A wet garage floor during or after rain can have several different causes, and identifying the correct one determines the most effective fix. Treating condensation with a door seal, or treating water ingress under the door with a damp-proof membrane, wastes money and effort. Diagnosis before treatment is essential.

Water Under the Garage Door

The most common cause of a wet garage floor after rain is water tracking in under the bottom of the garage door. Check by closing the door during rain and observing exactly where water enters. If water appears at the base of the door, the bottom seal has failed or the driveway slopes toward the door without a threshold seal to stop water pooling. Replace the bottom seal and add a threshold seal if the driveway directs water toward the door.

Water Through Wall Cracks

Cracks in the garage walls allow rain-driven water to penetrate, particularly on the windward side. These can be identified by wet patches appearing on the wall surface during rain. The fix is to fill external cracks with flexible exterior filler or mortar and to apply an appropriate masonry waterproofing treatment to the external wall face.

Rising Damp Through the Floor Slab

If the floor becomes wet in dry weather as well as during rain, the moisture may be rising from the ground beneath the slab rather than entering from above. This is most noticeable in older garages where the original slab was laid without a damp-proof membrane. Persistent moisture rising through the slab eventually requires either a damp-proof membrane overlay followed by a new screed, or a liquid damp-proofing treatment applied to the concrete surface.

Condensation

A wet-looking garage floor that dries completely once the door is opened is likely caused by condensation rather than water ingress. When warm humid air (from a rainstorm or a warm day following a cool night) enters a garage with a cold concrete floor, the moisture in the air condenses on the cold floor surface. This looks exactly like water ingress but has a completely different cause. Sealing the floor with a penetrating or surface sealer reduces condensation by slightly warming the floor surface and reducing the porosity that makes condensation more visible.

Summary

A wet garage floor after rain is caused by water under the door (fix: replace bottom seal and add threshold seal), water through wall cracks (fix: fill cracks and waterproof externally), rising damp through the slab (fix: damp-proof membrane), or condensation (fix: seal the floor, improve ventilation). Use the polythene test to distinguish ingress from below from ingress from above.