Why is my garage floor wet when it has not rained?
A garage floor that goes wet in mild damp weather is usually condensing moisture out of the air, not letting damp up from below. The two need opposite responses.
- Difficulty
- beginner
- Time
- 25 min
- Read
- 2 min
- Safety
- caution
Short answer
A cold slab plus warm humid air equals condensation on the surface, which is why it appears suddenly on mild days after cold weather and dries in a warm spell. Rising damp is more constant and worse in patches. Ventilation and stopping warm air reaching a cold slab help the first; a membrane question underlies the second.
This is one of the most misdiagnosed problems in a house, because the obvious conclusion — water is coming up through the floor — is usually wrong. A concrete slab has a great deal of thermal mass and lags behind air temperature by days. When mild moist air arrives after a cold spell, the slab is still cold, and the air deposits its moisture on it.
Safety
Step by step
- Note when it happens.Condensation appears on mild, humid days following cold weather, often within hours, and affects the whole floor fairly evenly. Rising damp is far more constant and usually worse in defined areas.
- Do the plastic sheet test.Tape a piece of clear plastic tightly to the floor on all four edges and leave it a couple of days. Moisture on the underside means it is coming from the slab. Moisture on the top means it is condensing out of the air.
- Check whether the garage is being warmed.A heater, a tumble drier, drying washing or a car brought in wet all raise the moisture and the temperature of the air while the slab stays cold. That combination is exactly what causes it.
- Ventilate rather than seal.The instinct is to close the garage up. That traps the moist air against the cold slab. Airflow — a vent, or simply leaving the door open on a dry day — removes the moisture instead.
- Keep the source of moisture out where you can.A wet car brought in and shut up in a sealed garage releases a great deal of water. So does storing damp material or drying washing there.
- Consider insulating the slab surface only if it is worth it.Raising the surface temperature stops condensation, but insulating a garage floor is a substantial job. Mats or a raised floor system in the area that matters is often the practical answer.
- Treat genuine rising damp differently.If the sheet test says the moisture is from below, the question is whether the slab has a damp proof membrane. That is a bigger decision and GuideHQ covers damp on concrete floors separately.
- Do not coat a floor that is still wetting.Any coating applied over a slab with moisture coming through it will lift. The moisture test comes before the paint decision, always.
Questions people ask
Will sealing the floor stop it?
Not if it is condensation — the water is arriving from the air above, so sealing the concrete changes nothing except making the wet surface more slippery.
Is a dehumidifier the answer?
It can help in a closed garage, but it is treating symptoms continuously. Ventilation and keeping wet things out are cheaper and usually more effective.