April Showers Bring May Flowers, Along with Concrete Subfloor Moisture

By Jack Boesch

The effect of seasonal change in concrete subfloor moisture correlates directly with rainfall and ground water tables. Surface water during rainy season also plays a part. Even the driest of climates can have an unusual amount of rain that can turn a dry creek bed into a raging river, and metropolitan areas have streets and parking lots that can be inundated during heavy downpours. Cyclic changes of damp soil vary, and since concrete is porous, capillary moisture can rise and fall in a slab just as it does in the soil.  

Inadequate water drainage away from buildings also can be the culprit. Clogged, overflowing gutters, improper downspouts, and low spots around a foundation, all can compound wet slab-on-grade or basement problems. 

To minimize concrete moisture issues, consider taking these steps: 

  • Install a quality Class I impermeable vapor retarder, making sure there are no holes or cracks.
  • Vapor retarders may include underlayment sheet-goods products, liquid applied membranes, multi-functional adhesives, or multi-layered systems. 
  • After newly poured concrete has set, scarify the surface of the concrete to meet the necessary CSP (Concrete Surface Profile) and to obtain a good bond if applying flooring sealants or adhesives.
  • Have a properly operating HVAC system or dehumidifier to meet the flooring manufacturer required environmental conditions.
  • In extreme cases, a sump pump could be needed.

Moisture Vapor Drive

This graphic shows how moisture vapor works through a concrete slab.

It is important to evaluate and address moisture issues in your concrete subfloors, as they can adversely affect your floor coverings for many years. Floor covering manufacturers of all types generally post the maximum allowable moisture content of the concrete that their flooring products are being installed over. Refer to their installation instructions or warranties. Where these don’t exist, refer to the NWFA Installation Guidelines to address these troublesome issues.

The number one enemy to wood flooring is harmful moisture. Concrete subfloors containing moisture generally release it as a vapor. It remains a vapor until it encounters an obstacle that causes it to form droplets or liquid. Liquid generally is what causes warping of wood. It is for this reason that you want a suitable barrier beneath a wood floor that will inhibit the moisture from damaging the flooring. The NWFA recommends a “class 1 vapor retarding membrane over concrete slabs to help minimize the flow of moisture into and through the installed flooring system.” 

Jack Boesch is the Director of Marketing for MP Global Products in Norfolk, Nebraska. He can be reached at jboesch@mpglobalproducts.com

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