How to Lay Ceramic Tile Using Thin Set Mortar

by Jim Bessey


If you’re ready to take the new bathroom floor installation plunge, chances are you’re considering using ceramic or porcelain tile. If you’re following current floor design trends, then you’re likely using one of the larger floor tile sizes. These big tiles are best installed using latex-modified thin set mortar, rather than a mastic-type adhesive.

Tile floor in progress, using spacers

This floor uses 12" square tiles and 3/16" spacers

Why bother with all that mortar mixing and mess?

It’s certainly easier to work with mastic. You just open the bucket and get to work. For bathroom floors, however, you should avoid using organic adhesives.   It’s true you’ll need more tools, more buckets, and more patience to work with thin set mortar. The difference in leveling, adhesion, and durability of your new tile floor is well worth the mess.

Thin set mortar is a cement product. Applied correctly, it’s incredibly strong and highly resistant to water damage over time. The bond between cement-based sub-flooring and ceramic or porcelain tile is nearly indestructible. Done right, your floor will stay dry, your tiles will stay put, and your grout won’t crack. A high-quality tile floor should last at least 30 years.

Which mortar should you use?

For typical application on cement-based underlayment like HardiBacker® or DUROCK™, you can safely use any basic polymer/latex-modified thin set mortar labelled for floors. For non-standard installation (over wood, old tile, or vinyl, for instance) choose a high-bond product like TEC Super Flex™.

Thin set mortars are available in gray or white. Use gray for dark tiles and white for lighter-toned tile. All thin set products carry clear labelling showing acceptable uses.

Whenever you’re working with products that dry quickly you need to have your ducks lined up before you mix any mortar.  Use these steps as a guideline:

1.    Prepare the room itself–“empty” is best.
2.    Remove or prepare the existing floor–“solid sub-floor” is ideal.
3.    Install cement board or other acceptable underlayment (called “substrate”).
4.    Determine and mark your layout. Dry-fit your chosen tile.
5.    Be sure you have enough tile to complete the job; mix tile from several boxes.
6.    Purchase or rent a good tile-cutting tool. Check stores in your area for options.
7.    Gather other needed tools and supplies. (More on this shortly)
8.    Pre-cut edge tiles, if possible (it usually is).
9.    Mix thin set.

If it seems like this job is mostly about preparation, you’re right. When you’re working with a clean room, a good layout, tiles at hand, the right tools, and a smooth mix–then the final laying of tile is the easy part.

Tools to gather before you continue:
●    Knee-pads, piece of foam board, carpet sample, and/or an old towel–whatever you prefer.
●    Notched trowel. See mortar bag for guidelines, based on tile size. Buy a good trowel!
●    4″ plastic “putty knife,” also sold as an economy drywall knife ($2, paint department).
●    Rubber mallet, for vibrating the tiles into place.
●    Spacers: usually 3/16″ or ¼” for floor tile.
●    Tile nippers, for cutting irregular shapes–around the toilet shoe, for instance.

TIP: While you might begin the ceramic tile layout from the room’s center, you probably won’t want to begin laying tile from there. You need one good, long straight line to work with, often one and a half (or so) tiles away from the longest wall (or from the tub or shower). Start in a section farthest from the doorway, so you can “back out” of the room as you go. It’s important that you don’t walk on freshly-laid tile.

For “square” layouts (a grid), you can screw a straight board to the floor. A scrap piece of baseboard works well for this. Double-check your lines for parallel and square–to the room, and to each other. Now start with an area about two feet square, or slightly larger. Work between your straight-edge (or line), away from yourself, toward the wall or tub.

Now you can begin laying tile.

First, use your notched trowel or plastic knife to put some thin set mortar into your working section. Then, holding the smooth edge of the trowel at an angle to the floor, “butter” the floor with a thin layer of mortar. Next, reverse the trowel and use the notched side to “comb” the mortar. This is important.

The notches create a perfectly uniform bed of adhesive beneath the tiles–never too much or too little. Push any excess mortar into the section where you’ll be working next. At this point, if your mortar won’t hold the shape of the notches, it’s too thin. If it won’t ‘comb’, it’s too thick.

Choose a tile, check it for defects, and lay it snug to your edge/line. Settle it with firm finger-pressure, then use the rubber mallet to gently tap the tile into the mortar. It’s vibration, not pounding, that properly seats the tile. Now lay another tile beside the first, settle it until the two tiles form a flush surface, then softly slide that tile away from the first. Insert two spacers to set the gap. Add two more cut or full tiles in the same manner to form a “set”–two tiles by two tiles. This forms a stable group of tiles which square themselves when properly spaced.

Continue laying more tile this way in groups of four, always checking by feel to ensure a smooth, flat tile surface. As you go along:

●    If one tile seats too low, pull it up immediately and re-butter it.
●    Use your plastic knife to keep both trowel and tiles clean.
●    Use a folded piece of cardboard (tile wrappers work fine) to remove any excess mortar from the grout joints.
●    Watch for tile defects! Be aware of repetitive surface patterns in your tiles–mix ’em up!
●    Try to make and use your largest cuts before smaller cuts. In case of error, you’ll have a good chance of re-using a bigger cut later on.
●    Double-check spacing and alignment by standing up and looking at the whole floor.
●    Finish into a threshold or Schluter™-type edging at doorway(s).
●    Mark your new floor OFF LIMITS for about 24 hours.

What’s left? Let your new tile floor cure for at least a day. Once you can safely walk on it, you’re ready for baseboard, grout, paint or paper, and fixtures. We’ll talk about all of these topics in upcoming articles.