# How do you lay a concrete slab?

Excavate and compact the subgrade, set forms to the finished elevation with a slope for drainage, add a vapor retarder for interior slabs, then place, screed, float, and edge the concrete. Cut control joints within the first day, and cure the slab for 3 to 7 days afterward.

## Short version

- Everything except the concrete has to be finished before the truck arrives.
- Compact the subgrade uniformly and slope exterior slabs so water runs off.
- Order 3 to 5 inch slump for flatwork and never add water on site to make it easier.
- Wait for bleed water to disappear before floating or troweling.
- Joints every 8 to 12 feet on a 4-inch slab, cut about 25% of the depth.

## What goes under a concrete slab?

Under a slab you need a uniformly compacted subgrade, and for interior work, a vapor retarder. NRMCA's flatwork guidance is blunt about sequencing: finish all excavation, compaction, formwork, and placement of mesh, reinforcement, and embedments before the concrete is delivered.

Compact the subgrade uniformly and slope it as required for drainage. Avoid wet spots, and do not place concrete on a cold or frozen subgrade. Uniformity matters more than raw density — a soft pocket next to an over-compacted area creates the differential support that cracks slabs.

Interior slabs that will receive flooring need a vapor retarder. Vapor retarders are specified under ASTM E1745, which requires permeance no greater than 0.1 US perms, and a minimum thickness of 10 mil (0.25 mm) is recommended for vapor performance and for surviving installation. Seal the laps and penetrations; a torn retarder is a conduit, not a barrier.

Skip the granular blotter layer directly under an interior slab that will get moisture-sensitive flooring — NRMCA advises against it, because that layer takes on water and feeds it to the slab.

*General information, not engineering advice. Structural work should be designed by a licensed engineer.*

## How do you set the forms and grade?

Forms establish the finished elevation, the slab thickness, and the drainage, so set them before anything else moves. Use straight lumber or metal forms staked at close enough intervals that the pressure of fresh concrete cannot bow them outward — a form that deflects costs concrete and leaves a wavy edge.

Set the top of the form at the finished surface elevation. For exterior slabs, build in a slope so water sheds away from buildings; flat exterior slabs pond, and ponded water is what saturates concrete before a freeze.

Check the layout by measuring both diagonals of the rectangle. Equal diagonals means square, and square panels crack less than skewed ones.

Confirm the excavation depth across the whole footprint, not just at the edges. Low spots absorb concrete volume you did not order, and NRMCA notes that even a 1/8-inch difference in a 4-inch slab is about 3% of the volume.

Set isolation joint material now, wherever the slab meets a foundation wall, a garage slab, steps, columns, or light poles.

## How do you place and screed the concrete?

Place concrete as close to its final position as possible and work from the far end back. Deposit it directly from the chute where you can, or use wheelbarrows, buggies, or a pump; then spread it with a short-handled square-ended shovel or a come-along, always placing new concrete against concrete already in place.

Do not use a rake, and do not move concrete long distances across the slab — both segregate the mix and pull the coarse aggregate away from the paste.

Order the right slump instead of fixing consistency on site. Most flatwork calls for 3 to 5 inches, and on sloped work a lower slump and placing up the slope is the correct approach. Adding water beyond the design mixing water delays setting, reduces strength, hurts durability, and increases cracking.

Consolidate as you go, then strike off the surface with a screed board riding on the forms, moving it with a sawing motion to fill low spots. Follow immediately with a bull float to close the surface and embed coarse aggregate.

Everything after the bull float waits.

## When do you float, edge, and finish?

Wait until the bleed water has left the surface before doing any further finishing — this is the single most consequential timing decision on the slab. Finishing while bleed water is present, or before bleeding has finished, works that water back into the top quarter inch and creates a very high water-cement ratio there.

The result is a weak, low-strength wearing surface that dusts under traffic and can be scratched with a nail. Sprinkling water on the surface to ease finishing or broadcasting dry cement to speed it up cause the same damage.

Once the sheen is gone and the slab supports a footprint of about a quarter inch, edge the perimeter, then float. Exterior slabs usually finish with a broom texture, which gives traction on a surface that must not be slippery when wet.

Steel troweling belongs on interior floors. Troweling exterior air-entrained concrete too early or too aggressively causes delamination and scaling, and interior slabs troweled while moisture is condensing on cold concrete dust badly.

Avoid excessive finishing in general. More passes do not make a better slab.

## When do you cut the control joints?

Cut control joints as soon as the slab will support the saw without the edges raveling, and always within the first day. Joints are pre-planned cracks: concrete shrinks, restraint creates tensile stress that exceeds its tensile strength, and it cracks. The joint decides where.

Timing is practice, not a published rule — early-entry saws cut within hours of finishing, conventional wet saws generally the same day or by the following morning. Cut too soon and the edges ravel; cut too late and the slab has already cracked on its own.

| Slab thickness | Maximum joint spacing | Joint depth |
|---|---|---|
| 4 in (10 cm) | 8–12 ft | about 1 in |
| 5 in (13 cm) | 10–15 ft | about 1¼ in |
| 6 in (15 cm) | 12–18 ft | about 1½ in |

Keep panels close to square. Long, narrow panels crack across the middle regardless of joint spacing.

Isolation joints are separate from control joints and go in before the pour, wherever the slab abuts another element that will move independently.

## How do you cure the slab?

Start curing immediately after finishing and keep it going for 3 to 7 days. Curing means keeping the concrete moist and above 50 °F (10 °C) so hydration continues — it is what actually builds the strength the mix was designed to deliver.

Use water methods where you can: fogging, sprinkling, soaker hoses, wet burlap, or absorbent coverings kept continuously wet. Plastic sheeting and curing paper retain the concrete's own moisture; a membrane-forming curing compound is the practical option on large pours or where re-wetting is not realistic.

Watch the first 24 hours especially. Before and during finishing, minimize moisture loss with fogging or an evaporation retarder to prevent plastic shrinkage cracking.

Do not let coverings dry out and re-wet repeatedly — intermittent wetting can mark the surface, and it cures worse than a consistent method.

## Common mistakes

- **Placing on a subgrade that was never uniformly compacted** — differential support cracks a slab no matter how thick it is.
- **Adding water to the truck to make placing easier** — it delays set, cuts strength, and raises the odds of cracking.
- **Finishing while bleed water is still on the surface** — that water gets worked in and leaves a weak, dusting wearing course.
- **Waiting too long to cut control joints** — the slab cracks on its own schedule, and joints cut afterward do nothing.
- **Stopping the cure after a day** — the strength never developed in that first week does not appear later.