Preparing for Plaster Walls: Showers

Below is the prep guide that I give to most clients when they ask about how to prepare a shower for plaster. The biggest problem I've come accross is people preparing the shower as if it will be tiled. While the two finished preparred showers look similar, there are many differences. The biggest difference between tile prep and plaster prep for showers is that every square millimeter of wall space for plaster must be solid and backed. while tile prep can leave soft spots in taped seams, corners can be gapped, and cement board can float off the floor 1/4". Because tile is a rigid bodied finish material it can easily span gaps and hid blemishes. Tadelakt and shower plaster is typically 3/16" to 1/4" thick and wet applied, it won't span gaps, it won't perform with out solid backing. See below, and e-mail me if you have questions or need further guidance. Like many things in life, there are may right ways to prepare a shower for plaster; there are even more wrong ways.

Concave Tadelakt shower wall.

How to Prepare a Shower for Tadelakt and Plaster Showers

by Lime Wood Clay

Plaster prep is not tile prep. All surfaces must be solid with zero gaps. Following the steps below will help prevent cracking along seams and joints of your underlying substrate. (There are many ways to successfully prepare a wet area for plaster and I’m happy to discuss your specific project with you.)

Cement Board

1.  Studs spacing no larger than 16” O.C.

2.  1/2 inch cement board (Hardie, etc.) (Durock and Permbase not recommended)

3.  2x6 blocking framed in at all cement board seams.

4.  Cement board joints must be fiberglass taped embedded in thin set.

5.  Fill screw holes with thin set

6.  Water proof using Hydroban, Redguard, or Aqua Defense

Kerdi Board

1. Studs spacing no larger than 16” O.C.

2. 1/2 inch Kerdi board

3. 2x6 blocking framed in at all Kerdi board seams.

4. Finish out seams and screws with Kerdi band as directed by Schluter.

**UNLIKE TILE PREP

Plaster must have a solid backing on which to adhere. Any gaps at seams or edges must be filled with thin-set and fiberglass tape prior to waterproofing. Corners require bead or must be built up and re-enforced with mesh and completely solid. If plaster is meeting a tile shower floor, cement board must run to within 1/8” of tile floor.

**Tub interface - If you are going to Tadelakt around a tub, the upper lip on the tub basin is designed to keep water out of the wall and to allow tile to hang over it. Since plaster cannot “hang down” like a tile to cover the tub flange, install your backer board to within an 1/8 of an inch of the tub flange. Then use fiberglass tape to straddle the backer board and the tub flange. Run the tape to precisely where you want the plaster to stop. Use a modified thin set to embed the tape and fill any voids.