Ultimate Performance Through Optimum Drying

The most common technical enquiries encountered by screed contractors are, “What are the expected drying times?” and “When can my floor covering be installed?”. 


The simple fact is that final covering (vinyl, tiles, wood panelling, carpet, cork, parquet etc) cannot be laid to perfection unless the screed on which it is laid has dried optimally.

In keeping with good workmanship and the specific standards governing the installation of floor screeds, the primary method of testing if the screed has fully dried to the required level is to conduct a simple moisture test.

One cannot stress enough the importance of this measurement, as a premature installation of the final floor covering will adversely affect the quality of the entire flooring project.

According to BS 5385-3:2014, screed drying time is approximately 1mm per day up to 40mm thickness in adequate temperatures and drying conditions (sample drying times are given in Table 1).

However, this can be greatly affected by actual conditions. This period should be increased for screeds thicker than 40mm and in poor drying conditions.

Good drying conditions should be provided as soon as the screed is laid. The screed should be protected from very rapid drying or draughts on the first day but, thereafter, atmospheric humidity should be low (ie not greater than 65% RH), and the air temperature should be adequate (eg 20°C) so that moisture can evaporate.


There are several practical methods for assessing the moisture content in screeds.

• The BS 8203:2001+A 1 :2009 method for measuring the moisture condition of a base to receive a floor covering is to use an insulated impermeable box on top of the screed and measure the equilibrium relative humidity of the trapped air inside the box using an electronic probe, or hair hygrometer.

• The DIN 18560-4 method for measuring the moisture content of a base to receive a floor covering using a CM (carbide method) testing device.

• The moisture content of the screed may be determined by drying a sample of the screed in an oven. The sample is weighed before and after the oven drying to determine the weight loss as a percentage of the dry weight.

Moisture testing by hygrometer (RH): The British Standard for testing a base to receive a resilient floor covering is to use a hair hygrometer to the method defined in BS8203: 2001. This provides a non-destructive test method and will give results for relative humidity near to 75% (which is the usual required limit for floor finishes). Above this level of moisture, the hair hygrometer may not always provide a meaningful reading. For correct results, the BS 8023 method must be strictly adhered to, including the use of a correctly sized and insulated box sealed to the floor, a sufficiently long test for equilibrium to be reached and the use (where appropriate) of an impervious sheet around the instrument.

Moisture testing by CM tester (CM): The CM tester works according to the carbide method, using the destruction of calcium carbide in water. During this reaction, acetylene gas is formed which causes an increase in pressure in the vessel. From the measured pressure and the test portion of the material to be tested, the water content of the sample is read from a table or directly from the manometer. A representative sample should be taken from the dry screed floor. This should cover the whole thickness of the floor material. For parquet, the sample is taken from the lower to middle area of the screed. Before filling the vessel, the material must be crushed and slightly milled. The larger pieces of mineral aggregates should be removed. By shaking the vessel well, with the inclusion of steel balls, the desired final crushing will be achieved. Use 100g of crushed screed (depending on the type of CM tester).


For calcium sulphate-based floor screeds, the requirements are for a maximum of 1% moisture content for moisture permeable floorings (eg carpets) and 0.5% moisture content for impermeable floorings (eg vinyl, ceramic and stone).


With cement-based floor screeds, the requirements are for a maximum of 2% moisture content for moisture permeable floorings (eg carpets) and 2% moisture content for impermeable floorings (eg vinyl, ceramic and stone).


An electronic CM device is helpful to locate the highest moisture content location of the floor. The reading can then be confirmed by testing to the carbide method.


BS 5385-3: 2014 Wall and floor tiling – Part 3 states: The assessment of residual moisture levels, is the responsibility of the main contractor.


When it’s not dry enough, there are steps you can take – either wait; force dry (calcium sulphate-based floor screeds only); or apply a surface DPM.


Unlike cement-based screed, calcium sulphate screed can be forced-dried, by commissioning the underfloor heating system (if applicable) or by utilising a dehumidifier.

Commissioning Underfloor Heating:

• Five days after installation of the screed, the commissioning process starts with a water temperature (UFH manifold) of 25°C, which is maintained for three days. The water temperature is then raised to the maximum value (max 55°C) and kept at this level for at least four days.

• Allow for plenty of ventilation by opening windows on each side of the building. Please note: it is essential that the building receives sufficient air changes in order to achieve low air humidity (< 65% RH).

Utilising a dehumidifier:

• Five days after the installation of screed, introduce heat and utilise a dehumidifier with correct capacity for the m³ area of the building. Use several dehumidifiers if required.

• Keep windows and doors closed to allow the dehumidifier to work efficiently.

• Continue with the above procedure until a moisture content of 0.5% (tiling/vinyl) or 1% (carpet) is achieved.


In general, surface-applied DPM systems do not stop the passage of moisture – they actually allow the passage of moisture from the screed to the floor finishes, but at a greatly reduced and controlled rate. It will always be best practice and economical to dry the screed rather than use a surface DPM. However, because of time constraints or site conditions, cement and calcium sulphate screeds can be covered with a surface DPM, providing certain criteria are met.

With calcium sulphate-based screeds, care must be taken regarding the amount of moisture trapped in the screed, as this will have an effect on the strength gain. Applying a moisture-suppressing surface DPM will effectively cap the strength of the screed. It should not, therefore, be applied until the screed has gained sufficient strength in order to install floor finishes. Without forced drying methods of the screed (as earlier), the screed should be at least four weeks’ cured before applying a surface DPM. Moisture content must be < 1.5% CM or 87% RH, tested as described.

We recommend the use of Merlin Barrier Coat as DPM which is suitable for use on both cement-based and calcium sulphate-based floor screeds. Merlin Barrier Coat has a successful track record using its surface DPM on cement and calcium sulphate screeds for many years.

From the handheld electronic to CM Professional kits, SMET has a range of German-made Gann moisture testing equipment and liquid Damp Proof Membrane available to purchase.

Get in touch to discuss your flooring project:

Smet Building Products Ltd | 93A Belfast Road | Newry | BT34 1QH | Northern Ireland | T: +44 (0) 28 3026 6833 | F: +44 (0) 28 3025 2777


www.smetbuildingproducts.com or www.smet.ie