CTI CTI Waterproofing & Moisture Management 2 — Questions and Answers
Question 1: Which type of backer board is inherently waterproof and does not require an additional waterproofing membrane in most wet area applications?
- Standard cement board
- Fiber cement board
- Foam-core waterproof tile backer (e.g., Wedi, Schluter Kerdi-Board) (Correct answer)
- Gypsum drywall
Correct answer: Foam-core waterproof tile backer (e.g., Wedi, Schluter Kerdi-Board)
Foam-core tile backers like Wedi or Schluter Kerdi-Board are manufactured with a built-in waterproof surface, eliminating the need for a separate membrane.
Question 2: What is the correct way to waterproof the seams and corners of a cement board shower installation?
- Apply grout along all seams
- Tape seams with alkali-resistant mesh tape and apply a topical waterproofing membrane over the tape (Correct answer)
- Use silicone caulk only
- Leave seams untreated as cement board is water-resistant
Correct answer: Tape seams with alkali-resistant mesh tape and apply a topical waterproofing membrane over the tape
Seams must be reinforced with mesh tape and then covered with a liquid membrane to create a continuous waterproof barrier at the most vulnerable joints.
Question 3: When installing a tile floor over a wood subfloor in a bathroom, what product helps prevent substrate movement from cracking the grout joints?
- Extra thin-set mortar
- An uncoupling membrane (e.g., Schluter Ditra) (Correct answer)
- A second layer of plywood
- Cement board alone
Correct answer: An uncoupling membrane (e.g., Schluter Ditra)
An uncoupling membrane absorbs differential movement between the wood subfloor and the tile, preventing stress cracks in the grout and tile.
Question 4: What is the minimum curing time recommended before flood-testing a newly waterproofed shower?
- 1 hour
- 4 hours
- 24 hours
- As specified by the membrane manufacturer, typically 24–72 hours (Correct answer)
Correct answer: As specified by the membrane manufacturer, typically 24–72 hours
Waterproofing membranes must be fully cured before flood testing; manufacturers typically specify 24–72 hours depending on temperature and humidity.
Question 5: How long should a flood test for a shower pan be conducted to verify waterproofing integrity?
- 30 minutes
- 2 hours
- A minimum of 24 hours (Correct answer)
- 72 hours
Correct answer: A minimum of 24 hours
Industry standard requires the shower pan to hold water for at least 24 hours with the water level marked to detect any drop that indicates leakage.
Question 6: Which sealant is recommended at the change of plane between a tiled wall and tiled floor in a wet area?
- Sanded grout
- Unsanded grout
- 100% silicone caulk (Correct answer)
- Epoxy grout
Correct answer: 100% silicone caulk
Silicone caulk is required at all changes of plane (floor-to-wall, inside corners) because it flexes with building movement without cracking, unlike grout.
CTI CTI Waterproofing & Moisture Management 2
Which type of backer board is inherently waterproof and does not require an additional waterproofing membrane in most wet area applications?