Residual moisture and moisture measurement before flooring

Concrete releases moisture for a long time after pouring and a concrete floor can look bone-dry on the surface while still wet in the core. If a tight wooden floor is laid too early the moisture becomes trapped and the result is bumps discoloration loose joints and in the worst case mold. Therefore moisture measurement is not a formality — it is a prerequisite for the floor to last.

§The limit: 85 % relative humidity

Before flooring is laid, the residual moisture in the substrate should generally not exceed 85% relative humidity (RH). This is the common limit for laying wooden flooring on a concrete substrate, and it must be observed regardless of how busy the construction site is. Some floorings and adhesive types have even stricter requirements, so the supplier's laying instructions must always be checked.

§Borehole measurement — the recognized method

The most accurate and recognized method for determining residual moisture in concrete is borehole measurement (in situ RF measurement). A hole is drilled to a specified depth, a measurement tube is inserted and left until the moisture in the tube is in equilibrium with the concrete. Then the relative humidity is read with a moisture measurement instrument. It is the depth of the moisture, not the surface, that matters, so a simple surface meter does not give a reliable picture.

§Moisture content in the wood floor itself

Not just the substrate must be in order — the flooring material must also have the right moisture content. Solid wooden floors and parquet are typically supplied with a moisture content of around 8% (±2%), corresponding to a normal heated indoor climate. If the floor is laid in a room that is much more humid or drier, the wood will seek equilibrium and either swell or shrink. Therefore, the material is acclimatised in the room before laying.

• 01Measure residual moisture several places — especially at outer walls, pipe penetrations and thick pours
• 02Document measurement, date, depth and result — it's your safety net in a later damage claim
• 03Check supplier's laying guide for enhanced requirements specific to that surface
• 04Ensure stable indoor climate before, during and after installation so the floor doesn't move in humidity

§Documentation is your insurance

A floor damage often shows up months after laying and then the discussion begins about who bears the responsibility. If the floor layer has a moisture measurement report showing the substrate was dry enough at the time of laying one stands on much firmer ground. Moisture measurement is therefore both good craftsmanship and good business — and a fixed part of quality assurance on any floor work.