Concrete slab moisture signs the other trades see first

Concrete slab moisture signs are easy to read without a single tool. Look for a white, chalky bloom under anything stored flat on the floor, or a dark ring at the base of a wall. Watch for beads of water on the slab in humid weather. A coating that came up in sheets is another one. If the slab was poured on the ground before 1990, odds are decent that no vapor barrier went under it.

If you pull wire, set fixtures, hang duct, or frame walls, you’ve walked across every one of those. This page covers the read you can do by eye. A lab test comes later, and only when the read calls for one. Knowing the signs costs nothing, and it makes you the person in the room who can say what a floor is doing.

How do you read a slab without any tools?

Six signs cover almost everything you’ll see on a working floor. You can check all six in about 2 minutes on your way to the panel. The third column matters as much as the second, because it tells you what each sign does not mean.

Two of these, the bloom and the wall ring, are the ones nobody explains to the owner. Start there.

Why does white bloom show up under stored goods?

That white bloom has a name: efflorescence. Once you know how it forms, you can read any slab. Water moving up through concrete picks up calcium and other salts on the way. When it reaches the surface and dries, the water leaves and the salt stays behind as a white or gray crust. That’s all it is.

Why it piles up under a pallet, a stack of feed sacks, or a rubber mat is the part people get backward. The stack isn’t causing the moisture. It keeps that patch from drying, so the vapor collects underneath and drops its salt in one spot instead of spreading it thin across the floor. Move a stack that has sat for 3 years, and you’re looking at a slow-motion moisture meter that ran the whole time. Bare slab 12 inches away can look clean and be doing the exact same thing.

That’s why brushing it off fixes nothing for good. A stiff broom takes it off in 5 minutes, and it comes back each time the water does. It also warns the owner about any floor they plan to put down. Salt that forms crystals where a coating meets the concrete is one way a coating gets pushed off a slab from below.

The dark ring at the base of a wall

A band of darker concrete along the bottom of a wall is water wicking into the slab. It usually runs 2 to 8 inches up from the floor or out from the wall. Concrete, block, and brick all pull water sideways and upward through their tiny pores, which is called capillary action. The slab edge is the most exposed part of the pour, so the water shows up there first. Three things cause most of it:

• Grade at or above the slab edge. Grade is the level of the dirt outside. When it’s that high, the soil holds water against the concrete every time it rains. The fix is a shovel and a slope away from the building, not a floor product.
• A downspout or roof valley dumping in one place. This is the cheapest repair on this page. An extension and a splash block often end it.
• No capillary break between the footing and the slab. That’s a layer that stops water from wicking up. Leaving it out was standard building for decades. You can’t see it from inside, and you can’t add it later without opening up the edge of the building.

Telling this apart from a plumbing leak takes patience, not equipment. A supply leak stays wet through a dry week and stays in one area. On a hot line, it will often feel warm to the back of your hand. A grade problem follows the weather. It darkens after rain, fades in a dry stretch, and runs as a line along the wall instead of a puddle at one point. Tell the owner which pattern you see, and let them decide who to call.

Why does a slab sweat in the spring?

A slab sits on the ground and holds onto the cold like a battery, so it runs weeks behind the air. In a shop building, the floor can still be in the 50s when the first warm, humid days show up. Air at 70 degrees F with high humidity hits that cold surface, cools past its dew point, and turns to water. It’s the same thing a cold glass does on a summer porch.

That’s condensation from above. It’s a different problem from vapor drive, which is water vapor pushing up from the ground below. Telling the two apart in the field can save the owner a lot of money on the wrong fix. Condensation shows up in one kind of weather. It wets the open floor, even spots that never get covered, and it dries when the air dries or the slab warms. Vapor drive shows up under things, doesn’t care much about the weather, and never fully goes away.

Plenty of floors do both at once, so anyone about to spend money should confirm it instead of guessing. The owner can check it on their own with an 18-inch plastic square, taped down on all four edges for at least 16 hours. The full method is in our guide on how to test concrete for moisture.

The one vapor barrier fact worth passing along

This one fact explains most of the table above. A slab poured right on the ground needs a plastic sheet, the vapor barrier, sitting directly against the underside of the concrete. On new work, that’s usually a 15 mil sheet rated ASTM E1745 Class A. Sometimes it goes under the gravel instead, because wet plastic is slick to walk on and gravel isn’t. Then the gravel above the plastic turns into a tank. Ground water fills the gaps between the stones, sits against the slab, and never gets a way to dry out. That’s a source of moisture for the life of the building, and no product put on top of the floor shuts it off.

Every trade in the building has watched a floor finish fail over this, and almost nobody has heard why. The right order and what goes wrong are laid out in our post on vapor barrier placement. The regional version is in our concrete vapor barrier guide, including why clay soils and shallow water tables make it worse. If you work new construction, say this out loud the day before a pour. Once the truck leaves, the chance to fix it is gone.

What do you say to the owner, and what do you leave alone?

Say what you actually saw and what it means for the money they’re about to spend. Something close to this works:

“Moisture is moving up through this slab. That white powder is minerals it left behind. It will not hurt anything sitting on the floor, but it will take out most coatings or glued-down flooring until somebody deals with the water first.”

Then stop. Some calls aren’t yours to make, and we won’t make them from a photo either: that the foundation is failing, that the slab has to come out, or that the building has a mold problem. Different people make those calls after their own look. Guessing at them is how a tradesperson loses the trust the read just earned.

How do you confirm high moisture in a concrete slab?

If the owner asks what a real answer costs, the warning signs matter more than the test. A paid test is worth it when the slab shows one of these:

• It was poured on the ground before 1990, and you can’t see a vapor barrier at the edge.
• The building sits on a floodplain or a low spot.
• A coating on this floor already came loose.

With any of those, a calcium chloride test per ASTM F1869 is worth paying for. It gives a number called MVER, the moisture vapor emission rate, in pounds. Most coating makers set their limit at 3 pounds. A slab over that line can still take a coating, but a moisture barrier primer rated for that number goes down first. The probe test, ASTM F2170, reads inside the slab at 40% of its depth. That number does a better job of predicting how the slab will act over the long run.

With none of those signs, the free taped plastic square is a good first step, and the paid test can wait.

Why the read comes back to you

Nothing on this page means you have to send anyone our way. It’s for your own jobs. The owner who gets a straight answer from the electrician remembers the electrician.

We do return the favor, and it isn’t a lead program. We keep a list of the trades we work next to, sorted by territory. Nobody buys a spot on it. The only filter is years on a job site: who shows up, who leaves the place clean, and who still picks up the phone six months later. Concrete goes in the first week of a build, and the finished floor is one of the last things touched. So our crews end up working next to nearly every other trade on the job, and we notice who’s good.

If you want the concrete side of a floor question, call and ask. There’s no charge for the answer and no strings on our end.