How to inspect a new concrete slab before you pay

Learning how to inspect a new concrete slab takes 2 tools and about 30 minutes. Run a 10-foot straightedge across the slab in several directions, then wet it with a garden hose to find the low spots. Do it before the final payment clears, because after that your word counts for a lot less. And go in knowing that about half of what you notice will be normal.

That second part matters as much as the first. An owner who wrecks a good working bond with the crew over crazing pays a real price for nothing.

What do you check in roughly the first 24 hours?

Two things, and neither one takes a specialist.

Run the straightedge. Lay a 10-foot aluminum straightedge flat on the slab and walk it across in several directions, diagonals too, because waves run whichever way the screed went. Slide your fingers under it and find the biggest gap. Write the number down in sixteenths of an inch and mark the spot with a pencil. For now you’re just recording. Flatness is a contract spec, measured as an F-number per ASTM E1155. If your bid named a number, that measurement settles it. The FF flatness score page explains what the numbers mean, and why a slab headed for polish needs a higher one than code asks for.

Run the hose. Wet the whole slab, shut the water off, and come back an hour later. Anything still holding water is a birdbath. Mark the edge of each puddle with chalk, and take a photo with the straightedge laid across it for scale. This is also the easy way to check slab level, or really slope. On an outside slab, watch whether the water runs toward the drain or toward the door. Slope is a design choice somebody made, and it either got built or it didn’t.

Then walk the edges. Formed edges should be straight and unbroken. Corners should be square and solid. Every pipe collar, plumbing stub, conduit, or anchor bolt should sit clean in the concrete. Watch for a ring of honeycomb around it, which is rough, open concrete where the mix didn’t fill in.

How do you check joints without owning a saw?

A tape measure and a stiff piece of wire answer both joint questions.

Depth. One quarter of the slab thickness is the working minimum. A 4-inch pour wants joints about 1 inch deep, and a 6-inch pour about 1.5 inches. Push a stiff wire or a folded strip of card into the saw cut until it stops. Pinch it at the surface, pull it out, and measure. A joint cut too shallow won’t make a reliable weak line, so the slab lets go of its shrinking stress somewhere you didn’t pick.

Spacing. Figure about 2.5 times the slab thickness, read in feet. A 4-inch slab wants a grid of about 10 feet, and a 6-inch slab can stretch farther. Measure center to center both ways. Check the inside corners while you’re down there. An L-shaped slab, or a notch around a post, needs a diagonal cut to relieve that corner. When it’s missing, a crack shows up later, running out from the corner at 45 degrees.

Timing. You can’t measure this after the fact, but you can ask, and the answer is worth having in writing. Joints get sawed while the concrete is still young, in a green-cut window. That window runs 4 to 36 hours in mild weather and can close in 6 to 8 hours on a hot, windy day. Our guide to green-cut timing and crack control covers the full window and the spacing rules. If the joints went in three days after the pour, they’re just for looks. That’s a bigger problem than spacing.

What is not a defect?

Give this list as much weight as the one above it. A lot of what people argue about on a new slab is normal concrete doing normal things.

Crazing. This is the fine spiderweb pattern across the top. You see it best while the slab is damp and drying. It stays in the thin top layer and doesn’t run down into the slab, so it doesn’t weaken anything. On an outdoor broom finish, you’ll barely see it after a season.

Hairline shrinkage cracks. These are under 1/16 inch, with no step up or down between the two sides. They often start at an inside corner or wander near a joint. Every slab gives up water as it cures, and every slab shrinks doing it. There are two kinds of concrete: concrete that’s cracked, and concrete that hasn’t cracked yet.

Minor color differences. You might see a darker trowel shine or a cloudy patch where the cure blanket overlapped. The last truck may leave a slightly different tone. Concrete is mixed in batches, and batches vary. The color evens out a good deal over the first year.

Chalky dust on top. It’s common on an indoor slab, and it’s a pain. Most of the time it’s a soft top layer, and it doesn’t touch the slab’s strength.

What shows up at seven days, and what waits until 28?

There are three check-ins, each with its own list. The slab keeps telling you things after the first one.

At 7 days, walk it again with the hose. The slab has been shrinking all week, and any cracks that were coming have usually started. Look hard at the joints. The concrete on each side has pulled back a little, so the joints should have opened slightly, which is their whole job. Then look for a crack running alongside a joint, a few inches off it. That pattern says the joint was cut too shallow or too late.

At 28 days, the slab has reached its design strength, and the picture stops changing much. Run the straightedge again over any spot you marked. Photograph the same cracks from the same angles, so you have a before and after. If a crack has widened or one side has dropped in that time, the movement is coming from under the slab. That calls for a structural look at the subgrade, the packed ground under the slab, before anyone worries about the finish.

Twenty-eight days won’t tell you if the slab is dry enough to coat or seal. Strength and dryness run on separate clocks. A slab can reach full strength and still hold more water than a coating can sit on. If yours is headed for a coating, a moisture reading earns its cost when something raises a flag. Maybe it went in during a wet season, or nobody can tell you what went under it as a vapor barrier. If the slab is sound, its records are clear, and it sits on a Class A barrier (the strongest grade of plastic under a slab), the usual approach for your area tends to hold without a test.

How do you raise an issue with a crew you have to work with again?

Talk about the slab. You may want this crew back for the shop apron next spring. The foreman who cheerfully fixes a birdbath in July is not the one you called a hack in June. A 10-minute walk with the foreman next to you beats days of emails back and forth.

• “I ran a straightedge and I’m getting a gap right here. What do you usually do with one like that?” This gives him room to answer without defending himself.
• “Water is still standing in this corner an hour after the hose. Will it drain once the yard settles, or is that the slab?” It’s a question, and questions get answers.
• “The joints on this end look shallower than that end. Was this section cut later?” It names what you saw and asks why.
• “Here are the photos I took, so we’re both looking at the same thing.” A photo takes no side.

All four describe the slab. Nobody has to be wrong for the issue to get fixed, and that’s the whole trick to a punch list that actually gets worked.

Walk the slab before the final payment

The walk tells you what the slab is. The payment terms decide what you can still do about it.

Retainage (the share of the price held back until the end), the final draw, and the lien waiver all run on the same timeline. Once you sign an unconditional waiver, your punch list becomes a request for a favor. That says nothing about anyone’s honesty. It’s how the paperwork works on building jobs all over the country. Do the walk before the last draw. Put any fixes you both agree on into an email you both have. And know what you’re signing before you sign it. Our guide to how lien waivers and draws work on a concrete job walks through it.

If what you find really falls outside the contract, take it up with the crew that poured it. The contract’s terms on fixes decide what happens next. Sometimes the real question is a different one: what can a finish do with this slab as it sits? That takes someone checking surface hardness, flatness, joint condition, and moisture history in person. Nobody should answer it from photos, us included.

What do owner-builders miss on the first walk?

The first miss is walking the slab dry. A dry slab hides low spots completely, and the hose is the cheapest tool on the whole job. The second is walking it alone and then sending a list. The same notes, gone over on the concrete with the foreman, get fixed far more often, because a man looking at his own work will usually tell you straight what he sees.

Everything that came before this walk decided how the slab turned out. That means the ground under it, the vapor barrier, the mix, and the cure. Our guide on why slab condition matters covers what those choices do to any finish that goes on top later.

Thirty minutes, two tools, one walk over the slab

Buy the straightedge. A 10-foot aluminum one costs less than fifty dollars. It works on every slab you’ll ever own, and it turns an opinion into a number.

Walk the slab and write down what you find. Sort it into the short pile that matters and the much bigger pile that doesn’t. Then raise it while everybody is still on site. That’s the whole method, and it works just as well on our pours as on anybody else’s.