Q. As the flooring installer on a recent project we were at a crossroads. Prior to installing the flooring, we did our due diligence by performing a relative humidity (RH) test on the concrete, which involved inserting an RH probe to a depth of 40% in the slab. The results indicated the floor was too wet to safely install flooring. More time was needed for the concrete to dry. But the general contractor was keen to proceed in order to stay on schedule. We have installed hundreds of floors and knew the risks. We told the GC that we understood why he believed the project needed to keep moving, that time and money, not to mention business reputations, were on the line. How can we convince the GC to wait?

A. We asked Jason Spangler, the Rapid RH sales manager with Wagner Meters, to respond: The RH test you used was a smart choice. It provided a fast, reliable, and scientifically proven method for determining the actual moisture condition inside the concrete. Why test from the inside? Concrete surface tests, such as the calcium chloride test, are inadequate because they only reveal the condition at the surface and slabs invariably exhibit a moisture gradient prior to being sealed by a floor installation or coating. In addition, the surface layer is more vulnerable to the ambient temperature and RH of the room.

Too often, wood floors installed over a damp slab fail and end up being torn out.

Concrete drying times vary, although a typical rule of thumb is to allow at least 28 days for each inch in the concrete slab. Many other factors influence drying time as well, such as the way the concrete was mixed and the condition of the surface.

Given the concrete’s moisture condition, the GC could ignore the problem and go ahead with the flooring installation. Or, he could use a moisture mitigation system on top of the concrete prior to starting the flooring installation, adding additional cost to the project. Or he could continue RH testing until the readings are below the acceptable threshold of the floor manufacturer’s specifications, which could potentially cause a significant delay in the project.

As the flooring installer, you should emphasize that over $1 billion in damages occur every year due to excess moisture in concrete slabs. It sounds like you have seen your fair share of floors that had warped, blistered, peeled, and de-bonded. You know that if the project goes ahead, everyone, including the building owner, could be liable for damages which would cost much more than any delay now. You really need to insist that the installation of the floor wait until the slab is sufficiently dry.

If we take a look back, there are some steps that could have been taken to let the concrete dry more quickly. Here are some things that could have been done in the early planning stages:

● Enclose the structure as soon as possible after the concrete floor has been poured to control variable ambient conditions and keep the air circulating with the HVAC system.
● The finishing techniques used should keep the surface open while the concrete slab dries; hard troweling can seal off the natural capillaries that allow moisture to move through the slab.
● Choose the most appropriate flooring products for the construction schedule.
● Consider factoring potential moisture mitigation measures into the project budget.
● Maintain a low water-cement ratio; most recommend 0.50 or less.
● Install a subgrade vapor barrier below the concrete to stop the flow of moisture from the soil.

In an ideal scenario, letting concrete dry naturally is usually best, but construction project schedules often don’t allow enough time for this. Each floor installation has a unique set of variables, and RH testing is an absolute must for obtaining an accurate assessment of the slab’s moisture condition. At this point, you can take a few steps to try to increase the drying rate:
● Dehumidifying methods can help remove moisture in the air, increasing moisture removal from the concrete.
● Increased airflow across the concrete surface will help move the moisture into the air.
● Increasing the temperature in the enclosed environment will help move moisture out of the slab.

You should strongly recommend that the GC delay the floor installation until the concrete’s moisture condition meets the flooring product manufacturer’s specifications, otherwise this project is likely to be faced with the costly financial and legal implications of a failed flooring installation.