Installation and Bonding of Wooden Parquet Blocks

New blocks arrive from the supplier with an even thickness and moisture content. Solid blocks are usually roughly 14 to 22 mm thick. A thicker block can be sanded more times over the life of the floor. A thinner block suits underfloor heating better because heat passes through it more easily. Oak is the most common choice, followed by beech and ash. Beech looks lovely but reacts more to changes in humidity. It moves more, so it is a weaker choice for heated floors.

Original blocks are lifted carefully during restoration. The fitter cleans off old adhesive or bitumen, sorts the blocks and replaces damaged ones. Often part of a tongue or groove has broken off, or some blocks are already too thin after earlier sanding. The work is slower and the result depends on the condition of the wood. In return, the flat or house keeps its original character. If the old blocks sit in black bitumen adhesive, have it tested before anyone starts scraping. Older adhesives of this type can contain harmful substances, sometimes including asbestos. Removing them is a job for a licensed specialist.

Even the finest wood fails on a poor base. The screed must be firm and flat, and it must not crumble or give off dust. A common guideline is a deviation of no more than about 2 mm under a 2 m straightedge. Flatness matters even more with herringbone, because every dip shows at the joints.

The key figure is the residual moisture of the screed. It is measured with the CM (calcium carbide) method or with the hygrometer test, whichever the adhesive manufacturer specifies. For cement screed the usual limit is around 2 CM %, and for anhydrite (calcium sulphate) screed around 0.5 CM %. With underfloor heating, adhesive manufacturers often require even lower values. If the screed is damp, the wood absorbs the moisture and the floor can lift or come unstuck.

Uneven areas are levelled with a self-levelling compound. The fitter primes the subfloor before and after levelling, following the adhesive manufacturer's recommendations. The primer strengthens the surface and helps the adhesive grip. Finally the subfloor is vacuumed thoroughly.

Leave the blocks in the room for at least 48 hours before laying, and preferably several days. The room should be at roughly 18 to 22 °C with a relative humidity of around 45 to 60 %. It should also be free of building moisture from fresh plaster or screed.

Before gluing, the fitter draws up a laying plan. It sets the laying direction, the starting line and how the pattern continues from room to room. Blocks are usually laid along the longer wall or in the direction of the light from the window.

1. Spread the adhesive with the notched trowel the adhesive manufacturer specifies, only over an area that can be covered before the adhesive skins over.
2. Lay the blocks into the fresh adhesive following the laying plan and tap them home with a tapping block so the edges stay undamaged.
3. Leave an expansion gap at the walls, usually around 10 to 15 mm, using spacer wedges.
4. Check flatness, straight rows and tight joints as you go.
5. Cut the edge pieces at walls, door frames and pipes.
6. Leave the floor undisturbed until the adhesive has cured. Depending on the adhesive, you can usually walk on it after 12 to 24 hours, and sanding usually starts after 48 to 72 hours.

The most common adhesives are polyurethane and MS polymer or silane types. More flexible adhesives cope better with the natural movement of the wood, so they suit underfloor heating. Once cured, the floor is sanded in several passes. The joints are filled with filler mixed with the sanding dust, and the surface gets lacquer, oil or wax. These steps are usually listed as separate jobs.

A simple strip or brick bond is the quickest pattern to lay. Herringbone, double herringbone and chequerboard need precise setting out, more cuts and more time. Other things that slow the work down:

• An awkward room with alcoves, columns and many doorways
• An uneven subfloor that needs levelling first
• Hard exotic species that are tougher to cut
• Restoring original blocks, which adds cleaning and sorting

On the other hand, a flat, dry screed, a simple rectangular room and new blocks with an accurate profile make the job quicker.

With underfloor heating, the screed needs a heating-up cycle before laying to dry it out and test it. After laying, switch the heating back on gradually, a few degrees a day. The floor surface should usually not exceed around 27 °C. The exact values come from the parquet and heating manufacturers.

In a flat, check the building rules. Sanding and cutting are noisy, so agree suitable working hours with the building management or owners' association. Replacing a floor covering does not normally need permission. If the work changes the floor structure, check with your local building authority first.

• Bonding onto screed without measuring its moisture
• Skipping the primer, which weakens the bond
• A missing or blocked expansion gap at the walls, which makes the floor lift
• Unevenly spread adhesive, which leaves hollow spots under the blocks that knock or creak underfoot
• Skipping acclimatisation, which leads to gaps opening up later
• No movement joint or threshold strip on large areas, roughly over 8 to 10 m in one direction

Walk over the floor before sanding and again once it is finished, and check that:

• The blocks do not move underfoot and do not sound hollow when tapped
• No edges are raised and the joints are tight
• The pattern is regular and the rows look straight, even when you look along the floor towards the light
• The expansion gap around the edges is clear of adhesive and debris
• Transitions between rooms have a threshold strip or movement joint
• The fitter has given you a record of the measured subfloor moisture and the adhesive used
• With underfloor heating, you have the start-up procedure for the heating in writing