The development in wood as a choice for both commercial and domestic flooring has grown significantly over my time in the industry. Many years ago, the options for a wood floor were pretty much either a wood block herringbone or basketweave design, or a finger parquet pattern. Both these systems are still available today and, if you look around in older schools, churches, domestic properties etc., you will still find them performing well.
There are two reasons why these floors are so successful and have performed for many a year. The first reason is that the timber dimensions are relatively small. This characteristic means that expansion or shrinkage due to changes in ambient conditions (e.g. temperature variations and moisture/humidity fluctuations) has a much smaller effect than on larger dimension timber, as there are many “gaps” which can open or close with little visual impact. In addition, any movement introduces negligible stress on subfloors and the materials below.
The second reason is the types of adhesives used. These were predominantly bitumen based and had a small degree of flexibility, and crucially were unaffected by moisture. This makes these older systems very tolerant to tough, variable environments.
However, bitumen adhesives are not particularly user friendly and, although still available, have been pretty much superseded by alternative technologies. There are resin-based adhesives, which are generally solvent borne and may be used with timber floors (as there’s no introduction of moisture into the timber during installation). However, they offer a relatively rigid adhesive bond and so aren’t suitable for timber of larger dimensions, or difficult species of wood. As a result, the use of solvent-based options should generally be limited to small sized wood blocks or narrow engineered timber. Otherwise, timber movement will simply shear the subfloor up.
The modern adhesive types being recommended these days are neither solvent-based nor water-based. They’re moisture cure, polymer-based systems (also known as MS, PU, SMP) which offer tenacious bonds along with flexibility.
These can be used on a variety of substrates provided they are clean, grease free and, in some cases, textured. There is no necessity for an absorbent base to enable the adhesive to dry out and key, the adhesives simply cure under atmospheric conditions provided there is some moisture present in the atmosphere or substrates. The polymer systems bond extremely well to both the underside of the timber and to the substrates.
They also offer flexibility or deformability to enable larger timber and more difficult species of wood, such as Beech, to be bonded. In a similar way to silicone sealants, the adhesives are made up not just of “active” polymers but also of fillers and bulking out agents. This means not all adhesives are capable of the same degree of adhesion or have the ability to dissipate stresses and strains impacted on them by the timber. It is logical to realise that a wider board of sold timber will put more strain on a floor than a similar board width of an engineered timber. Similarly, a narrower solid board will be easier to bond. The species of timber can also play a part by either being more affected by moisture variation, or by simply being more difficult to adhere.
So how do you decide on which adhesive to use? Well, before offering a response to this I’d like to reiterate an even more important point. Make sure that before installing timber, you have it conditioned in the environment that’s likely to be experienced during the time the building will be in service. It’s easy to say as per the standards that you need 23oC and 50% relative humidity, but the real focus should be on installing in conditions expected to be met in service. So, once this has been assured, what adhesive should you choose?
The more difficult timber species and wider solid boards (wider can be subjective, but I think of ‘narrow’ as 140mm or less, ‘medium’ between 140mm and 180mm, and ‘wide’ being above 180mm) should be bonded with the highest specification adhesives. This means less filler and more polymer. This can normally be determined by looking at the SG (specific gravity) of the product. Filler is heavier than polymer, so as the SG increases then the polymer levels decrease.
When choosing an adhesive, always bear in mind that timber is a prestigious and expensive product, and should therefore be used with preparation and bonding products of a similar quality.
Finally, check on the cure time of the adhesive selected. Depending on your pattern, you may need a very rapid cure adhesive if you need to create a crown line to work from, otherwise you will potentially have to wait until the next day before continuing the bulk of the floor.