Flooring adhesives and poor performance
David Hayward, ATFA Technical Manager, takes a look at adhesives in the timber flooring industry and points out some things that need to considered with both product and installation.
IRRESPECTIVE OF whether it is solid timber, engineered or perhaps bamboo flooring there are a range of adhesives that are used to provide part or at times all the fixing.
One of the most common types of flooring adhesive is the moisture cured polyurethane which often comes in ‘sausages’ or ‘pails’ and are referred to by the industry as either the ‘non foaming’ type and the ‘foaming’ type. The ‘non foaming’ type is more flexible and cures to a rubbery like material and is able move to an extent when flooring expands or contracts. The foaming type is more rigid and can have the benefit of filling voids beneath the floor and subfloor reducing the incidence of drummy spots. Due to aspects such as this, some contractors prefer one over the other. Also entering the market are the polymer adhesives which too are flexible and like the polyurethane adhesives also moisture curing (from small amounts of water in the air, subfloor or product). Moisture curing adhesives, particularly in Europe, the USA and to a lesser degree in Australia are also being used which have properties to act as a moisture vapour retarder and with acoustic attenuating properties. So over the years we have seen many advances in flooring adhesives and we can be sure to see further developments in the future.
In most cases when we adhesive fix floors there are no issues and we go from job to job without thinking too much about it. But all products and fixing methods have their limitations so don’t get lulled into a false sense of security. All flooring products differ in terms of their expansion and shrinkage properties under changes in atmospheric humidity and all flooring products also differ in the forces they generate when it comes to expansion.
We need to remember that adhesive fixed floors differ in nature to nail down floors. When a floor is nailed down the floorboard is able to slide over the subfloor it is fixed to. Although the two are connected they act somewhat independent of each other. However, when we full bed adhesive fix a floorboard, it and the subfloor become one. The expansion properties of the floorboard differ from that of the subfloor and the adhesive can transfer very high forces between floor and subfloor.
Have you ever seen an engineered floor take on a crowned shape when laid in humidity conditions that are too high? The crowned shape occurs because the face lamella expands more than the core layer it is glued to and the board arches. This is shown in the first photo. Now when we full bed adhesive fix solid timber flooring on to say a plywood subfloor we are in effect creating an ‘engineered’ product that is the width of the plywood sheet. Just like the face lamella on the engineered floor the timber flooring expands more than the plywood beneath it. Consequently, if humidity conditions are too high the sheet subfloor can buckle off the joists. Photo 2 (Below right) demonstrates the result.
Now even though these failures occurred it does not mean that floors cannot be laid in high humidity environments. However, greater care is needed with choice of product, species and installation method. Note that engineered flooring manufacturers generally have a humidity range suited to their products and that recommended installation practice can also vary depending on environmental conditions. Low and medium density solid timber cannot generate the same expansion force as high density timbers. With solid timber the installation practice may include acclimatisation to reduce expansion forces after installation. So it is important to think about each job, the species and fixing method to be used.
It is also important to realise that particularly when adhesive fixing high density solid flooring the forces pass through the adhesive to the subfloor. It is not just the fixing of plywood subfloors to joists that is important but the likes of plywood subfloors to concrete slabs or if direct to slabs the integrity of the slab surface and any levelling compound used. One further aspect to note is that adhesive fixing of solid timber will also restrain the boards from expanding greater than with say nail fixing. This then generates greater pressure at board edges within a floor. Under such pressure a floor can be more prone to peaking (boards with a cupped appearance having raised board edges) and that high feature grade floors may also be more prone to shearing through the likes of gum veins.
It is important to realise that that we can lay many different types and sizes of timber floors because we have adhesives. However, it is also important to understand what the effects are of using adhesives with different flooring products and the limits of those products and the adhesives being used. It is important to follow manufacturer instructions and use industry recognised practices.