How to Waterproof Concrete before Laying Tile

What You'll Need
Mild Detergent
Warm Water
Lined Paint Tray
Paint Stirrer
Paint Rollers
Waterproof Sealer/Membrane
Paint Brush
Acid Roughening Solution

To waterproof concrete is a fairly simply procedure. Cement is actually a very porous material and can easily absorb water. Therefore, any concrete that may come into regular contact with water, such as under tile or for a pool, will need to be waterproofed. If cement is not waterproofed, then it can cause issues with the mortar used between the tiles. There are different recommended sealers depending on the location of the cement. Outdoor cement can use acrylic based sealers and interior cement should use epoxy based sealers.

Step 1 – Cleaning

Before applying anything, it is necessary to completely clean the cement surface. First sweep the cement to get rid of dirt, dust and other debris. Then scrub the cement with a light detergent solution. A stiff brush is best for scrubbing. Rinse this soapy cement with water and rags. Once the cement has been cleaned and rinsed allow it to dry naturally.

Step 2 – Roughen and Level

If the concrete is very smooth, then it may be necessary to roughen it up a bit using special acid based solutions. Roughening the surface is needed for the sealer to stick properly to the concrete. Also ensure that the concrete is level. Use filler to fill in any dips and apply a floor leveler for any humps in the floor. This is particularly import when laying tile, as the floor needs to be as level as possible.

Step 3 – Mixing

Before using the sealer, ensure that it is mixed thoroughly. This is particularly important if it has been sitting for a long time. Use a paint stirrer and really make sure the mixture is homogeneous. Many times sediment will have settled to the bottom of the can. Once the sealer is thoroughly mixed pour the sealer into a lined paint tray.

Step 4 – Painting

Use a paint brush to paint on the sealer near tight areas and edges. Smooth over these areas several times to avoid leaving any brush strokes. For large areas, apply the sealer using a paint roller. Allow the roller to absorb plenty of the sealer and then roll it onto the cement in thin coats. Make sure that each stroke is overlapped so no small areas are missed. The layers should be even but not so thick that the sealer pools anywhere.

Step 5 – Dry and 2nd Coat

Allow this first coat of sealer to completely dry. Depending on the weather, this can take several hours. Gently check with your finger. The sealer should not be tacky when touched. Once the 1st coat is dry apply a thin 2nd coat. Let the 2nd coat dry for a minimum of 2 days. Then it is possible to lay down tile.

Step 6 – Membrane

Instead of sealer it is also possible to lay a waterproof membrane over the cement. It is possible to apply the tile directly on top of this membrane. Many of the membranes can also be applied to freshly laid, or green, cement.  When it is applied to green cement it also acts as a curing membrane.