Dan Morrison | December 09, 2015

details // Glues, Sealants, Coatings, Kitchens & Baths, Tile, Flooring, Drywall

Tile Waterproofing for Cement Backer Board


Retiling a tub surround is a common remodeling project because tubs have a track record for quitting early


Cement tile backer is crumbly, porous, heavy, and dusty to cut. It is also affordable, durable, and extremely common. Here's how to make sure your installation keeps water out of the walls and in the tub.

If the surround includes an exterior wall, it is critical to insulate and air seal because bathtubs can amount to pretty big holes in the air barrier.

One way to do it is to pack the wall cavity with rigid insulation or spray foam. But there are lots of options.


Step by step:
  • A row of blocking between the studs acts as backing for the cement board and to makes a base for flashing the tub into the wall.
  • Begin flashing with vertical corner strips  of peel and stick membrane … and then fill in the back and sides peeling and sticking the membrane to the blocking and tub flange.
  • #30 Felt paper or house wrap is is a fail safe drainage plane for behind the backer board. Make sure to overlap the felt to direct water into the tub, not into the wall.
  • Next, install the cement board paying attention to the fastening schedule, and keeping the bottom at least a half inch off the tub deck to prevent wicking.
  • Tape the seams to prevent tile pops and feather the mesh tape with a wide layer of thin set being careful not to create a hump.
  • For added assurance, add a layer of pain-on waterproofing membrane before applying the tile.
  • Corners are a common leak spot, so use silicone caulk and backer rod instead of grout.
  • And leave gaps in the caulk the bottom of the tile, providing weep spaces for any leaks to drain into the tub, and down the drain.

This animated construction sequence is one part of a collection on tub and shower tile waterproofing videos.








The only real way to address this area is by using the Schluter membrane as the surface layer on the face of the wall bords.  30lb felt you got to be kidding.  Get with the 21 Century 


JS Construction 2, Miami, FL 33157

Tile pros use Schluter membrane to seal the cement board and moisture resistant sheetrock.

Daniel Morrison's picture

Many tile professionals go straight for Schluter or Laticrete or another system like that. Because many remodelers will use cement board, we wanted to begin the series with the most common denominator, cement board, and show how to install a drainage plane behind it.

Thanks for the comments, stay tuned.


oxymorons abound..

professionals or the typical new age home depot remodeler..

and simply using the 'common denominator' is no excuse, it's cementatious - means it sucks moisture.. isn't the article is about waterproofing - it is your title.. then ultimately that should mean 'waterproofing' before the opportunity to reach behind the substrate - by then it's already too late..

it's this typical 101 knowledge - then get a truck and some signs - which not only gives the industry a bad rep, but also separates the true professional..

ultimately, you never addressed or resolved the original problem that caused the failure in the first place.. i know, get more silicone...

Daniel Morrison's picture


You are absolutely correct.

Cement board without a drainage plane behind it and a waterproofing membrane on top of it could cause a lot of problems. 

In fact, we went back and forth on whether to include cement board in the series because there are more water-tolerant products available. In the end we decided that because there is a lot of cement tile backer sold (to more than just weekend warriors) that we ought to show how to do it well.

It is like a brick wall. Brick absorbs water, which is why there is a drainage plane behind it, through-wall flashing below it, and weep holes to let water drain out. 

We used a detail showing how to flash the tub flange to the wall and install a drainage plane over it. Weep gaps in the tile caulk allow any moisture to escape. Then we added a paint-on waterproofing membrane to the cement board. 

As I said, we will present several options. 

What is your ideal tile detail and product list?


Why do the videos say "the connection was reset", and I can't watch them.

Daniel Morrison's picture

Perhaps YouTube had a hiccup?

The video plays on my machine.


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