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Adhesives and wood flooring

Adhesives and wood flooring
Martin Cummins

Published by Martin Cummins - UK Technical Support Manager, Bostik

Martin has been involved in the industry for over 20 years. He’s passionate about flooring in all its guises, with his particular expertise being in subfloor preparation products and adhesives. Martin is an active member of the CFA manufacturers committee and he sits on the CFA council. He aims to keep abreast of all the changes in the industry and feeds this knowledge back to installers to make sure when floors are laid they perform to their maximum potential.

Most wood flooring manufacturers (for the purpose of this article I’m only referring to fully-bonded timber, not loose lay laminates or batten-bonded systems) will have their specifications and recommendations relating to factors such as the storage and transportation of their products, conditioning requirements before laying, compatibility with underfloor heating, general site conditions, as well as the evenness and humidity of the subfloor.

These should be followed in all cases as they’re there to ensure the wood flooring they’ve manufactured will be suitable for and will perform in the specific application. This is the same for manufacturers of other flooring products such as vinyl, LVT, rubber etc., but it’s easy to understand why, with wood flooring, following the criteria is perhaps even more important.

Wood is a natural product and will, if allowed, change shape and dimension based on external factors. The culprit is always moisture. Maintaining a controlled and constant(ish) moisture content in wood flooring means it will remain in its laid out state, with no cupping, curling or gapping. Moisture isn’t simply a concern owing to the spillage of water onto the floor or dampness from a substrate – temperature is also a controlling factor.

For example, there’ll be a certain amount of moisture in a given air space when the temperature is at say 15oC, on a day where air humidity is at 55%RH (relative humidity). Take the temperature up to 25oC on the same day, which can happen simply by having south facing windows, then suddenly that air can hold a lot more moisture and so can pull it from the surroundings, including the wood flooring, to get to the 55%RH. Similarly, unless you have dehumidifiers in your building then internal humidity can change significantly from day-to-day and season-to-season. If the relative humidity of air changes then moisture levels will too.

Fortunately, the wood flooring manufacturers and adhesive manufacturers such as Bostik understand these issues and look to mitigate them as much as possible.

When it comes to adhesive requirements the equation is pretty similar. How can we ensure the flooring remains bonded even under slight conditional changes and movement? Adhesive choice therefore depends on what may occur to the wood flooring in normal service after installation. If we assume all other fitting requirements are met and that the floor is sufficiently strong and flat to give good contact from wood to floor with the chosen adhesive, the answer lies in understanding what the adhesive needs to do.

Unlike textiles and resilient floor coverings that can be bonded to a floor with deviation owing to their flexible nature, this isn’t the case with wood flooring. The timber will simply bridge across high spots if they’re significantly bigger than the adhesive application depth. Fortunately, most of the higher specification adhesives can be applied in thick layers as they’re usually 100% solids, meaning no significant shrinkage occurs on curing.

Solid wood blocks (by definition small in dimension) and small engineered planks etc. won’t move significantly owing to changes in the room condition, so are relatively easy to stick down. Simply ensure a smooth and even floor is provided, then any wood flooring adhesives from resin alcohol and PUs through to MS polymers can be used.

As the dimensions of the flooring increase, particularly with solid wood flooring, the need to have both a high bond strength and a degree of flexibility in the adhesive becomes essential. A polymer type system can take the stress of the wood flooring and dissipate it into the subfloor, rather than allowing the wood to expand and pull on the smoothing compound, causing it to lift. These flexible adhesives don’t actually prevent movement, their unique ability is more to do with releasing stress and strain.

As different dimensions and species of wood flooring are installed, it’s important to consider the level of performance needed from the adhesive – which basically links to the product’s polymer content. Adhesives are made up of several ingredients, and generally speaking the ingredients that influence the performance parameters are the expensive ones.

It’s similar to silicone sealants where cheap, highly filled products offer very little flexibility and adhesion but might be sufficient around a splash back in a domestic kitchen – yet in vulnerable places with large movement, difficult substrates etc. they’re a waste of time. Therefore higher specification products are also available, at a price, for when jobs demand a bit more.

Adhesives such as resin alcohol products give good, strong adhesion to timber and the subfloor with very little flexibility. These can be thought of as a rigid adhesive designed mainly for small dimension engineered flooring or wood blocks.

High performance polymer systems, such as Bostik WOOD H200 ELASTIC, give a tenacious bond and a high degree of flexibility. They’re suitable for all types of engineered flooring and solid flooring in large widths as well as difficult species, such as beech.

Personally, with the cost associated in purchasing the wood flooring in the first place, I would always choose the highest specification product if I were doing a project for myself. Paying a few extra pounds for peace of mind would be worth it. Of course, from a contractors’ point of view overall unit cost per m2 is important.

So ensure you don’t scrimp and save on adhesives at the risk of the beautiful flooring you are laying coming up at a later date. It’s not only bad for the reputation but re-installing always costs a lot more than doing it right first time.