How to Use Thinset Mortar on RedGard Water Proofing

Updated February 21, 2017

RedGard Waterproofing and Crack Prevention Membrane is slightly different from older methods of waterproofing, or newer plasticised waterproofing membranes. With RedGard, you spray, paint, or roll the liquid on--and it works very well on walls, without stapling, nailing and caulking to seal the material to your surface like other membranes. The pink-coloured liquid begins to set once it is applied; after turning red, it is dry and ready for mortar and tile installation. Best of all, you can use it indoors or out on many surfaces.

Create a slope, if necessary, by spreading a mortar or cement surface with a trowel--especially outside or in wet locations. Slope as you spread material--about 1/4 inch per foot. Make a homemade gauge, marking a stick at 1/4-inch increments and moving every foot, spreading product to height indicated on gauge. Place a level on the hardened surface and check angle as well. Rough surface up lightly with a broom. Cure concrete at least 28 days before proceeding, according to Custom Building Products.

Wet surfaces slightly, but do not over-saturate. Force RedGard into cracks measuring less than 1/8 inch to fill. Continue spreading RedGard over all cracks with a trowel, brush or roller. Flatten and smooth RedGard as you work. Extend RedGard membrane beyond cracks a minimum of the diagonal width of the tile you will use. Fill gaps between flooring sheets and where walls meet the floors as well.

Use a trowel or paint roller to pre-coat corners and intersections where floors and walls or other objects meet, spreading RedGard heavily and extending 6 inches on either side of joints. Embed 6-inch-wide fibreglass mesh into the membrane as you work, especially over gaps, to create stronger applications. Use an airless sprayer alternately, coating surfaces with a flow rate of 1 to 1 1/2 gallons per minute at a 1,900 to 2,300 pounds-per-square-inch pressure. Make sure the tip orifice size of sprayer is 0.025 to 0.029, advises Custom Building Products. Overlap all work as you apply RedGard.

Wait for RedGard to dry--around an hour or two. Notice that the wet pink colour changes to dark red when dry. Inspect to see if any spots need more RedGard; holes or other voids can be covered by reapplying a second coat. Apply at a right angle to first coat. Periodically check the thickness with a wood gauge, measuring to see if it is at least 2 1/2 inches but no more than about 5 inches. (In showers use a minimum of 3 1/2 inches.) Consult product instructions for more information.

Mix your thin-set mortar according to coverage required and product instructions. Let stand a few minutes after mixing, then stir again. Stir occasionally while using to keep it fluffy, advises Custom Building Products. Do not add any water after first mix.

Use the flat side of a trowel to spread a skim coat of mortar on top of your RedGard surface. Spread in one direction only. Work very large areas by dividing into quarters to prevent mortar from drying before tile is installed. Do not spread more mortar than you can tile in 15 minutes (or until mortar loses its sticky texture.)

Press tile into place with a twisting motion. "Butter" the backs of some tiles, states Custom Building Products; consult product information and instructions to make sure. Adjust your tiles as you work and tap each after setting in place to help eliminate bubbles. Pull a tile back up periodically and check to ensure complete coverage.

Cure your tile and mortar for at least 24 hours. Apply grout to gaps and finish with tile and grout sealer.


Properly mixed mortar will stand up with ridges that do not slump--similar to meringue on pie. If mortar loses its stickiness and skins over, go back over it with a trowel. Mix a new batch if mortar dries out. Note that mortar will last only about four hours. RedGard bonds to metal, PVC, stainless steel and other surfaces. Apply under vinyl floors for moisture barriers as well.


Wear gloves and eye protection while working with RedGard. Wash hands thoroughly after completion. Apply for any building permits required for your installation.

Things You'll Need

  • Redgard Waterproofing Membrane
  • Tile
  • Sprayer, paint roller or brush
  • 6-inch fibreglass mesh
  • Thin-set mortar
  • Grout
  • Trowel
  • Grout sealer
  • Homemade gauge for depths
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About the Author

Karie Fay earned a Bachelor of Science in psychology with a minor in law from the University of Arkansas at Monticello. After growing up in construction and with more than 30 years in the field, she believes a girl can swing a hammer with the best of them. She enjoys "green" or innovative solutions and unusual construction.