FAQ

The surface of the screed needs to be completely clean of any residual adhesive from the previous substrate coating before installing a new wood flooring or flexible floor covering. Otherwise the bond between the flooring and the substrate will not be sufficient. Residues of adhesive on the surface of the screed also make it difficult to test the substrate, which is necessary and standard procedure.

In former times, adhesives containing solvents and PUR adhesives were often applied directly to the contaminated substrate. Usually a good bond is achieved with the coating or adhesive residues. But these adhesive residues present a weak point in the adhesive because its strength cannot withstand the strain of the wood flooring (especially solid wood flooring and floorboards). Old adhesive residues usually do not have the necessary tensile strength which means that as the wood works it separates from the substrate.

According to the manufacturer, only cube or herringbone mosaic patterns or multi-layered wood flooring should be used with pre-mixed screed. Only water-free adhesives should be used for bonding to plaster-based screed plates. Direct bonding of strip wood flooring or industrial wood flooring is thus not possible.

Our experience has shown that the strength of the surface  of the dry screed is completely sufficient for bonding wood flooring. However, the thinness of the screed and the accompanying relatively low flexural strength is problematic. If solid wood flooring of almost the same thickness is to be bonded to the screed, welling up of the entire floor due to seasonal climate fluctuations and the way the wood works needs to be taken into account.

The flexural strength of the substrate and thus also the surface resistance can be significantly improved by adhering and screwing down an additional OSB or particle board layer over the dry screed, which makes it then also possible to install solid wood flooring or floorboards.

The thickness of the particle board should be at least as thick as the wood flooring that will be installed. OSB boards can be somewhat smaller for the same strength. To bond flooring plates to the dry screed, it is recommended that the same adhesive be used as for bonding wood flooring. It is not necessary to use a primer on the pre-made screed.

We recommend using only reactive, water-free adhesive systems for bonding full surfaces. 1 or 2 component PUR adhesive, such as PU-455 or PUK-445 are best, although the SPU adhesive SPU-570 also significantly reduces the dimensional changes of the wood due to climate changes. The screed surface must be sufficiently resistant because the forces on solid floorboards are higher than with multi-layer wood flooring, for example. To reduce strain on the substrate, adhesives with hard elastic adhesion mechanics, such as SPU-460 or SMP-950 are recommended. It should be noted, however, that because of the somewhat higher flexibility in the glue joints, visible deformation (cupping) can occur under very unfavourable climatic conditions, particularly with wide floorboards.

In principle, yes! Mastic asphalt is a thermoplastic substrate and not an absorbent and non-polar one. When laying solid wood flooring, some special peculiarities should be observed: The usual abrading that first assures that many installation materials can be keyed must be done continuously evenly and completely. It should be noted that dispersion-based primers and adhesives cure significantly slower than typical because of a lack of absorption capacity which leads to more water penetrating the solid wood flooring and causing it to deform and change dimensionally.

Since mastic asphalt is a thermoplastic material, it can deform under very high point loads. Furthermore, the mastic asphalt can bulge when there is strong swelling in the solid wood flooring, especially if the installation strength is too low. In addition to the general mandatory testing of DIN 18356 and DIN 18560, a few peculiarities should be observed when laying wood flooring on mastic asphalt. The mastic asphalt must have sufficient distance from the wall when bonding wood flooring. This distance should be at least 10 mm or proportionally more for larger surfaces.

Usually new mastic asphalt is ready for installation and needs no other preparation except cleaning (vacuuming). The planner should adjust screed thickness and resistance classes accordingly. Our experience has shown that a screed thickness of at least 45 mm and resistance class IC15 or, even better, IC10, is sound when combined with 22 mm floorboards.

A cement screed with accelerating additives is a special construction under DIN 18560. There is no generally applicable valid limit for curing (residual moisture). Approval should be given by the accelerator manufacturer (CM measurement), screed installer or even better the planner or owner. There is also detailed information in the trade press as well as in the BEB fact sheet on CM measurement.

No! Wood flooring may be directly bonded to screed with STAUF SMP/SPU or PUK type adhesives without the surface needing to be additionally primed and filled. This approval applies to all properly cured substrates (i.e., resistant, firm, level, clean and dry) such as, for example, cement screed, anhydrite screed (both also for floor heating), OSB or V-100 flooring plates. The primers available for our PUK adhesive are only optional or for use on non-cured substrates otherwise.

In general, a screed cannot be compared to a levelling compound in terms of the surface properties after installation. Above all there are great differences in regards to absorption capacity, resilience and roughness (grain structure)! This is especially true with flexible flooring because even the smallest impurities or unevenness can be detected through the surface of the flooring. This is why all important trade associations recommend trowelling  before installing flexible flooring. Usually trowelling with a squeegee technique is recommended which is the current state of the art.

Excerpt from popular fact sheets: TKB Fact Sheet No. 8: ‘Mastic must always be applied before installing textile or elastic flooring’ and note to DIN 18365 “Flooring work”: “The substrate (...) must be smoothed with mastic.” For carpeting, depending on the backing of the carpet and the later use of it, isolated exceptions may occur, however we also recommend complete trowelling with a suitable levelling compound.

First, the tiles need to be cleaned of all contamination and cleaning residues that could cause a separation layer. Ensure that the cleaning agents used for this purpose do not contain any waxes that can build up a new layer of separation. Then, abrade roughly. If no hollow spots or loose tiles are discovered beforehand, remove any loose material after abrading.

Fill holes with filler compound. The entire surface can be primed with a primer such as STAUF VDP-160. Then either trowel the surface (especially important for flexible flooring) or bond it directly to the tile or stone. Multi-layer wood flooring is preferable because it has a rather low tendency to swell. 

In general, there are many methods for preparing the surface that remove different amounts of material. They are distinguished by the depth and strength of the material they can remove. There are different abrasives, from corundum to copper to diamond, and various types of grinders as well as machines for sand blasting and shot blasting.

Often coarse abrading with the help of a single-disc rotary machine with different types of discs is enough. Have the machine manufacturer advise you on what is best. It is no longer true that it is impossible to abrade (remove) some types of old installation materials. Usually the problem is just not having the right tool to achieve the necessary amount of abrasion.

Since elastic adhesives are soft when there is a firm bond between the wood flooring and substrate, techniques that cut open the adhesive joints are used to remove the flooring. This can be done with a sharp blade that is pushed under the wood flooring layer. Ideally, it should be done mechanically with specially shaped cutting tools and hydraulic, electric or compressed air powered tools. So-called "strippers" which are mostly used to remove textiles or flexible floorings are one possibility.

It is a good idea to cut cross-directionally into the wood compared to the direction of installation. The distance between the cuts depend on the width of the tool or the blade used. Without cutting into the surface, the machine can jam when attempting to remove larger flooring elements. Prying it up with lance-type machines operating on compressed air is usually not possible.

Bonding a covering directly onto an existing raw concrete subfloor is usually difficult. There are two things to consider:

• The concrete needs to be free of separation layers, or the floor must be abraded, ground, or shot blasted.
• There must also be sufficient insulation present especially for ground structures.
• In addition, there needs to be a vapour barrier below the raw concrete layer to prevent moisture penetrating from the substrate.

Without sufficient vapour barrier, moisture will migrate into the flooring. Primers that are ‘only’ vapour barrier are not suitable and should not be used for installing flooring. However, if there is an appropriately sufficient vapour barrier beneath the concrete, it is possible to apply a vapour barrier primer. According to today’s knowledge, a CM-value of 5 CM% is too high.

In general, it is recommended that a vapour barrier be added only starting at 3.5 CM% or 3 CM%. If there is radiant floor heating, up to 3 CM% is okay but 2.5 CM% is better. The only reliable test for residual moisture is, in our opinion, taking larger samples (usually with core drilling) and then taking a Darr measurement. This is especially important because a raw concrete subfloor can give off moisture for years. These details are particularly important when installing floor coverings that are sensitive to moisture and vapour.

It is possible to directly bond floorboards to a concrete subfloor if the surface of the concrete is similar to the surface of a cement screed. Otherwise, the surface needs to be prepared with grinding (sanding is usually not ideal), vacuuming and/or priming. For larger uneven areas, levelling compounds should be used. Note further under point 11. Can I put a barrier to a new (heated) concrete subfloor?

Reactive adhesives in the SMP, SPU series or PUK series are not hazardous when properly applied for either the installer or the user. The products contain only small to no elements that can migrate, such as softeners, and are label free except for the PUK series.

They contain no solvents and have the Emicode EC1RPlus label for ‘very low emissions’ from the Association for the Control of Emissions in Products for Flooring Installation, Adhesives and Building Materials e.V. (GEV). More information is available at the GEV website www.emicode.de. In addition, they are classified as GISCODE RS10 or RU1 (hazardous substance classification from the GISBAU) for solvent free products. The professional association recommends adhesives classified under RS10 and RU1 as substitutes for strong solvent-containing adhesives for bonding wood flooring.

These products also meet the requirements for use in living areas under the AgBB programme (Committee for health-related evaluation of building products) and are approved by the DIBt (German Institution for Building Technology).