Diagnosing the Cause Of, and Avoiding Cupping in Solid Wood Flooring
Cupping or dishing in solid hardwood means the boards raise slightly at their edges making the hardwood flooring uneven across its width. The cause of cupping is an imbalance of moisture through the timber. This can be a result of a spillage on the floor, or because of the hardwood flooring gaining moisture from the sub-floor material.
Taking moisture reading of the underside of the timber and comparing this with a reading taken from the upper side will confirm this.
Wood normally expands when it takes on moisture. It therefore follows that moisture on the underside of the wood will make the underside expand laterally. However, the upper side of the wood which has not absorbed any moisture, so this stays at near its original size. The immense strength of the expansion of the underside of the wood causes the edges of the wood curl.
On nearly every incidence of cupping that I have inspected, the flooring was fitted onto a solid floor with adhesive- meaning that the sub-floor was too high moisture content. A quick and simple way of identifying the problem is by taking a moisture reading of the sub-floor. Usually this is just a matter of drilling a small hole into the sub-floor between the gaps in the board, and inserting the probe from a moisture meter. The reading of the sub-floor should be around 9- 11% moisture content. If it is higher, then this may be the source of the moisture.
As part of an inspection process, I also look for evidence of leaks from appliances e.g. dishwashers, and leaks from heating pipes, and poor or zero ventilation in the room.
I also take a reading of the humidity at the time of the inspection, although this is merely a snapshot of one moment in time and does not necessarily give an indication of the humidity at an earlier time that may have caused the cupping.
However, another cause could be that the sub-floor was within the above parameters, but the wood could have been dried too aggressively before being installed. This would mean that the sub-floor was at the correct moisture content, but the wood was too dry to be installed. In a case like this, the humidity needs to be regulated quickly which can in most cases stabilize the problem.
How to Rectify the Cupping
Firstly check the integrity of the fixing of the floor. In extreme cases of buckling, the flooring can (in the case of adhesive installations), de-bond from the sub-floor. If this is the case, the floor is possibly beyond repair if in excess of 40% of the floor area is affected in this way.
Stabilize the humidity in the room as quickly as possible. If the relative humidity of the room is below 40% then this is too dry. A correct humidity level is between 45% and 60% R.H. This can be achieved by the use of a humidifier.
It may then ne necessary to have the floor sanded and re-finished, which should then eliminate any further problems and restore the appearance of the flooring.
It is imperative though, that the stabilization is undertaken gradually over a period, and that thorough moisture readings are taken on both sides of the timber before sanding. If the floor is sanded before the wood has stabilized, the edges of the board will be lower than the middle of the boards.
How To Avoid the Cupping
If you are about to lay a floor using a stick down method, take thorough moisture checks of the sub-floor and the humidity of the room that is to be fitted.
The sub-floor should be ideally below 12% moisture content and the humidity of the room should be between 40% and 60%.
If there is any doubt regarding the integrity of sub-floor, it may be necessary to install an epoxy resin damp proof membrane to the sub-floor.
If you are installing using the secret nailing technique, then an impervious layer should be laid over the sub-floor or battens -e.g. foil, polythene membrane or builders paper.
Make sure that an adequate acclimatization period is allowed for. This is normally achieved by storing the flooring in the room for at least two weeks prior to installation. The room should be heated and ventilated in the same conditions that the floor will be used i.e. temperature and relative humidity.
Final Considerations
1. Tangential cut hardwoods always have greater movement than radial cut or quarter sawn hardwoods.
2. Narrow width solid hardwoods are always more stable and less prone to movement than wider width flooring.
3. Engineered flooring will always cope better with fluctuations in moisture, and climate.
4. Wood floors will always move depending on the season- e.g. in winter months slight opening of joints and cracks are to be expected.