CTI CTI Substrate Preparation & Waterproofing 2 — Questions and Answers
Question 1: What is the minimum thickness of a Portland cement mortar bed for a floor tile installation per TCNA standards?
- 1/4 inch
- 3/4 inch (Correct answer)
- 1-1/2 inch
- 2 inches
Correct answer: 3/4 inch
TCNA specifies a minimum mortar bed thickness of 3/4 inch for floor installations to provide adequate structural support.
Question 2: Which ASTM standard governs the in-situ relative humidity probe test for concrete slab moisture?
- ASTM F1869
- ASTM F2170 (Correct answer)
- ASTM C627
- ASTM C1028
Correct answer: ASTM F2170
ASTM F2170 covers the in-situ relative humidity probe test, where probes are inserted into drilled holes in the slab.
Question 3: How should a CTI inspector recommend correcting a substrate with excessive high spots and low spots before tiling?
- Apply thicker mortar only to fill low areas
- Grind high spots and fill low spots with patching compound (Correct answer)
- Install a second layer of backer board over the uneven area
- Use larger tiles to bridge the variation
Correct answer: Grind high spots and fill low spots with patching compound
Both high spots must be ground down and low spots filled to achieve a flat substrate meeting the required tolerance.
Question 4: What is the maximum substrate flatness tolerance for large-format tiles with any side 15 inches or longer?
- 3/16 inch in 10 feet
- 1/4 inch in 10 feet
- 1/8 inch in 10 feet (Correct answer)
- 1/2 inch in 10 feet
Correct answer: 1/8 inch in 10 feet
TCNA recommends no more than 1/8 inch variation in 10 feet for large-format tiles to prevent visible lippage.
Question 5: What material is typically used to bond a secondary waterproofing membrane to a substrate around shower niches and corners?
- Silicone caulk alone
- Thinset mortar per the membrane manufacturer's specifications (Correct answer)
- Construction adhesive
- Unsanded grout
Correct answer: Thinset mortar per the membrane manufacturer's specifications
The membrane manufacturer specifies the bonding material — typically unmodified or modified thinset — to ensure compatibility and maintain warranty coverage.
Question 6: Most crack isolation membranes are rated to accommodate existing substrate cracks up to what maximum width?
- 1/16 inch
- 1/8 inch (Correct answer)
- 1/4 inch
- 1/2 inch
Correct answer: 1/8 inch
Most crack isolation membranes accommodate cracks up to 1/8 inch wide without transferring movement stress to the tile surface.
CTI CTI Substrate Preparation & Waterproofing 2
What is the minimum thickness of a Portland cement mortar bed for a floor tile installation per TCNA standards?