For adhesive application, sand off sealer and other irregularities.
First, choose a product suitable for the grade level of the slab. Solid wood flooring, engineered flooring, and parquet flooring, can all be installed on “above-grade” or “on-grade” slabs. Engineered products are the only products suitable for below grade applications. Remember, where even one edge of the slab is below the exterior grade level, the entire slab is considered below grade.

Initially, for the wood flooring installation, the jobsite conditions must be suitable for installation. The roof must be completed. All windows and doors must be up and in place with locking mechanisms. Drywall, plastering and other wet work are complete. A near occupied environment should have been established.

The general condition of the slab must be dry, flat, clean and sound.

Dry means the slab has been tested for extra moisture and has passed the manufacturers requirement. Remember to follow the manufacturer’s instructions. Testing should take place under near living conditions, particularly for temperature and humidity levels. Testing under other or adverse conditions will give inappropriate results. The slab should generally be older than 60 days for good results. For testing the slab, some recommendations are: use an appropriate moisture meter, a calcium chloride test kit, phenolphthalein for alkalinity, and a taped sheet of plastic. Using the moisture meter or the calcium chloride kits are generally considered the most trustworthy. The alkalinity test can give indications of a previous high moisture condition, and if positive, the alkalinity itself may interfere with proper performance of adhesives.

Flat means different requirements for different applications. The slab must be flatter for adhesive applications than for mechanically fastened flooring. For adhesive application, an overall variation of 3/16-inch in 10 feet or 1/8-inch in 6 feet is acceptable. The modern adhesives can perform adequately under these conditions. Remember, adhesive is not a leveling compound. For nail-down, mechanically fastened applications, an overall variation of 3/16-inch in 6 feet or 1/4-inch in 10 feet is acceptable. Uneven slabs out of tolerance must be flattened before installation. High areas and rough areas should be sanded, ground, chiseled etc. flat. Low areas should be filled with an appropriate material. Adhesive applications require the fill material to be compatible with the adhesive and have appropriate strength. Check with the manufacturer of the adhesive. Most any material that resists compression, such as vinyl, shingles, etc., can be used to flatten a slab with mechanically fastened applications. If using a cement type product, be sure to give adequate time for introduced moisture to evaporate. Gypsum type levelers retain moisture for extended periods of time.

Clean means get out the broom, flat hoe scraper, razor scraper, buffer with very coarse (16, 20, 24 grit) open coat sand paper or whatever is necessary, and clean the floor. If a sealer was used on the concrete and adhesive is to be applied, remove the sealer. Grind/sand/bead blast it off for good adhesive performance. With adhesive applications, the slab should be especially clean. All the dry wall compound, construction adhesive, paint, etc. should be removed.

Sound means the slab should be of proper strength for the application. For adhesive applications, this includes the term “dusty” slab. Apply an appropriate compatible compound to reduce the dusting. Also, lightweight concrete may not be strong enough to resist the forces of the adhesives and the wood flooring. A mechanically fastened and or floated application would then be an appropriate alternative.

Leveling the slab insures proper flatness prior to installation.
Solid Wood Product Installation
All solid products thicker than 1/2-inch require an appropriate vapor retarder placed on top of the slab. This includes parquet. As a vapor retarder, 6-mil polyethylene, or an equivalent, is considered adequate. For locations that represent high moisture conditions, gluing the poly to the slab is recommended. There are many choices for vapor retarders; remember they must be continuous and elastic and not crack with the inevitable cracks that form in slabs; again, they should be equivalent to the perm ratings for 6-mil poly. Number 15 asphalt saturated felt is not an equivalent retarder. Some adhesives may qualify for vapor retarders. They must generally be applied with a flat trowel and allowed to cure before any additional adhesive is applied.

In addition to the vapor retarder for solid wood thicker than 1/2-inch, a proper fastening base must be placed on the slab. Adhesive application of unidirectional flooring is not recommended. For 1/2-inch flooring, 5/8-inch plywood is the minimum thickness; for 3/4-inch flooring, 3/4-inch plywood is the minimum thickness. Where sleepers are used, 2 x 4s of dense soft wood species (southern pine, douglas fir, larch) spaced no more than 12 inches on center are required.

Remember, these are the basic parameters for slab installation. Many variables such as grade length, sound requirements, heated floors. sports flooring, etc. will modify these basic requirements.