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Hardwood flooring can be installed successfully over a slab which is on-grade or above grade. Below-grade installations are not recommended. The slab must be constructed properly (dry and flat with a trowel finish).

Watch out for water. New concrete is heavy with moisture, an inherent enemy of wood. Proper on-grade slab construction requires a vapor retarder such as 6 mil polyethylene film between the gravel fill and the slab. While this prevents moisture entry through the slab, this membrane also retards curing of the slab. So test for dryness, even if the slab has been in place over two years. Slabs younger than 60-days are generally too wet for flooring installation.


NOTE: Make tests in several areas of each room on both old and new slabs. When tests indicate too much moisture in the slab, do not install hardwood floors. For a moist slab, wait until it dries naturally, or accelerate drying with heat and ventilation then test again.
1. The Rubber Mat Test. Lay a smooth, non-corrugated rubber mat on the slab, place a weight on top to prevent moisture from escaping, and allow the mat to remain 24 hours. If the covered area shows water marks when the mat is removed too much moisture is present. This test is worthless if the slab surface is other than light in color originally.

2. The Polyethylene Film Test. Tape a one-foot square of 6 mil clear polyethylene film to the slab, sealing all edges with plastic moisture resistant tape. If, after 24 hours, there is no "clouding" or drops of moisture on the underside of the film, the slab can be considered dry enough to install wood floors.

3. The Calcium Chloride Test. Place a quarter teaspoonful of dry (anhydrous) Calcium Chloride crystals inside a 3-inch diameter putty ring on the slab. Cover with a glass so the crystals are totally sealed off from the air. If the crystals dissolve within 12 hours the slab is too wet.

4. The Phenolphthalein Test. Put several drops of a 3% Phenolphthalein solution in grain alcohol at various spots on the slab. If a red color develops in a few minutes, too much moisture is present."

Source NOFMA

NOFMA suggest 4 methods of testing. Three are very low tech: rubber mat, polyethylene film, and phenolphthalein (a controlled substance). The NOFMA does not make any recommendations about moisture detection meters like the Tramex. Moisture detection is one of the most critical aspect of a jobsite success or failure. If you don't test your slab you are playing Russian Roulette. In today high tech world installer should invest in the tools that can ensure successful installation.

The calcium chloride test is the only test that gives definitive results that recommend what type installation can proceed: engineered, nail down, or no wood acceptable without remedial action. I recommend that you use a Tramex meter in conjunction with you calcium chloride test. Use your Tramex to go over the slab then apply the calcium chloride test where your meter has highest readings. These areas will probably be potential problems areas. It is recommended that you do three calcium chloride test per 1000s.f. These test only cost $15.00 each. You cannot go back on a job for the cost of these tests. These test conform to ASTM F1869-04 Standard Test Method for Measuring Moisture Vapor Emission Rate of Concrete Subfloor Using Anhydrous Calcium Chloride this is the only standardized test for checking moisture in slab.

Click Here for testing procedures by NWFA.

If calcium chloride test results 3lbs or under it is okay to put in a wood floor.

If your results are between 3lbs and 7lbs. A vapor barrier is required before installing a wood floor. This means an engineered wood floor cannot be installed directly to a slab. A sub floor with a 6 mil moisture barrier must be installed with plywood before gluing the floor down. Another method is to to create a moisture barrier with a product like Bostik's MVP. Consequently, a traditional nail down floor can be installed with a 6 mil poly vapor barrier under plywood or screeds is acceptable also. Click on screeds to see where the vapor barrier goes with screed systems.

Finally a slab emitting over 7 lbs of pressure needs special attention. Products are available to reduce emission to an acceptable level. The Bostik's MVP is suitable for reducing emissions so an engineered floor can be installed. Bostik does offer a system for nail down floors. This is done on a job by job basis. Contact your sales person to see if Bostik's will written moisture warranty and procedures for your particular job.