What is the acceptable substrate moisture reading for vinyl floors and the recommended moisture test?

With more than 50 years’ experience in the local and international flooring industry, Denver Coleman, chairman of Polyflor SA, answers questions posed by installers, architects and readers. In this issue, he discusses the acceptable substrate moisture reading for vinyl floors and the recommended moisture test.

“What is the acceptable moisture reading for the installation of vinyl floors and what is the best moisture test that you would recommend?” Alfred B. Davids, Port Elizabeth

It is critically important to test for moisture in screeds or slabs before laying any moisture-sensitive type of flooring, such as vinyl, wood, rubber or laminates. We increasingly come across instances of “wet” screeds – both in new buildings and even in old buildings that are being refurbished. Even screeds that have previously been fitted with tiles or sheeting are sometimes declared too wet for refitting.

While this seems bizarre, both flooring materials and adhesive technology have changed in recent years, in many ways making them even more sensitive to moisture. Previously, screeds were quite often wood-floated or less densely power-floated. This allowed the top 10 mm to be much drier at the time of fitting, allowing the adhesive to fully cure before coming into contact with any rising moisture. When the old floorcovering is removed, however, the moisture that was trapped now sits in the screed and needs time to evaporate.

Often, moisture is also found in upper-floor suspended slabs. Whatever the reason might be for moisture in screeds or in old slabs, it is very important to obtain moisture readings prior to all installations. Unfortunately, no surface test or meter is sufficiently accurate. Testing the screed surface only, especially with a densely finished power/steel-floated screed, is dangerous.

Moisture vapour slowly permeates upwards through the dense top finish and, because moisture is released in small quantities, it evaporates quite quickly and often does not show significantly with surface testing methods. The only way to obtain an accurate reading, and tell what is happening below the surface, is by drilling at least 40% into the depth of the screed or slab.

Drilling into the screed in combination with the use of a moisture meter designed to test below-surface moisture is therefore critical.

When drilling, it is possible to get an indication of moisture by testing the dust from the drilling. These days even the dust is found to have moisture present, and it should be remembered that the drilling causes some heat and therefore slight evaporation of moisture from the dust, therefore it indicates high moisture if it shows in the dust.

Once the hole is drilled into the screed or slab, an electronic sensor can be inserted that will accurately read the % RH moisture in the screed, as well as measure the screed temperature, which has an effect on the speed at which moisture evaporates. Temperature is also important when applying adhesives and self-levelling screeds. Alternatively, one can choose from a variety of meters equipped with probes which are inserted into the hole, which give a comparable numeric reading that translates into an RH reading.

While many old-fashioned surface testing methods still exist, they are not as accurate as a below-surface test.

Acceptable readings for vinyl installations are between 70% and 75% RH – depending on the manufacturer. Testing methods and results should always be recorded in the site minutes. These days with the use of cellphone cameras, a dated photograph of the site, test method, temperatures and humidity can easily be taken and filed for future reference.

If you have any flooring question relating to design, installation, problems or commentary you wish to share with Denver, please e-mail him at marketing@polyflor.co.za with ASK DENVER in the subject line or phone Blythe at Polyflor on 011 609 3500.