Installation over underfloor heating

Above and beyond problems associated with residual moisture, both at subfloor level and penetrating from outside (infiltration, leaks, etc.), flooring that has been glued-down onto a heated concrete slab is subject to significantly greater humidity-related stresses than flooring glued down onto a “cold” subfloor.

These stresses cause the floor to flex at the joints. When it does not occur to excess, such flexing is normal and to be expected.

Flooring compatible with underfloor heating:

Only flooring with a thermal resistance of less than 0.15 m 20 Kw can be used.

Solid wood flooring:

Strip thickness: < = 15 mm

Strip width: Oak: < = 120 mm; Exotic woods: < = 120 mm

Installation method: Full glue-down onto screed, or onto a suitable insulating material using the double glue-down method

Engineered wooden flooring with a multi-layer core:

Strip thickness: < = 20 mm

Strip width: No restrictions

Installation method: Full glue-down onto screed, or onto a suitable insulating material using the double glue-down method

Laminate flooring:

Strip thickness: No restrictions

Strip width: No restrictions

Installation method: Floating installation onto a suitable subfloor

Prerequisites for laying flooring over underfloor heating

Your floor installation will be unsuccessful unless certain essential prerequisites are satisfied: storage conditions, site humidity, subfloor humidity, flatness and expansion space. For more information: Laying wooden flooring: understanding the essential prerequisites for a successful installation.

The underfloor heating must be of the “low temperature” type (up to 28°C). Heat distribution must be uniform, with no temperature variations. It is essential that the concrete or mortar screed is dry, with a moisture level of less than 3%.

Once the underfloor slab has reached a moisture level of 3% or more, the heating up process must commence. This must be carried out slowly and progressively until normal temperature levels are attained.

The temperature must then be maintained for three weeks, no matter what the season.

The heating must be turned off at least 48 h before floor laying commences. The longer the delay, the better the results will be.

Test the concrete/mortar slab for one last time before beginning the floor laying process. Store the flooring strips at the installation site for at least four days before laying.

Laying the floor

Use a polyurethane glue, either solvent-free or solvent-based (one or two-part), exclusively, applying it with a spatula. Avoid using lines of glue: they cause a thermal barrier to be created between the tiles and the flooring.

If installing sound insulation, it must be of a type suitable for use with underfloor heating and must be fully surface glued directly to the screed.

The flooring can only be laid using the full glue-down method, be it directly onto the screed or directly onto the insulation.

You should use the glue at a rate of 1 kg to 1.3 kg/m2.

The concrete/mortar underfloor slab must not be heated until seven days have passed after the floor has been installed and any finishes applied.

Using a serrated spatula, carefully paste the glue onto the subfloor in crisscrossing strokes. Paste no more than one square meter at a time to avoid the glue drying before the floor is laid. This technique is suitable for dry rooms, for humid rooms, and for laying flooring over underfloor heating and/or cooling systems.

Begin laying the flooring from the room’s central axis.

Place the strips in position BEFORE gluing.

Position a rule to mark the placement of the flooring strips as you glue them into place, repositioning it as you progress.

Using a length of wood, press each strip down firmly to ensure complete adhesion to the underfloor.

Use a cloth to remove any excess glue from the edges.

Work on one small area at a time, checking squareness and alignment as you progress, and aim to finish the whole room in one day.

Leave an expansion gap of the recommended size at the edges, along the bottom edges of walls and other obstacles.

Note: If you are gluing down an insulating layer separately (double glue-down), use 1000 g/m², then use a maximum of 1000 g/m² again for the flooring.

Finishing the installation

Hide the expansion gaps at the peripheries by installing skirting or beading matching your flooring.

Fill in the places where the flooring has been cut to fit around door jambs and pipes.

Adjacent rooms

When laying different varieties of flooring in adjacent rooms, use room thresholds as boundaries, or use junctions between different floor sections where present.

When using the same variety of flooring in adjacent rooms, a decorative effect can be achieved by laying a flooring strip in the opposing direction to the rest of the floor at the threshold between the rooms. It is entirely possible to lay 

the same flooring across several rooms as a single area, leaving thresholds unmarked, on condition that the flooring is properly parallel to the walls in each room. Where this is not the case, it is preferable to ensure the flooring is non-contiguous at room thresholds to avoid aesthetically undesirable results.

Finishing the floor 

Factory finished, ready to lay flooring: no specific treatment required unless otherwise indicated by the manufacturer. As a rule, flooring protected with natural oil requires one final and additional protective coat after laying.

Untreated wood flooring: oiled, varnished, stained. There is a wide range of finishes available designed to protect and enhance the appearance of this type of floor.  Before applying the protective finish, it is essential to sand the floor very carefully. This will need to be done at least twice, if not three times in some cases.

Important: it is advisable to apply any final finishes to the floor BEFORE fitting skirting or beading along the edges (wait until completely dry, about 48 h).

Time taken for the flooring to settle after laying: 8 days minimum.

This installation advice is based on French building code DTU 51.2. It is provided for information purposes only and does not entail the liability of Decoplus in any way. The advice given here will, nonetheless, enable you to efficiently carry out the installation of your new flooring yourself. However, if problems are encountered, it is advisable to engage the services of a professional rather than risk irreversibly modifying your newly purchased flooring.