waste time shoveling extra mud.

Better approach:

• Pour in strips ~6–8 ft wide (1.8–2.5 m).

• Screed each strip to the forms before moving on.

• Keep a two-person rhythm: one rodding/screeding, one raking/feeding.

• Aim to stay just ahead of initial set, not behind it.

Crew roles on pour day:

• Person A: Comes along the chute/pump, moves concrete with a come-along rake.

• Person B: Screeds with the 2×4/aluminum straightedge.

• Person C (nice to have): Bull floats immediately after each pass; edges boards; keeps tools clean.

Step 9: Place, screed, and bull float

1. Wet the forms lightly; keep base surface damp, not soggy.

2. Place concrete near grade; don’t drop from high—it segregates.

3. Screed by sawing the board back and forth while pulling along the forms. Knock off the highs, fill the lows.

4. Bull float immediately—steady passes perpendicular to screed direction to push down large aggregate and bring up paste (the “cream”).

5. Edge and joint (if using groover) as the surface tightens.

If the concrete is already firm enough to walk on while you’re still screeding, you’re behind. That’s the signal to reduce the pour rate or add another screeder.

Step 10: Finish (simple is best)

For sheds and utility pads, a bull float + magnesium hand float finish is usually appropriate. Skip mirror-polishing; it becomes slippery and can trap water.

• Wait for bleed water to evaporate before steel troweling. Working bleed water back in weakens the surface.

• If you want extra grip, broom finish lightly after floating.

• Don’t obsess over perfection; a flat, well-drained slab beats over-worked paste every time.

Bart didn’t get to edge the slab during the ideal window; if that happens, you can soften edges later by light grinding or leave them square for shed use.

Step 11: Cure properly

Concrete gains design strength at 28 days. The first week matters most.

Choose one:

• Wet cure: Mist and keep damp for 3–7 days (cover with plastic to retain moisture).

• Curing compound: Spray after finishing (follow manufacturer).

• Cover: Plastic sheeting weighted at the edges to minimize evaporation.

Remove forms after 48 hours if temps are moderate. Backfill the edges gently.

Step 12: Cut control joints

Concrete shrinks as it cures; microcracks form internally. Control joints create a planned weak plane so cracks follow your lines, not random paths.

• Timing: 6–48 hours depending on temp/mix; by 48 hours it still cuts cleanly.

• Depth: ¼ of slab thickness (e.g., 1 in / 25 mm for a 4 in slab).

• Spacing: 24–36× slab thickness (in mm/in). For a 4 in slab, keep joints 8–12 ft (2.4–3.6 m) apart.

• Saw in a grid so panels are as square as possible.

• If slab abuts existing concrete or posts, add isolation joints.

Cut three 1-in deep joints two days after the pour, making four tidy panels. That’s on the late side but worked.

What would do differently next time

• Screed in sections on large pours. Working strip by strip saves time and accuracy.

• Edge sooner (if you want rounded edges).

• Add help if the truck asks “how many people?” and you answer “me alone.” Don’t do that.

• Wear pants & knee pads. Cement is caustic; it can burn skin and embed in abrasions.

Common mistakes (and quick fixes)

• Forms out of square or bowed: Use long strings; adjust with stakes until the board kisses the string line.

• Soft subgrade: Re-compact. If you can leave footprints, you’re not ready.

• Mesh on the ground: Lift it with hooks or set on chairs; otherwise it adds almost no value.

• Pouring entire slab at once: Break it into strips. Screed as you go.

• Working bleed water back in: Wait. Finish after the sheen disappears.

• Skipping joints: Expect random cracking. Cut the joints.

Thickness, reinforcement, and loads

• Patios/walkways: 4 in (100 mm), WWM mid-depth, 2–4 in gravel base

• Sheds/storage: 4 in (100 mm) with #3/#4 rebar grid @ 18–24 in (450–600 mm), 4 in gravel base

• Driveways/light vehicle: 5–6 in (125–150 mm), #4 @ 18 in (450 mm), 6 in gravel base

• Posts/point loads: Thicken edge/footing or add isolated piers tied into slab steel

Always check local code for frost depth, setbacks, and vapor barriers for interior slabs.

Safety you can’t shrug off

• Eyes/ears/face: Grinders, saws, mixers—treat them with respect.

• Skin: Wet concrete is alkaline. Wear long pants, waterproof gloves, and wash off splashes fast. Knee pads are not optional.

• Back and hands: Lift with legs; swap tasks often.

• Tools & cords: Keep cords dry and GFCI-protected; clean slurry off tools as you go.

Calculators

• Concrete volume (US):(length_ft × width_ft × thickness_in / 12) / 27 = cubic yards

  
  

• Concrete volume (metric):length_m × width_m × thickness_m = cubic meters

  
  

• Gravel base (US):(length_ft × width_ft × base_depth_in / 12) / 27 = cubic yards

  
  

• Joint spacing (rule):panel_size_max ≈ 24–36 × slab_thickness (same units)

  
  

• Add 5–10% to both concrete and gravel orders for waste/irregularities.

Troubleshooting after the pour

• Random hairline cracks: Normal. If joints are cut, cracks usually follow them.

  
  

• Low spots/ponding: Feather with a polymer-modified resurfacer after 28 days, or live with a utility finish for sheds.

  
  

• Spalling at edges: Often freeze–thaw + salts. Keep water draining off; seal if it’s a patio/drive.

  
  

• Surface dusty/chalky: Could be over-worked bleed water. Use densifier or resurfacer depending on severity.

FAQ

Can I pour alone?

You can prep alone. For the pour, have at least one helper. Concrete waits for no one.

Rebar or mesh?

Either, but supported. Rebar is easier to place precisely; mesh often ends up at the bottom unless you’re vigilant.

Do I always need gravel?

In freeze–thaw or poorly draining soils, yes. In arid zones on undisturbed, well-drained soils, some skip it—but drainage and capillary break are cheap insurance.

How much slope do I need?

For exterior slabs, 1–2% (⅛–¼ in per foot / 10–20 mm per meter) sheds water. Bart used a subtler fall to the yard.

When do I cut joints?

As soon as you can walk on it without raveling edges—often 6–24 hours in warm weather, up to 48 hours in cool weather.

Can I polish it?

Utility slabs don’t need polish. If you want a decorative finish, plan it—power trowel timing is tight, and later grinding/polishing requires different tools.

Finish checklist

• Layout square with 3-4-5

• Subgrade compacted (no deep footprints)

• 4 in (100 mm) clean gravel base, compacted

• Forms straight, level, correct fall

• Rebar/mesh tied and on chairs

• Concrete + tools scheduled; helper confirmed

• Screed strips planned (6–8 ft / 1.8–2.5 m)

• Bull float ready; edges/groovers clean

• Cure plan ready (water/plastic/compound)

• Joints saw-cut to ¼ depth at proper spacing