No, concrete isn’t waterproof. It’s water-resistant, which isn’t the same thing. Concrete is full of tiny pores and capillaries left behind as it cures, and water moves through them slowly. A thick slab can hold back water for a while, but over time moisture works its way in, especially under pressure or constant exposure.
That’s why concrete foundations, decks, balconies, parking garages, and walls in South Florida need sealing or waterproofing to keep water out.
Why Concrete Absorbs Water
Concrete is a mix of cement, water, sand, and stone. As it cures, some of the water evaporates and leaves a network of microscopic pores. Cracks from shrinkage, settling, and movement add bigger paths. Water gets in through both, pulled in by capillary action or pushed in by pressure from wet soil or standing water.
What Happens When Water Gets Into Concrete
- Rebar corrosion: Water, especially salty water near the coast, reaches the steel reinforcing inside the concrete. The steel rusts and expands, which cracks the concrete from inside. That’s the main cause of spalling.
- Leaks: Water passes through slabs and walls into the rooms, garages, and units below.
- Efflorescence and stains: Water carries salts to the surface and leaves white deposits and dark stains.
- Mold: Damp concrete walls and slabs feed mold on the materials around them.
Water-Resistant vs. Waterproof
Water-resistant means the material slows water down. Waterproof means water can’t get through at all, even under pressure. Plain concrete is water-resistant. Concrete with a proper waterproofing system is waterproof.
How to Waterproof Concrete
- Penetrating sealers: Silane and siloxane sealers soak into the surface and repel water while letting the concrete breathe. Good for walls, exposed slabs, and garages as a water repellent, though they aren’t a membrane.
- Crystalline waterproofing: Treatments that react with moisture in the concrete to grow crystals that block the pores. Used on foundations, below-grade walls, and water-holding structures.
- Membranes and coatings: Liquid-applied or sheet membranes that form a continuous waterproof layer. Used on decks, balconies, roofs, planters, and foundations.
- Crack repair and joint sealing: Waterproofing only works if cracks are injected or repaired and joints are sealed first.
Is Sealed Concrete Waterproof?
It depends on the sealer. A penetrating sealer makes concrete much more water-repellent, but it won’t stop water under pressure, like a below-grade wall against wet soil. For that, and for any slab over occupied space, you need a true waterproofing membrane or crystalline system.
How Deluxe Waterproofing Helps
Deluxe Waterproofing has waterproofed concrete on South Florida buildings for more than 80 years, from foundations and elevator pits to balconies, decks, and parking garages. When concrete is already damaged, our concrete restoration team repairs it before waterproofing goes on.
Frequently Asked Questions
Does concrete absorb water?
Yes. Concrete is porous and absorbs water through its pores and cracks.
Can water come up through a concrete slab?
Yes. Moisture from wet soil can move up through a slab, especially with South Florida’s high water table.
How long does concrete sealer last?
Penetrating sealers typically need reapplying every few years, depending on the product and exposure. Membranes and coatings follow their own maintenance schedules.
Is thicker concrete more waterproof?
Thicker concrete slows water down, but it doesn’t make concrete waterproof. Cracks and pressure still let water through.