CTI CTI Substrate Preparation & Assessment 1 — Questions and Answers
Question 1: What is the maximum allowable deflection for a subfloor that will receive ceramic tile, according to the Tile Council of North America (TCNA) guidelines?
- L/180
- L/360 (Correct answer)
- L/720
- L/240
Correct answer: L/360
TCNA requires subfloor deflection to not exceed L/360 (span in inches divided by 360) under live load to prevent tile and grout cracking.
Question 2: Which moisture testing method measures the amount of moisture vapor emitting from a concrete slab using a sealed hood over a defined area?
- Calcium chloride test (Correct answer)
- Relative humidity probe test
- Plastic sheet test
- Moisture meter test
Correct answer: Calcium chloride test
The calcium chloride test (ASTM F1869) measures moisture vapor emission rate (MVER) in pounds per 1,000 sq ft per 24 hours from a concrete slab.
Question 3: What is the maximum acceptable moisture vapor emission rate (MVER) for a concrete substrate before installing tile with a standard latex-modified thinset mortar?
- 3 lbs/1,000 sq ft/24 hrs
- 5 lbs/1,000 sq ft/24 hrs
- 8 lbs/1,000 sq ft/24 hrs (Correct answer)
- 10 lbs/1,000 sq ft/24 hrs
Correct answer: 8 lbs/1,000 sq ft/24 hrs
Most mortar manufacturers specify a maximum MVER of 8 lbs per 1,000 sq ft per 24 hours, though always verify with the specific product's requirements.
Question 4: A CTI inspector finds a wood subfloor with a gap of 1/4 inch between plywood panels. What is the correct assessment?
- Acceptable — gaps allow for wood expansion
- Unacceptable — gaps must not exceed 1/8 inch (Correct answer)
- Acceptable if panels are tongue-and-groove
- Unacceptable — no gaps are permitted at any time
Correct answer: Unacceptable — gaps must not exceed 1/8 inch
TCNA guidelines require gaps between plywood subfloor panels not to exceed 1/8 inch to provide adequate support and prevent flexing at seams.
Question 5: When assessing an existing concrete slab for tile installation, which condition would require the inspector to recommend surface preparation before tiling?
- A surface that is dusty or has a laitance layer (Correct answer)
- A surface that has cured for more than 28 days
- A surface that tests at 65% relative humidity
- A surface that is perfectly level and flat
Correct answer: A surface that is dusty or has a laitance layer
Laitance — a weak, powdery layer on concrete surfaces — must be removed by grinding or shot-blasting to ensure proper mortar bond.
Question 6: What is the minimum thickness of a single layer of plywood used as a subfloor for ceramic tile installation over joists spaced 16 inches on center?
- 1/2 inch (12.7 mm)
- 5/8 inch (15.9 mm)
- 3/4 inch (19 mm) (Correct answer)
- 7/8 inch (22.2 mm)
Correct answer: 3/4 inch (19 mm)
For joist spacing of 16 inches o.c., a single-layer plywood subfloor must be at least 3/4 inch thick to provide sufficient stiffness for tile.
CTI CTI Substrate Preparation & Assessment 1
What is the maximum allowable deflection for a subfloor that will receive ceramic tile, according to the Tile Council of North America (TCNA) guidelines?