Waterproofing Tile Installations
Tubs
Standard bathtubs have a lip on three sides of the tub to capture water. The fourth side is of course the long side that you pass over getting into and out of the tub. Porcelain coated steel tubs have a much higher lip than cast iron tubs. Cast iron tubs have a very slight rise that will capture and divert water back into the tub, however, you must make sure the tub is installed level in both directions.
Capillary Attraction
Have you ever noticed that water can travel up a paper towel or rag that you suspend into a bowl of water? This is caused by capillary attraction. The same thing can happen with cement board or water resistant drywall that is allowed to 'sit' in a puddle of water along a tub edge.
When you install ceramic tile board around a tub you do NOT want it to contact the surface of the tub. I recommend a 1/4 to 3/8 inch space between the bottom of the board and the tub or shower surface. You can caulk this crack if you like with pure silicone caulk.
Tar Paper or Vapor Barriers
What happens if water somehow gets past your cement backer board? You can stop it cold in its tracks and divert it back to the tub or shower basin if you install tar paper or a plastic vapor barrier on the wall studs before you install your ceramic tile backer board. You must make sure that the vapor barrier or tar paper laps into the tub. If you use tar paper make sure that different layers overlap one another like roofing shingles. You don't want water running behind a sheet of tar paper. Don't trim off the excess material until after you have installed the ceramic backer board. Then carefully cut it back so that it is flush with the outer surface of the ceramic backer board. When the tile is installed, the vapor barrier or tar paper will be recessed 1/4 inch from the finished edge.
Caulk ALL Seams BEFORE Tilework
As you install each sheet of ceramic tile backer board, leave a 3/16 or 1/4 inch space between sheets. These cracks will be caulked with 100 percent pure silicone caulk BEFORE you begin to install ceramic tile. I have had great success doing this. If you then decide to tape the joints with thinset and fiberglass tape, you can do so AFTER you have caulked.
If you do tape with thinset, you must be careful NOT to create a hump at the taped seam. Ceramic tile is very unforgiving. The tile will not sit evenly as it crosses the humped area. Add to this the high gloss surface of most ceramic tile and you have a recipe for disaster. The high gloss surface will visually transmit the defect.
Grout is NOT Waterproof
Contrary to popular belief, grout is not waterproof. Water can travel through grout and especially past the contact point between the grout and the ceramic tile.
The joint between the tub and shower must never be filled with grout. This joint should be filled with silicone caulk only. I always grout the tub or shower areas and dig the grout out of this crack while it is still soft. It is very hard to try not to grout it. Caulking the crack before grouting is a huge mistake.
After grout has cured for approximately one week it is a great idea to seal it with a silicone sealant or better yet a silane-siloxane water repellent. Silicone is a film forming sealant that cannot breathe. You can get a great silane-siloxane water repellent from Saver Systems.
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Hi Tim,
Thank you for all of your knowledge and teaching tips. In the article Waterproofing Tile Installations, you write, "The joint between the TUB and SHOWER must never be filled with grout. This joint should be filled with silicone caulk only." I am a bit confused by this wording, as I am about to float a mud wall for my shower, which includes a 13 inch section of tile above the bathtub (all 3 walls). The bathtub is fiberglass and has a lip, which is secured with screws to the wall studs. I intend to use tar paper and wire mesh affixed to the studs and apply the floated mud on top of the tar paper and wire mesh. The tar paper catches any water that penetrates the tile and floated wall mud (I am using fat mud), allowing the water to drain into the tub. My question is if the joint between the tile and the bathtub is silicone caulk, it will not allow any water captured by the tar paper to drain back into the tub. If silicone caulk is used, the water will be trapped and eventually cause mold over time. Is my assumption correct? If so, would it be better to use grout for the joint between the tile and the bathtub? I appreciate your time to address this question.
No caulk will allow the water to drain.