How to Apply a Mortar Scratch Coat for Shower Wall Tiles

What You'll Need
A wheel barrow.
A full sized (9x4) square notch trowel, with either 1/4-inch notches or 3/8-inch notches.
A small (4X2) square notch trowel.
A shovel.
A full sized flat trowel.
1 roll of asphalt impregnated felt.
A contractors stapler and 1/2-inch staples
Wire mesh lath. Enough to cover the area to be scratch coated.
Washed masonry sand
Type S lime
Portland cement

You’re remodeling your shower and need to know how to apply a scratch coat to the wall in order to have a good base to apply your new tiles to. This article will present the tools needed and the process to follow.

      Step 1: Mix the Mortar

      There are 2 ways this can be accomplished. The easiest is to purchase a pre-mixed Type N masonry cement and add 2-1/4 parts washed masonry sand and mix thoroughly. Accepted practice in the masonry industry for extra strength and durability to combine:

      1. 1 part Portland cement
      2. 1 part Type S lime
      3. 2-1/4 parts washed masonry sand
      4. Water

      Mix in either a wheelbarrow or masonry mixer, though the latter makes the job much simpler. Begin by mixing the 3 solid ingredients thoroughly. Add 1 gallon of water once they dry ingredients are mixed and mix well, removing any lumps and chunks. Add more water slowly as needed. The desired consistency has been reached when the mixture doesn’t quickly slough off of a vertically held shovel blade.

        Step 2: Apply Water Barrier to Wall

        Start at the bottom and roll out the asphalt impregnated felt, stapling it to the studs as you go. Each layer going up the wall should overlap the layer below by at least 3 inches to provide the optimal water barrier. Felt should be applied to the wall horizontally. Be very generous with the number of staples you use.

        Step 3: Apply Lath

        Lath can be applied to the wall either vertically or horizontally. When trimming is required, use extreme caution as trimmed edges are very sharp. Lath should be secured to the wall with staples. Use minimum 1/2-inch staples, although 5/8-inch staples are recommended. Make sure you cut openings for valve handles and the shower spigot.

        Step 4: Scratch Coat Application

        Take the necessary precautions to protect existing floors and flooring from drips and scratches. Apply the mud from top down in strips using the flat trowel, being sure to force the mud into the gaps in the lath. Total thickness of the mortar, or mud should be approximately 1/2 to 3/4 of an inch. As the needed thickness is reached, use the notch trowel to form grooves and valleys running vertically. Again, start at the top and work down in 1 motion per column notched. Press hard, you’re still filling gaps in the lath. The bottom of the notches should be 1/8 inch to 1/4 inch above the lath for optimum bonding and strength. Allow to dry at least 6 hours before applying the tile. Overnight is best.