How to Properly Mix Thin Set Mortar for Great Results

by Jim Bessey

Mixing thin set mortar isn’t fun. It’s so much easier to buy pre-mixed products like mastic. You can even get “thin set” in a sealed bucket–for at least twice the price. However, there’s no substitute for the real thing.  If you’re going to install large-format porcelain or ceramic tiles, you should mix your own thin set. When you do it right, your tile project will last for decades.

image of typical setup for thinset mixing

a simple setup using buckets for mixing thin set mortar

Why is mixing mortar so important?

Thin set mortar is a cement product, similar to concrete. 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, once cured, is nearly indestructible. Mixing this mortar properly spells the difference between a great job and disaster.

Which mortar should you use?

For typical application on cement-based underlayment like HardiBacker® or DUROCK™, you can safely use any basic thinset 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™ . All mortars should be latex-modified, either right from the bag or during mixing, for best longevity.

Use gray mortar for darker tiles, and white thin set for lighter shades. Always check the bag and the supplier’s website for specific information before you begin laying tile.

Freshly-mixed thin set has a work window of between two and five hours, depending on batch size and other variables. Before you mix anything, use this checklist to be sure you’re ready to start:

  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 to physically “try it.”
  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. Read on...

The proof of the thin set is in the mixing.

Too thin, and your mortar will be sloppy and dry poorly. Mix too thick, and you’ll have trouble from start to finish. Here’s how to get your mortar mix just right

Tools to gather before you begin:

  • Safety glasses and dust mask
  • Wide bucket for mixing thin set mortar (an empty joint compound bucket is fine)
  • Similar bucket for clean-up of tools and mixing paddle
  • Smaller bucket for clean water (or use an empty plastic soda bottle)
  • Mixing paddle–check the paint department for less-expensive versions.
  • Electric or at least 14-volt battery drill to run the paddle
  • 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).
  • Knee-pads, piece of foam board, carpet sample, and/or an old towel–whatever you prefer.

A 50-pound bag of dry mortar mix is more than enough for a typical bathroom floor (40 to 100 square feet). Never mix an entire bag at once! Always leave some dry mortar in the bag, in case your mix gets too wet.

CAUTION:  Dry mortar mix is a skin and eye irritant, so avoid mixing in close quarters. In fact, whenever you can, mix mortar outdoors. Always avoid breathing the cloud of dust that arises when you first pour mortar into your mixing bucket. Protect your driveway, garage floor, or other finished surfaces from both dry and wet mortar–it stains!

Arrange your buckets: (see photo, above) One empty for mixing, one half-full of water for cleaning, plus the small bucket with clean water for the mortar. To help avoid back-strain, set your mixing bucket a foot or so off the floor. You could do this, for example, by putting it atop an unopened bucket of joint compound. Figure on mixing no more than a half-bucket of mortar at a time.

Pour a pint or so of clean water into your mixing bucket, just to get it wet. Now carefully pour in a few pounds of dry mix (wear a dust mask). Add another splash of water on top. Use your drill and mixing paddle, set on “slow speed,” to work the water into the dry mortar. TIP: Work from “too dry” to “just right”–it’s much easier to add a bit more water than it is to keep pouring in more dry mortar.

How will you know when it’s “just right”?

image of mixed thin set in bucket

notice the texture of properly mixed mortar

Mixed properly, your thin set mortar will look and feel like a good thick pudding. Not soup. Not Play-Doh™ . It shouldn’t pour, and it should hold the shape of the ‘waves’ stirred up by your paddle. If, instead, it feels like chunky peanut butter, add a little more water. Remember that you can always stir in more water later, even while you’re working. Trying to make the mix “more dry,” however, isn’t nearly as easy.

While you’re working with thin set, keep your buckets and tools clean. Spin your mixing paddle in wash water. Scrape-down the sides of your mortar bucket using the notched trowel’s smooth side. Use the plastic knife to keep the trowel clean. Pick up splashes or blobs right away. If the mortar dries on tools or tiles, it becomes very hard to remove later on.

Once you’re happy with the consistency of your mix, get right to work. All cement products begin to cure as soon as water’s added, so time is of the essence. If at any time while you’re working you feel the mix is no longer quite right, fix it or throw it away. Don’t let a bad batch of mortar ruin a beautiful tile job.

Helpful resources:

  1. Choosing the right thinset  http://www.floorstransformed.com/choosethinset.html
  2. DUROCK®™   website:  http://www.usg.com/durock-cement-board.html
  3. Hardibacker® http://www.jameshardie.com/homeowner/products_backerboard_halfInch.shtml
  4. TEC™  thinset    http://www.nstile.com/tec_mortar.html
  5. Schluter®  DITRA   http://www.schluter.com/6_1_ditra.aspx