· by Michael Blair, Nashville's Garage Floor
Water on a garage floor has three usual explanations, and only one of them is the slab. Working out which changes the fix entirely.
Coming in under the door
The most common by far. A worn bottom seal, an uneven threshold or a driveway sloping toward the garage all let driving rain and runoff in.
A threshold seal bonded to the floor gives a positive stop and is inexpensive. Regrading the apron is the more thorough fix.
Condensation
In spring, warm humid Middle Tennessee air meets a slab still cold from winter, and moisture condenses across the whole floor. It looks like a leak and appears everywhere at once.
Ventilation and time solve it. Closing the garage up tight during a warm humid spell makes it worse.
- Check the bottom seal and threshold first
- Look at whether the apron slopes toward the door
- Condensation appears uniformly, leaks appear locally
- Water at the back wall suggests groundwater or grading
- Vehicles bring in a surprising amount of water and snowmelt
Groundwater and the back wall
Water appearing along the back or side wall, particularly after prolonged rain, suggests grading or groundwater rather than the door.
The fix is outside: gutters, downspout routing and grading away from that wall, exactly as for a basement.
Why it matters for a coating
A floor that gets wet regularly is a poor candidate for a coating, both for adhesion and because water under a car sits on it.
Fixing the water first is not a delay to the project — it is part of it.