If the subfloor is concrete, it will cause your readings to be higher than the actual moisture in the wood flooring. This is because the concrete is very dense. If the subfloor is a wood composite such as OSB or plywood, and there is not a huge amount of moisture in the subfloor (let’s say from flooding or another event), then the actual moisture measurement of the wood flooring will not be hugely affected.
Keep in mind, that if you are trying to use your Wagner Meters unit as a forensic tool – such as determining moisture after the fact on a failed wood floor job – the meter’s readings are not telling you anything about what the moisture content was in the flooring before it was installed.
All wood flooring, wood-based subflooring and concrete slabs should be measured (or at depth relative humidity in the case of a concrete slab) prior to wood flooring being installed.