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Any installation of wooden flooring over a concrete slab must be carried out only after having a polyethylene vapor barrier on the ground where the concrete slab is laid and on the top of the concrete slab. This is to prevent the moisture penetration from the ground into the concrete slab, which can cause swelling or buckling of the wood floor.

Installation of Wooden Flooring over Concrete Slab

Test on Concrete Slab for Moisture

Before carrying out strip or plank flooring over the concrete slab, it is necessary to carry out a test on the concrete to know the moisture content present in it. As wood is more prone to moisture disruption, a test will ensure the durability of wooden flooring on the concrete surface.

If the concrete floor fails in the test, certain provision must be provided to correct the moisture penetration problem or else the plan for wooden flooring has to be dropped.

The tests mentioned below are the ones recommended by the Oak Flooring Institute of National Oak Flooring Manufactures Association. They have bought the best suitable test from the different practices.

Rubber Mat Test

The test is carried out by laying a flat rubber mat on the slab. The rubber mat should not have any sort of corrugations and a weight is placed on it to prevent moisture loss. Leave the whole arrangement undisturbed overnight.

Next day, if the covered area shows water marks when the rubber mat is removed, it implies that there is trapped moisture within the concrete. The method goes worthless if the slab surface is other than light color.

Calcium Chloride Test

The calcium chloride test makes use of one quarter tea spoon of dry calcium chloride crystals. This anhydrous calcium chloride is placed inside a three inch diameter putty ring on the slab.

This is covered with a glass so that the crystals are completely sealed from the air. It is kept undisturbed for 12 hours. If after 12 hours the crystals dissolve, then it means that the slab is too wet to have hardwood flooring over it.

Phenolphthalein Test

Three percent of the phenolphthalein solution in grain alcohol in drops is put on various spots on the slab. If within few minutes, the red color is developed, then it shows the presence of moist alkaline substance over it. So, it is advised not to have hard flooring over it.

Polyethylene Film Test

A clear polyethylene film measured to one square foot in placed over the slab. The ends are sealed by a tape. The arrangement is left for 24 hours. After this time, if clouding or drops are observed on the polyethylene cover, then the slab is not suitable for wooden flooring. Else the wooden flooring can be carried out.

Concrete Slab Preparation for Wooden Flooring

The two ways to prepare the concrete slab for wooden flooring are the plywood on slab method and method using wood screeds. These methods are commonly used for residential purpose only.

Plywood on Slab Method

A 6-mil polyethylene plastic film is placed over a slab and all the edges are lapped at 4 to 6 inches. The film is laid on the slab and not bonded with the mastic. The 19mm plywood subfloor is laid over the film of polyethylene. It is secured to the slab with the help of a power actuated concrete nails.

The nailing must be started from the middle then to the edges. At least 9 nails should be used per panel. The use of additional nails will make the system stay flat. The figure-1 shows the installation procedure.

Concrete Slab Preparation for Wooden Flooring

Fig.1: Plywood on Slab Method-Installation of Wooden Flooring on Concrete Slab

Then the panels are arranged so that the end joints are staggered at every 4 feet. A space of half an inch is provided at the walls. Between the edges and the ends of the panels, a space of 6mm is provided.

Areas or spots where there is no base or shoe, for example, areas at a door opening, the plywood has to be cut so that it fit within 3mm of the space provided.

Screed Method

The screed method involves the use of asphalt primer to coat the concrete slab. This treatment is allowed to dry completely. Later over the same hot or cold asphalt mastic is applied, almost covering 100 percent of the whole floor area. Then Wood of .1 to 10.2cm screeds of 8 to 48 inches in length are laid over the mastic.

The ends of the screeds are placed by lapping at 4 inches. The Wall ends must have a gap of 1.9cm. The screeds are spaced at 12 to 16 inches. The figure-2 shows the installation as per screed method.

Screed Method of Wooden Flooring

Fig.2: Screed method of placing the wooden strips over the concrete slab

After this, a polyethylene film vapor barrier of 6-mil is placed over the screeds. The ends are lapped at 6 inches. Nailing or stapling the film in place is not appreciated as this procedure will create holes in it. This will reduce its moisture resisting properties.

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About Neenu ArjunVerified

Neenu is a Civil and Structural Engineer and has experience in Design of Concrete and Steel Structures. She is an Author, Editor and Partner at