In spite of the availability of installation guidance, codes of practice and a plethora of technical literature, the embarrassing reality is that there are several recurring problems within the wood flooring sector.
Excessive sub-floor or “latent” moisture
This surely has to be the “daddy of them all”, a recurring problem which continues to create havoc within the building process. In truth, it is likely that the problem of latent moisture arises as a consequence of scheduling and time pressures on site, rather than through ignorance. Whatever the reason, the outcome is always the same – excessive moisture uptake and subsequent swelling and disruption of the floor, usually in the form of ridging. There is no excuse for this, given that latent moisture within the slab can be measured hygrometrically to ensure that it is not above 75%rh before the floor is installed. The increasing use of stick-down systems makes it even more important to ensure that latent moisture is kept within allowable limits. In fact, manufacturers recommend that sub-floor moisture should be no greater than 65% or even 60%rh for glued systems.
Lack of, or inappropriate, vapour membrane
It may seem obvious, but it is astonishing how often floor disruption problems arise through an omission of a vapour membrane placed over the sub-floor or where inadequate alternative moisture protection strategies have been used in its place. And how often are stick-down floor systems installed with no moisture protection measures in place, where the contractor has assumed that the application of an adhesive to the sub-floor can double as an effective damp-proof membrane? Nor does a rubber solution applied to the sub-floor with the flooring stuck directly onto it serve as substitute for an epoxy-based DPM inserted beneath a thin surface screed.
Lack of, or inappropriate, pre-conditioning
How many times have we heard the expression “Wood is a hygroscopic material and will either take up or lose moisture in response to its surroundings”? Yet the incidence of in-service distortion and movement of wood floors is as prevalent today as it has always been. The reason may, in part, be due to conflicting views held on the need, or otherwise, of acclimatising flooring prior to laying. It is generally accepted that where boards are factory conditioned to 8-9% moisture content, on-site storage should be kept to a minimum and packs should be kept in their wrappers until required. While this will work for most environments where conditions are dry and stable, where greater in-service moisture values are anticipated, or if the boards have been delivered unwrapped, they will need to be pre-conditioned to the regime expected during service. The need to thoroughly research the anticipated in-service conditions for any given contract is likely to create a mental obstacle for many contractors, although doing so will obviate the risk of in-service swelling or shrinkage.
Rafting, “lightning strike” or cumulative shrinkage
Rafting occurs when the floor behaves as a single unit rather than as individual boards, where in-service shrinkage stresses are built up across the floor and are released along lines of greatest weakness in the form of a single or a few large gaps. The phenomenon usually occurs with floating floors which have been edge-bonded and where drying stresses are relieved by breakage at the weakest joint. The phenomenon is exacerbated in engineered flooring systems, where the groove of interlocking joints consists of end grain that can draw away low-viscosity adhesives from the bonding surface, or through in situ application of some lacquers which can permeate between adjacent boards causing them to stick together. The use of high viscosity adhesives and lacquers and proper acclimatisation will help minimise the risk of cumulative shrinkage.
“Thermal shock”; underfloor heating
Thermal shock is a common cause of excessive shrinkage and distortion of flooring. It happens when newly installed flooring is subjected to abnormally high regimes of heating immediately after its installation. This can be accidental, if the heating system is initially run without adequate primary checks of thermal output, or if the building owner/manager has not been advised to increase the heating gradually, on a “step-change” basis. There are numerous views on the correct preliminary heating regime to minimise shrinkage, although most require an initial period of gradual heating, with the system running between half to two-thirds of its maximum output for several weeks, followed by two to seven days at maximum power prior to installation of the floor. The surface temperature of floors made from engineered boards should reach no more than 27°C. For wider solid hardwood boards, which are more susceptible to changes in moisture, a maximum top surface temperature of 22-24°C may be more appropriate.
For further information see Wood Flooring - A guide to installation available from the TRADA bookshop. Visit www.trada.co.uk/Bookshop
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Latent moisture in the slab should not be above 75%rh before the floor is installed |
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underfloor heating: excessive shrinkage and distortion of flooring happen when newly installed flooring is subjected to abnormally high regimes of heating immediately after installation |
