CFI Subfloor Assessment & Preparation 2 — Questions and Answers
Question 1: When using a moisture meter on a concrete slab, what reading (in %RH) is the maximum acceptable level for most resilient flooring installations?
- 75% RH
- 80% RH (Correct answer)
- 85% RH
- 90% RH
Correct answer: 80% RH
ASTM F2170 sets 80% RH as the standard upper limit for most resilient flooring adhesive systems.
Question 2: A wood subfloor deflects excessively between joists. What is the most common corrective action before installing ceramic tile?
- Apply floor leveling compound
- Sister the joists or add blocking (Correct answer)
- Install an additional layer of underlayment
- Use a flexible tile adhesive only
Correct answer: Sister the joists or add blocking
Sistering joists or adding blocking addresses the root structural cause of excessive deflection rather than masking it.
Question 3: Which tool is used to detect low spots and high spots across a subfloor surface?
- Digital caliper
- Straightedge or floor flatness gauge (Correct answer)
- Moisture pin meter
- Feeler gauge
Correct answer: Straightedge or floor flatness gauge
A long straightedge (typically 10 ft) reveals high and low deviations across the subfloor plane.
Question 4: What is the purpose of skim-coating a concrete subfloor with Portland cement-based filler?
- To increase moisture vapor transmission
- To fill surface voids and achieve flatness (Correct answer)
- To prime the surface for adhesive bonding
- To reduce thermal expansion of the slab
Correct answer: To fill surface voids and achieve flatness
Skim-coating fills bug holes, trowel marks, and low spots to achieve the required flatness tolerance.
Question 5: OSB used as a subfloor panel should have its face grain oriented in which direction relative to floor joists?
- Parallel to joists
- Perpendicular to joists (Correct answer)
- At a 45-degree angle to joists
- Direction does not matter for OSB
Correct answer: Perpendicular to joists
OSB panels are engineered with the face strand oriented along the long dimension, which must span perpendicular to joists for maximum strength.
Question 6: A calcium chloride moisture test on a concrete slab yields 8 lbs/1,000 sq ft/24 hrs. What does this indicate?
- The slab is within acceptable range for all flooring types
- The slab exceeds the 3 lb limit and requires moisture mitigation (Correct answer)
- The slab is too dry and needs pre-wetting
- The reading is invalid and must be retested
Correct answer: The slab exceeds the 3 lb limit and requires moisture mitigation
Most flooring adhesives allow a maximum of 3–5 lbs per ASTM F1869; 8 lbs requires a vapor retarder or moisture mitigation system.
Question 7: When installing hardwood flooring, what is the minimum recommended subfloor thickness for joists spaced 19.2 inches on center?
- 1/2 inch
- 5/8 inch
- 3/4 inch (Correct answer)
- 1 inch
Correct answer: 3/4 inch
NWFA guidelines recommend 3/4-inch minimum subfloor thickness for joists at 19.2" OC to limit deflection under hardwood.
CFI Subfloor Assessment & Preparation 2
When using a moisture meter on a concrete slab, what reading (in %RH) is the maximum acceptable level for most resilient flooring installations?