WLFI Moisture Testing & Subfloor Preparation 3 — Questions and Answers
Question 1: An inspector uses a pin-type moisture meter on a plywood subfloor and gets a reading of 15%. What should the inspector do next?
- Immediately approve the subfloor for installation
- Verify the reading using a calibrated secondary test method (Correct answer)
- Add two layers of rosin paper as a vapor barrier
- Schedule installation for the following day
Correct answer: Verify the reading using a calibrated secondary test method
A single meter reading should be verified with a secondary method because pin meters can give false readings depending on species and temperature.
Question 2: What does a pinless (electromagnetic) moisture meter measure compared to a pin meter?
- It measures the same property but from a greater depth
- It detects moisture based on changes in electrical capacitance rather than resistance (Correct answer)
- It is only suitable for engineered hardwood, not solid wood
- It measures surface temperature, not moisture
Correct answer: It detects moisture based on changes in electrical capacitance rather than resistance
Pinless meters use electromagnetic fields to detect changes in electrical capacitance caused by moisture, allowing non-invasive scanning without penetrating the surface.
Question 3: When is it necessary to test concrete slabs that are built above grade (i.e., elevated slabs) for moisture?
- Never — elevated slabs cannot retain moisture
- Only when the building is located in a humid climate
- Always, because elevated slabs can still trap and emit moisture vapor (Correct answer)
- Only if the slab is less than 6 months old
Correct answer: Always, because elevated slabs can still trap and emit moisture vapor
Elevated slabs can still accumulate and emit moisture vapor, especially if they were exposed to weather during construction or contain residual mix water.
Question 4: Per ASTM F2170, how deep should the RH probe hole be drilled in a concrete slab of 4-inch thickness when the slab is drying from one side only?
- 25% of slab depth (approx. 1")
- 40% of slab depth (approx. 1.6") (Correct answer)
- 50% of slab depth (approx. 2")
- 75% of slab depth (approx. 3")
Correct answer: 40% of slab depth (approx. 1.6")
ASTM F2170 requires drilling to 40% of slab depth for slabs drying from one side, which represents the equilibrium point for moisture distribution.
Question 5: Which type of subfloor panel is most resistant to moisture-related swelling and is preferred in high-humidity environments as a subfloor for wood flooring?
- Standard OSB (Oriented Strand Board)
- Tongue-and-groove plywood rated Exposure 1 (Correct answer)
- Concrete backer board
- Underlayment-grade MDF
Correct answer: Tongue-and-groove plywood rated Exposure 1
Exposure 1 tongue-and-groove plywood resists delamination and swelling better than OSB in high-humidity subfloor applications.
Question 6: An inspector notices white crystalline deposits on the surface of a concrete slab. This condition is called:
- Carbonation
- Efflorescence (Correct answer)
- Alkali-silica reaction
- Laitance
Correct answer: Efflorescence
Efflorescence is the migration of salts to the surface carried by moisture, indicating ongoing moisture movement through the slab.
Question 7: What is the minimum equilibration time required for an in-situ relative humidity probe before a reading is taken per ASTM F2170?
- 1 hour
- 24 hours (Correct answer)
- 72 hours
- 7 days
Correct answer: 24 hours
ASTM F2170 requires a minimum 24-hour equilibration period for the sleeve and probe in the hole before recording the RH reading.
WLFI Moisture Testing & Subfloor Preparation 3
An inspector uses a pin-type moisture meter on a plywood subfloor and gets a reading of 15%.
What should the inspector do next?