Subfloor Assessment & Preparation Flashcards

When using a moisture meter on a concrete slab, what reading (in %RH) is the maximum acceptable level for most resilient flooring installations?

Answer: 80% RH

ASTM F2170 sets 80% RH as the standard upper limit for most resilient flooring adhesive systems.

A wood subfloor deflects excessively between joists. What is the most common corrective action before installing ceramic tile?

Answer: Sister the joists or add blocking

Sistering joists or adding blocking addresses the root structural cause of excessive deflection rather than masking it.

Which tool is used to detect low spots and high spots across a subfloor surface?

Answer: Straightedge or floor flatness gauge

A long straightedge (typically 10 ft) reveals high and low deviations across the subfloor plane.

What is the purpose of skim-coating a concrete subfloor with Portland cement-based filler?

Answer: To fill surface voids and achieve flatness

Skim-coating fills bug holes, trowel marks, and low spots to achieve the required flatness tolerance.

OSB used as a subfloor panel should have its face grain oriented in which direction relative to floor joists?

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.

A calcium chloride moisture test on a concrete slab yields 8 lbs/1,000 sq ft/24 hrs. What does this indicate?

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.

When installing hardwood flooring, what is the minimum recommended subfloor thickness for joists spaced 19.2 inches on center?

Answer: 3/4 inch

NWFA guidelines recommend 3/4-inch minimum subfloor thickness for joists at 19.2" OC to limit deflection under hardwood.