CTI Substrate Preparation & Assessment Flashcards

What is the maximum allowable deflection for a subfloor that will receive ceramic tile, according to the Tile Council of North America (TCNA) guidelines?

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.

Which moisture testing method measures the amount of moisture vapor emitting from a concrete slab using a sealed hood over a defined area?

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.

What is the maximum acceptable moisture vapor emission rate (MVER) for a concrete substrate before installing tile with a standard latex-modified thinset mortar?

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.

A CTI inspector finds a wood subfloor with a gap of 1/4 inch between plywood panels. What is the correct assessment?

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.

When assessing an existing concrete slab for tile installation, which condition would require the inspector to recommend surface preparation before tiling?

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.

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?

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.