There are some aspects of tile installation that seem small or insignificant but they aren’t. They really have a big impact.
The way you treat your backer board seams is one of them. Doing them improperly can lead to big problems down the road. But these problems can be avoided by this one simple step.
Estimated reading time: 4 minutes
Alkali-Resistant Mesh Tape
For tile backer boards such as Durock, Wonderboard, Hardiebacker, Permabase, and others the seams between the board need to be treated with 2-inch alkali-resistant mesh tape.
Then a coat of thinset gets flat troweled over the seam.
Additionally, if you are tiling over drywall it’s perfectly OK to tape the drywall seams this way instead of using joint compound.
What this does is tie the two boards together and help prevent flexing. It helps prevent one board from flexing in a different way than the other board. It helps to make the walls that are to be tile more monolithic.
What happens if you don’t use mesh tape?
Sometimes nothing happens.
But I’ve walked into bathrooms where all the walls have cracks in them that follow the backer board joints.
Sometimes tiles will pop loose where there’s movement. Sometimes it allows an unimpeded way for water to get behind the board and into the wall.
It doesn’t cost very much extra to carry out this step. It doesn’t take very much more time. But it’s a necessary part of a successful tile installation.
What exactly needs to be taped and mudded?
Basically, a good rule of thumb is to tape anywhere two panels come together. In the photo above you can see that I’ve taped between the panels but also in the corners.
You can sort of fold and squeeze the tape into the corner so that there is still one inch on each side. Make sure the tape is tight into the corner. Otherwise, that can cause problems later.
You’ll want to tape where the cement board backer board meets the sheetrock wallboard. You can see in the photo that I even installed some around the plumbing fixtures where the cut left a gap.
For the thinset step, you should look up the instructions for whichever backer board that you are using. Typically, almost any modified mortar will work. To install the thinset you can use the backside (flat side) of a notched trowel for tile.
One downside to this is that sometimes the notches will catch the tape in the corners so you’ll just have to be careful. Personally, I’ve found that a 4-6 inch flat putty knife works great for working the thinset into the mesh taped seams.
If you are going to be applying a liquid waterproofing membrane over the wall then you’ll put some thinset over the screw heads also and flat trowel it flat with the wall. You don’t need to use mesh tape over the screw heads though.
Outside Corners
Outside corners can be difficult with 2-inch mesh tape. It’s hard to get the tape to fold over the corner and stay long enough for the thinset to set up.
They make 4-inch alkali-resistant mesh tape and this stuff works great. If you have outside corners to do then you may decide to just get the 4-inch mesh tape only and use it anywhere you would use the 2-inch.
If you can’t get your hands on the 4-inch stuff I’ve had success using two layers of 2-inch mesh tape for an outside corner.
I’ve done this by overlapping them maybe 3/4 of an inch so the tape ends up being about 3 1/4 inch wide total. It’s not as easy as one layer of 4-inch but it works if it’s your only option.
I felt that this was a simple thing but it’s important enough to warrant its own post. By skipping this step one could really mess up an otherwise good installation. I’ll have more of these types of posts down the road.
Cement Board installed on Floors
For floor applications where cement board is installed over a wood subfloor with a crawl space underneath please refer to this post.
Sealing a bathtub to the backer board
For information on how to treat the joint between a bathtub or shower and backer board please see this post.
