Moisture Testing Subfloors for Wood Flooring
For Concrete & Cementitious Screed
Drying Times
Concrete can take 28 days to reach its full strength. You only have to look at the amount of water mixed in to make concrete/screed to realise quite how much water there is to dry out.
The rule of thumb is:
- One day per mm for screeds up to 50mm thick
- Two days for each mm over 50mm thickness.
So, for example, a screed with a full depth of 75mm could take up to 100 days (50 days for the first 50mm, plus 50 days for the remaining 25mm). This estimate assumes ideal drying conditions.
We always recommend checking suppliers’ recommendations to ensure the fast-drying screed you have chosen is suitable for its intended application and will dry out in the time you have allowed for it. It’s also good to note that temperature and environmental moisture levels will affect drying times, e.g. open or closed windows, heaters, etc.
You or your fitter must check existing screeds/concrete for moisture before laying flooring to avoid disastrous problems like your flooring cupping and crowning. You can easily do this by using a specialist moisture meter.
Moisture Testing Subfloors
Chaunceys recommend using a Tramex Concrete Moisture Meter as a non-intrusive method of measuring moisture in your screed or concrete. A more intrusive but highly accurate moisture testing method involves drilling small holes in several places in the concrete substrate, then leaving a hygrometer probe in the holes overnight to read the moisture level.
A very rough yet qualitative way of approximating the concrete’s moisture content is by setting a humidity meter under a tent-like structure created by taping a 2m2 piece of non-permeable plastic over the concrete. When left overnight, the meter will indicate the level of dryness of the screed/concrete. Please note that this will only give a very approximate measure, and you should always do proper testing using a hygrometer if you have any concerns about moisture
in the subfloor.
Either the Tramex concrete moisture meter or hygrometer should give an accurate reading for the screed’s depth (please refer to the relevant manufacturer’s meter instructions). Tramex meters have scales for MC% moisture content and scales for the equivalent Carbide Method (CM) tests for concrete and anhydrite.
Excess Subfloor Moisture
When excess moisture is present in the concrete/screed subfloor, you should always rectify the problem before the wooden floor is laid. A reading greater than 2.5% on the Carbide Method test (~ 4% Tramex) for a concrete subfloor indicates that you should install a suitable damp proof membrane with a two-coat/two-part liquid layer (e.g. Mapei Mapeproof ESM 2 Part or Mapei Mapeiproof Turbo).
In the case of a floating floor, we suggest a well-installed heavy-duty, non-permeable plastic layer (make sure the plastic ends are continued up into the walls to fully protect the wooden floor).
Moisture Readings for Underfloor Heating
If you have chosen to have underfloor heating, a CM reading greater than 1.8% (~ 3% Tramex) for a concrete subfloor indicates that you should install a suitable damp-proof membrane.
In addition, we usually recommend gluing engineered boards to screed containing underfloor heating pipes as this will give a better transfer of heat. It is essential to use a flexible adhesive (such as Pallmann P5), adequately acclimatised wood flooring and a fully commissioned underfloor heating system.
Although there is no exact comparison between moisture content by weight and relative humidity, a concrete screed with a CM moisture content equal to or below 2.5% (1.8% if using underfloor heating) and a room relative humidity of 40% – 65% (at the time of fixing) should indicate suitable conditions for laying wooden flooring. However, it is essential to note that due to differences in size, number, and distribution of pores in the concrete, the CM moisture range of 2-3% may still exceed 75% RH or 65% with underfloor heating, in which case additional drying time or a damp-proof membrane will be necessary.
Laitance
Laitance is the weak, powdery layer of cement dust, lime, and fine sand on the surface of the concrete. It will rise to the surface of over-watered or over-trowelled concrete. The stability of your flooring will be dependent on its adhesion to laitance-free concrete. If the flooring has adhered to loose, crumbly laitance, then impact will cause the laitance layer to delaminate, along with the adhesive.
For Anhydrite (Gypsum) Screed
Measuring Moisture
Moisture content must be checked with a hygrometer. An anhydrite screed should have a CM moisture reading of less than or equal to 0.5% before it is suitable for installing a wooden floor. If excess moisture is found in the anhydrite screed subfloor (i.e., a CM meter reading greater than 0.5%*), you should allocate additional time to enable the screed to reach an acceptable level of dryness.
If you are using underfloor heating, a CM reading of greater than 0.3% for the anhydrite screed indicates that more time will be required to let the anhydrite screed dry. This can be accelerated by careful use of the underfloor heating system.
We would usually recommend gluing engineered boards to screed containing underfloor heating pipes as this will give a better transfer of heat. Remember, it is essential to use a flexible adhesive (such as Pallmann P5) when fixing wooden boards.
Never apply a vapour-proof membrane over a gypsum-based screed as these types of screed could potentially rot.
Laitance
Anhydrite (gypsum) type screeds should always be sound, smooth and dry. All possible laitance should be removed during the initial grinding stage. If laitance still exists, this should be mechanically removed by further grinding/sanding and vacuuming up all dust.
Need More Advice?
If you need any advice or further help regarding moisture readings or subfloors, please feel free to call our Customer Services Team on 0117 971 3131.
