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Flooring and Carpeting/preparation of slab for engineered wood floating floor


"preparation of slab for engineered wood floating floor"

hi Chris - I'm getting ready to lay down a wood floating engineered floor over a concrete slab. I started to pull back the carpeting and noticed that there is considerable moisture under the pad. The carpeting has been in place for 10 years with a very good pad, and we haven't had a problem. Anyway, I was going to put down 5 or 6 mil plastic, but now I'm thinking of a sealer on the slap as well to be on the safe side. The slab is on grade, not a basement.

I contacted one sealer manufacturer, bostik, and they recommended 2 different sealers, but said that I should check the amount of moisture coming from the slab. What if I just use the best sealer that I can find?

Won't just plastic by itself be enough? How could moisture penetrate that?

If I want to do a moisture test, is there a way to rent the equipment, or do you have a recommendation how I could accomplish that?

I hope you can give me some direction in terms of preparation before laying the flooring.

I appreciate any help that you can offer!​


What part of California are you in? I hail from the San Gabriel Valley and later moved East to the Inland Empire. All my experience basically comes from working in LA and OC counties. I moved to Utah 5 years ago. I suppose I miss Calif. But not the traffic and congestion.

There are 3 ways that we test the slab for moisture. But remember every season will give varying results.

# 1 quick DIY test with 2x2 piece of plastic and duct tape.
# 2 Calcium Chloride test kit
# 3 professional meters " Moisture Encounter" or WAGNER

#1 example

# 2 example now sold in the big box stores a complete kit with instructions

# 3 METERS 100.00-400.00

The moisture that you noticed after you pulled the carpet bad was it ...did it evaporate..if you do the quick DIY pre test will it return?

Moisture has to escape via transpiration and evaporation. If it is impeded by a barrier such as padding and carpet it is less likely to be a serious problem due to the fact those coverings breath somewhat better than solid products such as wood and vinyl. Vinyl is basically plastic with a felt back and for years moisture was the worst enemy and still is.

Wood was rarely laid on concrete until the advent of PERGO and laminates. This led to a series of new problems for the industry and the installer. Many new standards were written and many new disclaimers from the manufacturer came about.

A plastic barrier was one method to attempt to stop minor issues, typically seasonal. Whereas the moisture would not be constant and would dissipate on its own. But with poor conditions and lack of subsoil moisture barriers.. below the slab... constant moisture would eventually cause serious damage to a wood floor. Many older homes had moisture barriers installed during the 50's and 60's that were now ineffective. Even newer tract homes for example in Chino Hills were built on slopes and had extremely high water tables under the graded compacted soil which led to disaster for retailers and installers.

What I am trying to say is you really need to test and get a PSI reading via the calcium chloride test kit. Once you have the PSI reading you can compare that to the specs of the manufacture and determine the best solution. Manufactures state no more than 3.5 -4 lbs PSI of vapor transmission.

The most appealing solution that I have used is the following product made by Dri-Tac. A 2 part epoxy applied to the floor with a roller ...allowed to dry over night and can withstand moisture up to 12 lbs PSI ... Bostick MPV-4 is my second choice and I use that for direct glue down to the slab. But your install is going to be floating.

Weigh the costs of the products versus the reduction in moisture and make your choice. For example lets say the test results of the calcium chloride kit shows 8-9 PSI vapor transmission in 1 area and 6 PSI in another and 5 in the final area. Your solution must reduce the vapor transmission to an effective 3 lbs PSI overall ...or less.

Please read about Dri-Tac here

Feel free to contact them and ask any questions. I like their products immensely
I like Bostick too ... but in your case I think this would be a product you could just put down and walk away knowing there would be no future issues.

Please get back to me anytime regarding prep and tips



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20+ years flooring installation ...carpet,wood,tile and vinyl. Residential and commercial. I do not sell the products , just install what ever the shop / customer has purchased. I actually love seeing the finished project completed and it gives me great satisfaction to help others acheive that goal. If I don't know the answer I will say so, and then I will recommend another expert for you. I may even research the subject and answer to the best of my ability ...including links to my sources. I wish you all success Chris


20 years as an independant contractor

H.S. diploma A.S. agriculture Mt San Antonio JR. College

Past/Present Clients
Gene Bonas and Kevin Churnock Inc. Sheward&Son&Sons Century 21 Forte Const. Famous Footwear BOSE Corporation Homebodies General Contractor Perry Floors, Tri Cord Flooring T.J.'S Supplies

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