This is a post that describes installing large format porcelain ceramic tile for a 200-ft2 kitchen space on a concrete subfloor with some significant fractures.
If you want to make sure that your expensive new 12×12 inch or larger tile doesn’t crack, then there are several things you can do:
- Make sure the substrate (concrete slab or plywood subfloor) is sound and flex-free. If the plywood subfloor has any bounce to it or if the concrete slab has a minor crack or two, then…
- Make sure the tile installer uses a highly modified thinset that’s formulated for installing large tiles on a slightly flexy subfloor. But if the plywood subfloor has a good bit of bounce or if the concrete slab has lots of minor cracks or several large ones, then…
- Make sure the tile installer uses a crack isolation membrane to completely isolate the surface tile from shifts in the substrate base
This was an install of the third variety.
The client in this case had already done most of the hard work by removing existing cabinets and scraping off the original tile themselves. This exposed the concrete foundation slab of the early 60s vintage home, and exposed some pretty wide cracks.
The foundation had previously been leveled with a couple corners of the midsize home being raised an inch or two in the process. Although the foundation repair folks guaranteed that the slab would be stable from then on, the client (reasonably) wanted some added peace of mind to absolutely ensure that their expensive imported porcelain 12×24 inch kitchen floor tiles wouldn’t crack in the future.
A solution? The best crack prevention membrane material available. Aside from sinkholes or earthquakes, this will ensure no future cracks for tile or grout.
With the client already taking care of the demo and prep, the remaining tile install steps were very straightforward:
- Install DITRA crack prevention membrane material
- Install the tile
- Install the grout
And, done! Straightforward in theory, and in this case also straightforward in practice. You get lucky on that sometimes.
The DITRA Install
The DITRA crack prevention membrane material comes in rolls roughly 4-ft wide. It installs on concrete with basic unmodified thinset. The biggest install challenge for this is using the proper thinset thickness (a 1/8-in V-notch works well) and making sure all the excess thinset and air bubbles are all pushed out from underneath the membrane.
For pressing the membrane pieces fully into the thinset, start from the middle and push outward with a grout float. Then, once the membrane piece is in place and major air bubbles are all pushed out, just do a RiverDance dance-off. You’ll be able to tell that it’s well set in the thinset when almost all of the membrane has a nice grey tinge (assuming you’re following directions and using inexpensive unmodified thinset for the install on a concrete slab).
It’s easy in theory. In practice, the hardest part is the jumping around. Just think of it as a really rewarding cardio workout.
The Tips (for DIY)
Here are some additional do-it-yourself tips for installing DITRA crack prevention membranes:
- Precut and lay out all the DITRA pieces beforehand. This will give all the rolled-up membrane bits time to unwind and flatten out, and make things a lot faster once you’re working with the thinset.
- FOLLOW THE INSTRUCTIONS for what kind of thinset to use. For example, if installing DITRA on concrete then use plain unmodified thinset. There’s a lot of chemistry explanation for why this is important, but here’s the quick takeaway — USE THE THINSET TYPE THAT THE DIRECTIONS SPECIFY! Seriously, don’t overthink this. Just follow the directions.
- Use plain unmodified thinset for installing the tile on top of the DITRA membrane. Don’t worry about general rules of thumb for using highly modified thinset for large format tile installs. Just use plain jane cheap unmodified JUST LIKE THE DIRECTIONS SPECIFY!
Seriously, do not overthink the installation directions. For example, using modified thinset for a large format tile install on DITRA would be a big mistake. Here’s the quick chemistry explanation for that. Modified thinset has polymer additives that need to dry through evaporation before it can properly cure, while unmodified thinset ‘dries’ and cures through a hydrous chemical reaction where the water in the cement mix is incorporated into the material.
The bottom line? If you spend extra money to use modified thinset for installing tile on top of a DITRA membrane, then you will need to wait a week (or three) for the thinset to dry and cure underneath large format tile. If you follow directions and use cheap unmodified thinset, then you’ll be grouting the next day (and still be getting all the crack prevention goodness of the DITRA).
The Tile Install
The tile went in very easily with the DITRA membrane providing a nice level installation surface. Often, installing rectangular large-format expensive hand-fired porcelain ceramic tiles can be quite a challenge. This is because porcelain tiles are often not actually completely flat. In industrial production, the kiln firing process creates stress in the tile material, which then causes the tiles to bow upward or downward somewhat while cooling.
In practice, this means that some of the 12×24 inch tiles will be slightly concave (bowed up at the short ends by up to 1/16-in) or slightly convex (bowed down at the ends). This makes installing the tiles flat to one another kinda difficult. But, having a nice flat surface to start with makes it possible to at least get the tiles as flat to each other as can be.
As you can see, the supervisor on this particular project definitely approved the results.
More Tips (for DIY)
If you’re planning on installing tile yourself over a DITRA membrane, then here are some things to keep in mind.
First, buy double the amount of thinset you think you’ll need. Remember that you’re going to need to fill up all those waffle holes in the membrane before even getting to troweling on the normal ~1/4 to 1/2-in of thinset that’ll go under the tiles themselves. DITRA membranes are thirsty. But, since you’ll be using inexpensive $6/bag unmodified thinset for the job, it won’t add much to the install price.
Second, be certain to burn enough thinset into the membrane to completely fill up all the square waffle holes! This’ll slow down your tile install speed but is very critical to ensuring that the membrane can do it’s job of preventing the tile from cracking. A trick for this is to mix the thinset wetter than usual so it will flow more easily. This will also help keep the thinset from sticking to your trowel and pulling out of the waffle holes.
And third, if time isn’t a factor then think about a two step process for your install. First, go across the entire surface with thinset to fill up all the DITRA membrane waffle holes with thinset (scraping it all flat as you go). Wait overnight for that to fully dry and set, and then install your tile the next day.
Grouting and Finishing!
The client chose a light grey platinum-colored grout that really nicely accentuated the dark slate color and slight texture of the tiles. Since it’s a kitchen, it got sanded grout despite the skinny 1/8-in grout line width.
You always want to use sanded grout for tiles in high-traffic (and likely spill) areas like kitchen, hallway, and bathroom floors. If a contractor tries to sell you on unsanded grout for a floor install, give ’em a side-eye and ask ’em why.
There wasn’t much more to the finish for this project, since the client handled installing the new cabinets and baseboard trim bits. They just didn’t want to hassle with the tile flooring install, which was a good choice for a straightforward task to sub out.
A do-it-yourself install of crack prevention membrane and large format flooring tile install on ~200-ft2 with a pantry closet is definitely completely do-able, but would probably take a first-timer at least a full week. In contrast, an experienced skilled installer can make it happen in three days. The time savings can be key.
Judge for yourself — flat tiles with grout lines straight and true:
The job supervisor was definitely pleased with the finish!