You have the pour scheduled, the crew booked and a clear forecast for three days. Nothing about that week says rain.

Then the sealer goes down on a surface that looked dry, and a fortnight later it is blushed, patchy and lifting at the edges.

Clear does not mean dry. A slab can go damp under a cloudless sky, and clear skies are in fact the condition that produces it.

Concrete is unusually good at getting cold

Two properties combine badly here.

It radiates. A slab is a large horizontal surface with an unobstructed view of the sky, which makes it very efficient at losing heat by radiation on a clear night. Like a car roof, it ends up several degrees below the air temperature.

It lags. Concrete has a large thermal mass, so it warms and cools far more slowly than the air. Come morning, the air temperature climbs quickly and the slab does not — which means the gap between them widens just as the sun comes up, and the slab can be collecting water at 8am on a day that feels perfectly pleasant.

That lag is why "wait until mid-morning" is often not enough.

The test that actually settles it

Tape a square of clear plastic sheet flat to the slab, sealed on all four edges, and leave it for several hours or overnight.

  • Condensation on the underside — moisture is still moving through the slab, or condensing on it. Not ready.
  • A darkened patch of concrete underneath — same conclusion.
  • Dry and unchanged — you are probably clear.

It is crude, it costs nothing, and it answers the question the forecast cannot.

Reading the forecast properly

The figure to watch is the spread between the air temperature and the dew point overnight:

  • Under 5°F — the slab will almost certainly be wet by morning.
  • 5–9°F — likely on a clear, still night.
  • Over 15°F — the slab would have to lose an implausible amount of heat.

The condensation calculator will tell you the surface temperature at which it starts, and the hourly forecast shows when the gap closes.

The summer version

The same problem arrives from the opposite direction in warm weather.

A slab in shade, in a basement, or under a building stays cool while the outdoor air turns humid. Warm air at a 70°F dew point meeting concrete at 65°F produces condensation on a hot day — which is the same effect as water on the outside of an air-conditioned building's windows.

Opening the doors to "air it out" makes this actively worse, because it brings more humid air onto a cold surface. The instinct is exactly backwards.

The practical sequence

  1. Check the overnight dew point before committing to a date.
  2. Pour or place early enough in the day that the surface can take a real set before evening.
  3. Before sealing, run the plastic-sheet test — not the eye test.
  4. On a humid day, keep a cool slab shut rather than ventilated.
  5. If the spread is under about 5°F for the next two nights, wait.