How to Moisture Seal a Concrete Slab

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All concrete slabs are porous. The breathable surface allows moisture and air to penetrate and exit the slab. Sealing the concrete can only be done after the slab has fully cured. Depending on your particular environmental conditions, that can take anywhere from six months on up to a full year. Once the cement is fully cured, it can then be sealed. Various sealants will have different application methods. The best sealant to place on a concrete slab is a non-breathable, penetrating sealant. This type of sealant will work down into the concrete and create a permanent moisture barrier.

Things You'll Need

  • Five-gallon bucket
  • Detergent for concrete surfaces
  • Long-handled scrub brush
  • Garden hose w/sprayer
  • Dehumidifier (optional)
  • Box fan (optional)
  • Penetrating sealant
  • Roller applicator
  • Paintbrush
  • Fill the five-gallon bucket with warm water. Mix in the concrete detergent according to the label instructions. Wash the concrete with the long-handled scrub brush. Remove all dirt, grease and surface debris. Thoroughly rinse the surface with a garden hose and a sprayer attachment. Remove all soap and dirt residue.

  • Allow the slab to completely dry. This may take a couple of days depending upon conditions. Use a dehumidifier for enclosed spaces and rooms. Place a box fan to the edge of the slab. Position the fan so it can blow across the surface to speed up the drying process. You will have to reposition the fan several times to dry the entire surface.

  • Apply the penetrating sealant to the dry, clean concrete surface. Observe the manufacturer's recommended techniques. Pay special attention to any weather and temperature requirements. Typically most applications should occur on low-humidity days with temperatures above 68 degrees F.

  • Use a roller applicator to cover large surfaces. Push the sealant into small corners using the paintbrush. Follow the manufacturer's instructions when applying a second or third coat. In most cases, additional coats will be applied before the subsequent coat has a chance to fully dry.

Tips & Warnings

  • If a wood floor covering is being installed over the concrete, a thick rubber membrane maybe glued to the concrete surface. Consult the wood flooring manufacturer as to the type of membrane they recommend.
  • Certain tile adhesives will not bond to unsealed concrete floors. In some cases only manufacturer specific sealants can be applied for use in particular adhesives. Consult adhesive instructions for any special applications.

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