Installation - Stuck Down Hardwood Flooring

A hardwood flooring is most often installed as a floating floor. However, if you want to reduce, for example, the stamping sound or install larger uniform surfaces, the hardwood flooring can also be glued to the subfloor. When using this installation method, please take into consideration the insulation requirements of impact sounds.

Gluing the hardwood flooring to the subfloor is a more demanding installation method and it is recommended that it should be done by a professional.

A hardwood flooring absorbs impact sounds of approximately 11 dB ∆Lw when glued directly onto the subfloor, or approximately 13 dB ∆Lw when glued onto a 2-mm cork layer stuck to the subfloor.

Subfloor preparation
Technically, the most functional subfloor is a construction plate attached on top of a supporting structure, for example, 9 mm plywood. The sheets are installed at an angle of 45° from the laying direction of the hardwood board.

On concrete subfloors, and especially on subfloors above a damp space, consider the maximum humidity (80% RH or 2.5 weight %) and the surface grinding (= cement glue off, fine mineral aggregate to show) required by the surface strength requirement (1.5 N/mm2), strength category of concrete K30.

Caseine-free screeds are recommended. To make sure that the screed sticks and forms a strong surface, use the additives recommended by the manufacturer. The required evenness of the ground is 3 mm/2000, no dents are allowed.

Floor heating
Hardwood flooring and floor heating complement each other. Installation instructions are available from the manufacturers and salesmen of floor-heating equipment.

When using floor heating, special attention must be paid to correct conditions: the humidity of the air indoors must be 40 - 60 % and the maximum temperature of the floor surface +28 °C. The most even heat emission is obtained with a continuously-operated water-circulation system. This does not raise the temperature too high, as may be the case with so-called storage systems. Continuously used, it also reduces the relative humidity of the air indoors. Wood insulates heat rather well, but in this case, the hardwood flooring gradually adapts to the conditions.

A couple of days before installation, the floor temperature should be lowered to approximately 20 °C, and only returned to normal a couple of days after the installation.

The different species of wood do not have significant differences in thermal conductivity, but the heat is directed more quickly to the surface of a hardwood flooring in a stuck down rather than a floating installation.

Changes in the temperature also affect humidity. Beech and Maple are the most sensitive to changes in temperature and humidity. For this reason, these woods are not recommended for installation over floor heating.

Gluing/Installation

Start laying the hardwood floor in a normal fashion in the direction of the long wall, the walking direction or towards the incoming light. With this installation method, an expansion gap of 5 mm will suffice for fixed structures and penetrations. Expansion joints (abt. 5 mm) are recommended for areas between separate rooms or on uniform surfaces of over 12 m crosswise. Cover the expansion joint with an appropriate skirting. To make the start of the installation easier, secure the start row with wedges or when necessary, shape it according to the wall line. The overlap of adjacent board ends must be a minimum of 50 cm.

Start by applying the glue on the subfloor alongside the boards in an area of one or two rows of boards at a time, using a tongued trowel of 4 – 5 mm that is slightly narrower than the hardwood board.

Installation tip
Install the first two pre-shaped rows by gluing them to each other from the tongue, lift up the plate according to figure and start applying the glue. Place the plate on the layer of glue and check the straightness of the line. Let the glue dry for approximately 30 min and continue the installation in the normal fashion.

First, place the boards as close to each other as possible, the end tongue first. Place the boards in the tongue with a shaped striking block. After this, place them onto the layer of glue. Be careful not to stain the surface of the hardwood flooring with the glue.

It is advisable to let the first rows of boards settle for approximately 30 min before continuing the gluing. Make sure no excess glue is applied in the tongue, because it prevents tight installation. Remove any glue stains while fresh with, for example, white spirit.

Continue installation by fitting the boards in advance. Consider the limitations of time required for the glue to set, particularly when working with penetrations and the last row of boards.

If the board is longitudinally curved or arched, you can cut the ground veneer from a couple of points at an angle of approximately 45°. In this way, the board settles better on the glue surface lengthwise. If some point is loose from the gluing, especially at the start or against the end walls or, for example, because of deviations on the ground, you can use a weight until the glue is dry.

Finally, apply the finishing putty if necessary, and protect the floor with clean, porous cardboard during the other finishing tasks. A pre-finished hardwood floor does not need any surface treatment, as the boards are varnished or oiled at the factory.

Recommended installation glues are solvent-free polyurethane glues. These include Kiilto Flex hardwood flooring glue, Casco Parkett Elastic 3476 and Henkell Thomsit R710(PU) or P618. The total consumption is approximately 1 litre/2 m². For concrete floors in particular, make sure that the glue is suitable for the hardwood flooring. Products intended for gluing the tongues in a hardwood board are not suitable.