Mold on the Garage Floor

Concrete Is Not Waterproof

Concrete is porous. Ground moisture moves up through a slab continuously as vapor, a process called moisture vapor emission. On Houston's clay soils with a high water table, that rate can be substantial.

Where a slab was poured without an intact vapor barrier beneath it — common in older construction and not unheard of in newer — that moisture arrives at the surface indefinitely.

Then Condensation Adds To It

In spring and early summer, the slab stays cool from ground contact while incoming air is warm and humid. That air hits the cool concrete and condenses, sometimes enough to make the floor visibly wet.

Homeowners often assume a leak. It is usually the floor sweating, and it happens most on the first warm humid days after a cool stretch.

Mold, Efflorescence, or Stain?

These get confused constantly and the fixes differ.

Efflorescence is a white crystalline deposit — mineral salts left behind as water evaporates out of the concrete. It is not biological. It brushes off as powder and indicates moisture moving through the slab.

Mold is typically green, black, or gray, may look fuzzy, and usually appears where something organic is in contact with the slab — under cardboard, mats, carpet, or stored items. Bare clean concrete alone is a poor food source.

What Actually Fixes It

Before You Coat the Floor

Epoxy and similar coatings fail on slabs with high moisture vapor emission — they blister and delaminate. Any competent installer tests slab moisture before coating.

If you are considering a garage floor coating and the slab is damp, resolve the moisture first or the coating will not last.