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Sub Floor Moisture Testing – Mastering the Art

By June 8, 2023 November 10th, 2023 No Comments

concrete self levelling floor moisture test

Sub Floor Moisture Testing Done Right

  • Moisture testing is crucial for proper subfloor preparation in flooring installation.
  • Modern floor coverings have lower moisture vapor transmission rates, increasing the risk of moisture-related damage.
  • Construction practices have become faster, leading to floor coverings being installed quickly, even on moist subfloors.
  • Common moisture tests include hand-held pin meters, moisture encounter meters, hygrometers, probe testing, and carbide bomb tests.
  • Moisture tests determine the need for damp proof membranes (DPMs) and the appropriate product type.
  • Subfloor moisture testing helps ensure successful floor covering installation and minimizes imperfections.

Sub Floor Moisture Testing is done properly by Dr Flatpack. Here we will explain the importance of moisture testing before laying floors. For our self levelling and smoothing service, moisture testing is of the utmost importance.

Moisture testing plays a vital role in preparing new floors. Although manufacturers’ technical teams often provide these tests, it is beneficial to be familiar with the most commonly conducted ones.

Why is moisture testing necessary? In the past, newly installed flooring was less susceptible to moisture damage. Certain floor coverings had high moisture vapor transmission rates, allowing better breathability, and bitumen-based adhesives were relatively tolerant to moisture. However, modern buildings now frequently utilize floor coverings like rubber and highly flexible vinyl, which have significantly lower moisture vapor transmission rates. Additionally, while solvent-free adhesives offer health and environmental benefits, they do not effectively tolerate moisture.

Furthermore, construction practices have changed over time. In the past, buildings were constructed at a slower pace, but today there is pressure to expedite projects. As a result, floor coverings are often installed rapidly, even when the subfloors still contain considerable residual moisture.

To prevent failures, it is crucial to measure and interpret moisture levels, even if surface-applied damp proof membranes (DPMs) can be installed on moist floors. Conducting moisture tests helps determine the need for a DPM and the appropriate product type, such as a two-part epoxy or a water-based acrylic.

Various moisture tests are available to assess the dryness of a new floor slab, and these tests can also be used on older subfloors. They are particularly helpful when uncertainty exists regarding the presence of a moisture barrier.

Hand-held pin meter or moisture encounter meter

Pin meters or moisture encounter meters use pins to measure moisture content. These devices provide instant numerical readings indicating the dryness of the screed. Multiple readings can be taken in various locations, providing a comprehensive understanding of the slab’s moisture content and identifying areas at higher risk.

A pin meter is primarily used to measure the wood moisture equivalent percentage in timber and measures the top surface of a screed to a depth of approximately 1mm. It employs a traffic light system to indicate the moisture content. On the other hand, a moisture encounter meter, like a Tramex meter, sends a signal deeper into the floor, reaching depths of up to 19mm, depending on the substrate.

While these initial tests are not recognized as British Standards tests, conducting them in multiple areas helps determine where to focus further moisture tests that align with British Standards. The following are more suited to sub floor moisture testing.


A hygrometer is a non-invasive test that measures the water vapor present in a subfloor, providing a relative humidity (RH) reading. To conduct this test, a hygrometer box is sealed to the floor using putty or butyl tape and left in place for 3 to 7 days. The box captures air and moisture, allowing equilibrium to be reached.

Moisture readings are displayed on the hygrometer’s gauge or through connection to a meter. Some hygrometers can store historical data, including the highest reading obtained during the testing period.

The hygrometer provides a %RH reading, and according to British Standard BS-8203:2017 Annex B, a floor is considered dry enough for applying resilient floor coverings when a hygrometer reading of 75%RH or lower is obtained. For timber floor installations, the acceptable reading is 65%RH or lower.

As the surrounding areas of a hygrometer are open and ventilated while the area beneath the hygrometer is not, it is recommended to move the hygrometer once equilibrium has been achieved, usually every few days.

Unfortunately, on active construction sites, hygrometers can be accidentally knocked or moved by individuals who are unaware of their purpose, as evidenced by instances where fitters find the boxes moved aside upon arrival.

Probe testing in-situ RH

The choice between a hygrometer and a probe test depends on preference and convenience for a particular location or environment. Probe testing, relatively new to British Standards, is a destructive method that involves drilling a small hole in the subfloor.

A hole is drilled to a depth of 40% of the slab thickness. A probe hole liner is then inserted and capped. After 48 hours, the cap is removed, and a moisture-measuring probe is inserted into the hole. The probe is left for at least 20 minutes and then connected to a meter reader that displays the %RH of the slab.

Using probes allows frequent testing to monitor the drying progress of a screed. It eliminates the need to leave expensive equipment, like a hygrometer, on-site and enables drilling and plugging the floor at multiple locations.

Carbide bomb/Calcium chloride carbide measurements (CM) moisture testing

Unlike the previous tests, carbide bomb tests do not measure moisture vapor transmission but instead assess the moisture trapped within the screed formulation. Although less commonly used today, some screed manufacturers consider carbide bomb tests as the only suitable method for their screeds.

Carbide bomb tests are destructive and involve removing a sample of the slab or substrate by hammering and chiseling. The sample is crushed to dust, weighed to a specific amount, and placed into a CM testing vessel. Calcium carbide is added to the vessel, which is then sealed and shaken.

Upon contact with moisture, calcium carbide produces acetylene gas, and the gauge on the testing vessel displays the amount of gas generated. A higher reading indicates a wetter floor, with a reading of 0.5% for a 50g sample roughly equivalent to 75%RH.

The advantage of the carbide bomb test is the instant result it provides. If the moisture reading falls within acceptable limits, there is no need for a return visit to the site.

Sub floor moisture testing is crucial for effective subfloor preparation in modern construction. They are essential in mitigating the risk of residual construction moisture causing problems. Whether you perform these tests independently or with the assistance of a manufacturer’s technical team, understanding their principles and acting accordingly based on the results will help ensure a successful installation of floor coverings while minimizing imperfections.

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