HIGHVELD WINTER GAPS IN WOOD FLOORS

By the end of winter many people living inland in SA are looking at their wood floors and wondering why there are such large gaps between the floorboards and our phones start ringing off the hook.  This occurs not primarily because of poor installation but because this is what wood does and this has been particularly noticeable this year because of the prolonged hot and dry spell that suddenly arrived after winter as the current period of drought set in.

Even homes of 60-100 years with wood floors will find that there may even be permanent gaps between floorboards which have become filled with dirt over the years but even these gaps will get larger in winter months.

Wood is naturally hygroscopic and contracts when it loses moisture and expands when it absorbs moisture, even the dead piece of tree that has become your floorboard.  As the atmospheric humidity decreases, so moisture is pulled from the wood until it reaches a level of equilibrium. As the wood dries out, it shrinks and thus gaps occur between floorboards and your windows and doors close more easily. This happens to all wood, not just floorboards and includes all types of wood floors whether they are laminate, engineered or solid wood.

The extremely dry Hiveld winters make this cracking or splitting between floorboards quite pronounced and it often causes height variations between boards to further aggravate the problem and can even cause splits in the boards.  

Sometimes floors do not show an even gapping between floorboards because boards become bonded together by adhesive or surface finish that has penetrated between the boards and this causes the shrinkage to be transferred across several boards at once until a large gap opens at the weakest point. This gap is the sum of all the gaps that would have occurred between the group of boards and is known as rafting or panelling.

As the humidity levels increase, the wood absorbs moisture in order to get back to its equilibrium level and as it absorbs moisture, so it expands. Generally this goes on in an unnoticeable fashion but sudden increases in humidity can also cause extreme expansion problems and you may even see your floor developing small ripples in it as the gaps close and the wood continues to expand. This is the reason for leaving expansion gaps around the room which are covered by the skirtings and quarter rounds.

Different species and types of floor behave differently – a wide board solid floor will expand/shrink more than a narrower board; engineered floors less than solid; oak will settle down after one to two seasons whereas maple continues to shrink and swell with every seasonal change.

It is important to understand a little of the science behind this, as here in Johannesburg we have seen humidity levels range from 6% to 80% over a week period as rains have come and are currently around  25% at the time of writing. At the coast, humidity levels remain more constant and it is much easier for a wood floor to remain balanced.

WOOD MOISTURE

A living tree uses water to convey food between the roots and leaves and typically can contain more than 50% water. Some of this water is contained between the cells (known as free water) and the rest inside the cells themselves (known as bound water). Free water can generally be removed naturally once a tree is cut and this process is known as air-drying and will reduce the moisture content to somewhere between 25-35% (the fibre saturation point) depending on the tree.  The bound water is held in the cell by hydrogen bonding and can only be removed by heat and this process is known as kiln drying and will reduce the overall moisture level in the wood to between 8-12%, depending on its application. When the bound water leaves the cell, the wood shrinks.

Wood does not shrink or swell evenly in all directions and will move more in the direction parallel to the growth rings (width of the floorboard generally).

The moisture content of wood below the fibre saturation point becomes a function of relative humidity and surrounding temperature and wood will absorb or lose moisture until it reaches and equilibrium moisture content (EMC). The changes in wood are usually gradual and short term fluctuations in humidity would affect the surface only. The following table illustrates typical wood EMC levels at various temperatures and external relative humidity’s:

Moisture Content of wood in equilibrium at stated dry bulb temperature & relative humidity levels

Source: The Wood Handbook” Wood as an Engineering Material 1999”, Forest Products Library USDA Forest Service

Moisture content changes can be retarded by sealers and sub-floors such as ply wood as well as recommended vapour barriers.

GAP PREVENTION:

In SA, many floor installers are neglectful over taking due care over job site conditions – it is critical that attention is paid to installation MC checks on the floors and substrate of the site as well as due checks on RH levels combined with correct acclimation of floors to the particular sites. One building's internal humidity levels can be very different to the next.

Likewise, most homeowners are not aware of the fact that ideal humidity levels in the home for the protection of their floor should ideally be around the 30-50% level which means using humidifiers in winter on the Hiveld and theoretically, a dehumidifier should be used for those moister times in summer. This does become unduly onerous on the home owner as one does not want to spend one’s time checking RH levels every day but as a rule of thumb, if your skin feels good, so does your floor. When your nose, throat or skin feels dry then your floor is busy shrinking and needs moisturising !

Regardless of what moisture environment a floor has been exposed to, removal or replacement of the floor  because of gaps is usually unnecessary as the replacement flooring material is more than likely to have a different MC to that of the existing flooring that has already begun acclimating to the house.

The best solution is to leave the floor in place for at least one or two full season cycles whereafter it is most likely stable.  Any repairs then felt necessary should be undertaken and gaps filled by wedging or filling and then resanding as required. Filling is problematic if the floors are still moving as it can be squeezed out of the gaps when the boards expand again, creating further unsightly issues.

Factors to consider when laying a floor are as follows:

• Engineered flooring is generally more stable than solid wood
• Narrower boards will shrink less than wide boards for a given change in MC – a 180mm board will shrink/expand twice that of 90mm width. More joints between narrower boards also allows more spaces to distribute movement
• Species – some species of wood are dimensionally more stable than others
• Quarter sawn vs flat sawn floorboards differ in expansion because of the position that they came out of the tree when sawn
• Address the moisture issues - one must accept the physics that there is a direct relationship between temperature, moisture and the resultant Relative Humidity level. Bringing cold dry air into a house and warming it up will further reduce the RH level. To increase this RH level, one must reduce the ventilation or add moisture by means of a humidifier. Adding too much moisture will create further issues and the primary indicator of this will be condensation on the windows.

So what is the real answer to this problem bearing in mind that wood does what wood does and we cannot alter the laws of physics – ignore fashion and use narrower floorboards or opt for engineered floors or accept that winter weather coupled with uncharacteristic dry drought weather causes extreme seasonal gapping which should go away with time as the floor settles.

Wood is a natural product and develops its own character and shape with age which you most likely admired in older houses as part of their charm. Allow it to happen in your own home as the floors follow the laws of nature.

REFERENCES:

“Behaviour of Flooring” – NOFMA - National Wood Flooring Association

Wood Handbook – Ch. 4 “Moisture Relations & Physical Properties of Wood” Forest Products Laboratory, Glass & Zelinka