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The Screed Scientist is back by popular demand. This month he discusses how to assess moisture levels in screeds to prevent dampness; the most common cause of bubbles and ripples in vinyl flooring.

The decision to shut down one section of the new Clitheroe Hospital due to flooring faults came as an unpleasant surprise to the community. Just three months after the opening of the £7.8million hospital, areas of the floor de-bonded causing the vinyl to bubble. Inspection revealed serious problems with the floor screed.

According to the National Patient Safety Agency (NPSA), approximately 1,000 hospital workers and patients sustain a severe fall-related injury in hospitals across the UK each year. Therefore, implementing appropriate safety measures in medical institutions has become critical.

Considering the risks associated with trips and falls due to uneven floors, closing the affected areas of Clitheroe Community Hospital for remedial work was the right thing to do. Besides causing accidents, an irregular floor can make it difficult for the hospital staff to move equipment around. It also causes uneven, rapid wear in bubbled areas, typically resulting in premature replacement of floor finishes and unexpected expenses.


IN VINYL FLOORING The independent contractor hired to investigate flooring problems at the hospital indicated residual moisture as the prime culprit responsible for vinyl bubbling. This wasn’t surprising, considering that dampness is one of the most common causes of most floor problems.

If residual dampness is not completely eliminated before floor coverings are installed, it will slowly rise up to the screed/vinyl interface, degrading the adhesive which turns into a wet, gooey, milky consistency. The vinyl would subsequently de-bond and start to bubble and ripple.


IN SCREEDS Water is drawn up through the screed and evaporates off from the surface, making the screed drier at the top than at the bottom. This is called a moisture profile and can be affected by heat, ventilation and humidity. Using dielectric methods to measure moisture in the screed will only give an indication of the screeds dryness at the surface. To get a more accurate reading, a cross sectional sample would be required.

Currently, the calcium carbide test is considered the most accurate method to assess moisture levels in screeds. To perform this test, a screed sample must be crushed, mixed with calcium carbide, and placed in a special pressure cylinder. Inside the cylinder, the sample will react with calcium carbide, releasing acetylene gas that creates a measurable pressure. The expert performing the test will use an appropriately calibrated gauge to measure the pressure exerted by the gas. Then, gauge readings can be used to calculate the actual moisture content in screeds.

Consequently, one thing you can do to get a perfect floor and avoid time- consuming, expensive remedial work is to ensure that the screed is fully dry prior to laying vinyl flooring.

Another method to determine the level of moisture in screeds is the Tramex meter. Though this electronic device only measures moisture at the surface of the screed and is unable to deliver precise readings of moisture content in screeds, it provides a convenient, non-destructive testing method and offers instant results which is good for preliminary measurements. When greater accuracy is required (for example, moisture in screeds should not exceed 2% when the project includes sensitive floor coverings such as vinyl), the calcium carbide test can be used to check the readings from the Tramex meter.

Recommended Moisture levels in screed prior to laying floor covering.

% Moisture 2%

1.8% 0.8% 0.5%

Anhydrite Screed

In the case of Clitheroe Community Hospital damage to reputations and delays in providing patient healthcare could have been reduced or avoided if the screed moisture had been checked accurately. In instance of arbitration or uncertainty, we would always recommend calling in the appropriate experts to ascertain where the problems started. However, the Screed Scientist thinks that the problems described by the independent contractor would have been entirely eliminated by a quick, easy, accurate test before the floor covering had been laid. Heated

Cement Based Screed

Anhydrite Screed

Not Heated

Cement Based Screed

Do you have a question for the screed scientist? Email sarah@ and look out for the answer in future issues.


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