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Gapping on Wood Flooring - Part 3 - Cumulative Shrinkage
Rafting is best described as an un-natural or non-random appearance of gapping of installed wood flooring. Cumulative shrinkage can occur in most types of floors including solid strip, planks, wood block and mosaic just to name a few. Sometimes one or several large gaps may appear in a floor. Alternatively, you may see a regular pattern with gaps opening up at consistent intervals. Between the gaps, the groups of boards or “rafts” appear tight together.
Under normal circumstances, shrinkage only results in the appearance of small gaps between boards. Rafting occurs when boards that are bonded together either by the deliberate application of an adhesive to the tongue and groove joint, or by surface finish that penetrates between the boards. This bonds the boards together causing that group of boards to move as one panel.
In these circumstances, shrinkage is transferred across the floor as the boards move together as a single panel. Accumulative shrinkage causes pressure to build progressively across the floor until a gap opens up at the weakest point, usually on a joint. Each gap is the aggregate or total shrinkage of that particular group of boards.
Edge bonding caused by adhesive applied to the tongue and groove.
Edge bonding caused by site-applied surface finish penetrating between elements / failure to fill gaps between boards where appropriate.
Secondary Contributory Factors
A number of factors can contribute to the problem of rafting, but these are secondary and will not cause cumulative shrinkage in the absence of edge-bonding.
The use of a timber specie which has a high degree of movement with moisture loss.
Excess moisture content of flooring materials.
Acclimatising in unsuitably humid / damp site conditions causing increase in moisture content of materials.
Large area of flooring (across the width of the board.)
Excessively dry atmosphere which is anything below 35% relative humidity)
You must be careful when identifying the problem if you want to discover the best solution for rectifying the fault. You must check a number of things including:
Are the gaps occasional large gaps, or frequent small gaps?
Has the product had adhesive applied to the tongue and groove joint?
Has the product been site finished?
Was the product sanded & filled prior to finishing?
Was the product bevelled / chamfered edge?
A) Shows direction of cumulative movement due to side bonding by PVA adhesive (could alternatively be caused by finish between planks).
B) Negligible change in width of strip/ plank.
C) Large gaps are the result of cumulative shrinkage.
Simplified drawing – all flooring is same width, cutting type & specie.(solid)
On Site Diagnosis
If floors have rafting, the large gaps will not be due to excessive shrinkage of individual boards. Measure the boards next to gaps. The shrinkage from the specified size at time of installation will not correspond with the size of gaps, as these are the aggregate shrinkage of a group of boards.
Engineered floating floors products
Is the area of the flooring large across the width of the floors?
Are there any source of environmental heat likely to cause shrinkage, such as under-floor heating or hot water pipes?
Is the moisture content of the flooring excessively dry, indicating moisture loss?
Has the tongue and groove joint been glued?
Have any additional finished been applied on site?
Unfinished (site finished)
Has adhesive been applied to the tongue and groove joint?
Have the gaps between boards been filled prior to finishing?
When carrying out remedial work on rafted floors, always consider whether the boards will expand in the future. It is equally useful to know if the boards are replaced or if the gaps have been filled as well as whether the floor has adequate space to expand?
If unfavourable environmental conditions have contributed to rafting, changes should be made. Given enough time, gaps can reduce in size without other action.
Where a gap has occurred between flooring elements but near a wall, the boards between the gap and the wall may be removed and replaced, in order to close the gap, if adequate provision for expansion is maintained.
Filling may be adequate to reinstate the floor where there are multiple small gaps or a small gap in the middle of the floor. Filling is best carried out when the moisture content of the flooring is at its median point.
Except with oiled or waxed finishes, finishing of the filled area alone is unlikely to produce a satisfactory result, in which case the whole floor area can be fine sanded to create a key, followed by application of two or more applications of finish. Ensure the lacquer of choice will bond to the existing finish.
Avoiding Cumulative Shrinkage
Ensure the moisture content of the timber flooring is suitable for the specific site conditions.
Flooring must be acclimatised to the manufacturer’s recommendation, when humidity levels and sub-floor materials are within the norms for flooring installation.
Flooring should usually be laid in the direction to minimise the dimension across the width of the boards, adhering to the supplier’s recommendations for maximum room sizes etc.
Be aware that sources of heat and strong light can cause shrinkage
When site finishing newly installed products, these should be sanded and all joints thoroughly filled prior to finishing so that finishes cannot penetrate between flooring elements and bond the boards together.
When site finishing a floor where gaps must remain, or where shrinkage is expected, consider using a finish that does not have side-bonding characteristics.
Where using polyurethane finishes, consider using an acrylic primer which can lessen the side-bonding potential of PU finishes