How to Install Tile Underlayment
Ceramic tile is more brittle than it seems. If the material underneath the tile is too soft, its movement under the tiles will cause them to crack eventually.
On what can you install tiles?
- NEVER: particle board, veneered composite panels, luan plywood, soft wood plywood
- With specific extra materials, adhesives, and methods (not recommended for beginners): hard wood, hard plywood, vinyl flooring, metal, painted surfaces, acrylic surfaces, fiberglass surfaces
- Recommended: cement board ("cementuous backer units"), drywall/greenboard (in dry areas only), tile underlayment isolation membrane, extremely flat cement (you may need to flatten it with self-leveler), or other tile (as long as it is level, in good condition, and with a roughened surface)
As mentioned, while tile can be installed on drywall or greenboard, this should only be done for dry tile installations. In bathrooms or other areas that will be exposed to moisture frequently, you should use only cement board or underlayment membrane. Greenboard may have some waterproofing, but it is not enough for tile.
The nice thing about underlayment membrane is that it is easy to put anywhere, even around corners and in tight spaces. It gives you the waterproofing you need for shower and tub surrounds.
Preparation
You can install underlayment membrane on top of plywood, OSB, concrete, lightweight concrete, or gypsum.
You will need:
- Modified thinset mortar -- we recommend Tec mortars such as 3N1 Lightweight
- A small v-notched trowel (1/4" x 3/16")
- A flat trowel
- Underlayment
- Seam strips (get the same brand as your underlayment)
- For tile installation on top of the underlayment, unmodified thinset mortar; again, we recommend Tec
How To Install
Step 1: Modified Thinset
Spread modified thinset mortar over your substrate using the flat edge of the v-notched trowel. Then use the notched edge of your trowel to "notch over" the mortar, or draw grooves in it, both left-and-right and up-and-down.
Step 2: Seam Strips
Put seam strips into the corners of the installation. Smooth them down firmly into the mortar using the flat trowel.
Step 3: Press Underlayment
Put the underlayment on top of the mortar. Smooth the underlayment down firmly into the mortar using the flat trowel. Lift the underlayment and look at its underside to be sure that all of it has been coated with mortar, then smooth it again. Make sure to squish out any bubbles.
Step 4: Add More Underlayment
Consult the underlayment instructions as to whether pieces of underlayment should overlap each other or be butted up next to each other. If you need the installation to be waterproof, add more thinset on the seam using the v-notched trowel as in Step 1, and smooth a seam strip over the seam using the flat trowel. Smooth each piece and the seam strip as in Step 3.
Step 5: Install Tile
Spread the unmodified thinset mortar over the top, completely filling in the cavities or mesh of the underlayment, as in Step 1.
