Full Surface Bonding of Wood Flooring - Click/Clack

A floating floor lies on a thin underlay and is not fixed to the subfloor. It goes down faster, but it can click and feel slightly springy underfoot. A bonded floor is fixed firmly to the subfloor, so it sounds fuller and feels like one solid surface.

Gluing makes most sense in these situations:

• Solid boards and parquet blocks: solid wood moves more as humidity changes, and when laid loose it can twist or open at the joints
• Underfloor heating: with no air gap and no underlay, heat reaches the room more easily
• Large, connected spaces: a bonded floor often lets you run from room to room without threshold strips, if the floor manufacturer allows it
• Busy areas: hallways, living rooms or offices, where a floating floor would put strain on its joints over time

Engineered boards with a click joint can also be glued. Whether a particular manufacturer allows it, and with which adhesive, is stated in its technical data sheet. Not every click floor is made to be glued down.

Adhesive only holds as well as the surface underneath it. If the screed crumbles or is damp, the problem will eventually show across the whole floor. The subfloor must be:

• Dry: moisture is measured with the CM (calcium carbide) method, which tests a sample taken from the screed, not just a meter held against the surface. Cement screed usually needs to be around 2 %, calcium sulphate (anhydrite) screed around 0.5 %, and less still with underfloor heating. The exact limits come from the floor and adhesive manufacturers
• Sound: no crumbling layers, cracks or loose patches
• Flat: a deviation of roughly 2 mm under a 2 m straightedge is usually accepted. Anything worse needs a levelling compound
• Clean: free of dust, grease, paint and old adhesive

Anhydrite screed has to be sanded and thoroughly vacuumed. With underfloor heating, the screed is dried out following a heating-up protocol before laying. The heating installer should hand this record to the floor fitter as well.

A primer almost always goes on before the adhesive. It evens out how much the subfloor absorbs, binds remaining dust and improves grip. Use the primer from the system the adhesive manufacturer recommends.

When gluing over an old floor, such as tiles or old parquet, it must be firm, free of hollow spots and degreased. Loose parquet blocks have to come out first, otherwise the new floor will lift along with them.

General-purpose adhesives are not used for wood. In practice you will mostly come across three groups:

• MS polymer and silane adhesives: they stay flexible, take up the natural movement of the wood and contain no solvents. They suit most engineered floors and underfloor heating
• One-component PU adhesives: strong yet elastic, also suitable for solid parquet blocks
• Two-component PU adhesives: very strong and resistant to moisture, used for demanding species and heavier loads. They need accurate mixing and quick work

With exotic species, for example oily woods, manufacturers often specify a particular adhesive. The simple rule: the floor's data sheet and the adhesive's data sheet must agree with each other.

1. Leave the boards to acclimatise in the room where they will be laid, usually for at least 48 hours in unopened packs.
2. Measure the moisture of the subfloor and check that it is sound and flat.
3. Sand and vacuum the subfloor, level it if needed and let it dry.
4. Apply the primer and keep to the drying time given by the manufacturer.
5. Plan the layout: board direction, joint pattern and starting wall. Boards usually run along the longer wall or in the direction the light falls.
6. Spread the adhesive with a notched trowel of the size the manufacturer specifies, only over an area you can cover within the adhesive's open time.
7. Press the boards into the adhesive, join them and knock them tight with a tapping block and rubber mallet. Leave an expansion gap at the walls as specified, commonly around 10 mm.
8. Keep checking flatness and joints as you go, and wipe off any adhesive that squeezes up straight away.
9. Leave the floor undisturbed. Light foot traffic is usually fine after 24 hours, furniture and full load typically after 48 to 72 hours depending on the adhesive.
10. Fit the skirting boards to cover the expansion gaps.

During the work and for a few days afterwards, the room should be at roughly 18 to 22 °C with relative humidity around 45 to 60 %. Underfloor heating is switched off or turned down before gluing, and after the adhesive has cured it is brought back up gradually.

How much work is involved depends mainly on:

• Condition of the subfloor: a flat, dry new screed is the simple case. Old uneven concrete, or removing old parquet and adhesive, adds many hours
• Room shape: alcoves, columns, radiator pipes and lots of door frames mean many precise cuts
• Type of floor: engineered click boards go down faster than solid blocks laid in a pattern such as herringbone
• Species and adhesive: hardwood is slower to cut, and two-component adhesives need more experience

In a block of flats, let the building management or owners' association know in advance, mainly because sanding and removing an old floor is noisy. Some buildings also have rules on floor build-up and impact sound. Replacing a floor covering does not usually need permission, but if it is part of a bigger renovation, check your local building regulations with the local building authority.

• Damp subfloor: the wood draws moisture from below, swells and the adhesive can lose its grip. Measuring moisture is essential
• Skipped primer or dust on the surface: the adhesive then sticks to dust, not to the screed
• Too little adhesive or the wrong trowel: hollow spots remain under the boards and sound dull when tapped
• Open time exceeded: the adhesive skins over and the board no longer bonds properly
• No expansion gaps: even a bonded floor moves slightly and can lift at a wall with no gap
• Wood not acclimatised: boards straight from a cold store keep changing after laying, and gaps can open at the joints

Walk over the whole floor at handover and look at these points:

• No hollow sound anywhere when tapped, and no boards moving underfoot
• Even joints between boards, all pushed tight
• Surface free of adhesive residue, scratches and mallet marks
• An expansion gap at the walls, fully covered by the skirting
• Clean cuts around door frames and pipes
• A record of the subfloor moisture reading and the names of the adhesive and primer used
• Care instructions and clear information on when the floor can take full load and when the heating can go back on