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Concrete Slab Moisture Content

Installation of an MFMA flooring system shall not commence until the subfloor is determined to be "dry" by industry standard testing procedures. MFMA does not recommend installation of any maple flooring system over a newly installed concrete slab within 60 days of pour. Concrete slab drying time is affected by the type of construction, concrete formulation, location of the building and climatic conditions. Vapor barriers or under-slab membranes will likely increase slab drying times.

When conducting slab moisture testing, conduct tests in several areas -- regardless of the age of the slab. If the test(s) indicate a wet slab, document the results and delay installation of the maple system.

It is the flooring contractor's responsibility to inspect slab conditions prior to the commencement of any maple flooring system installation to ensure compliance with flooring manufacturer specifications. The use of an electronic concrete moisture meter to determine slab moisture emission is not recommended. Electronic meters measure moisture content in the slab, usually in terms of an overall percentage. Some meters register moisture content only at the surface, while other meters register an average in a vertical direction where used. Metered moisture content values may indicate degree of saturation of a slab, but not the emission rate that is occurring at the time.

Following are several industry-recognized concrete slab moisture tests that provide measurement of emission rate:

1. Polyethylene Film Test
   Tape a 2' x 2' square of 6 mil. clear polyethylene film flat to the slab, sealing all edges with moisture resistant tape. Suspend a 250-375 watt heat lamp 2' above the plastic film. After 24 hours, check the film -- if no condensation or "clouding" develops on the underside of the polyethylene, the test area shall be considered dry enough to install wood flooring.
2. Phenolphthalein Test
   Apply several drops of a 3 percent Phenolphthalein solution in grain alcohol at various spots on the slab. If a red color develops in a few minutes, the slab is too wet to begin installation of the finished flooring system.
3. Calcium Chloride Test
   Use a prepackaged calcium chloride test (widely available) and follow the manufacturer's instructions. The Calcium Chloride Test measures the quantity of moisture passing through a concrete floor. This measurement is stated as pounds of moisture over a 1,000 sq. ft. area during a 24-hour period. An acceptable level is 4.5 lbs. or less.

If you have additional questions, please contact MFMA's Technical Director at 847-480-9138.

Rev. February 2005

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