Richard Aylen provides a comprehensive analysis on how wood moves – and why this is normal.

I was recently watching a TV programme about the restoration of Notre Dame cathedral in Paris, following the disastrous fire in 2019, and it inspired me to revisit a topic that I talked about in this column a few years ago.

At Notre Dame the medieval carpenters built a complicated roof structure using ‘green oak’. Green oak is freshly cut from the tree and so it will have a far higher moisture content than any oak flooring or joinery that we have in our homes.

Green oak is far easier to cut and shape compared to oak that has been fully dried. Over time, as it dries, green oak will shrink, but the carpenters of the day were able to accommodate this when they built the structure.

I was impressed by their knowledge of their material and their deep understanding of how the wood moves as its moisture content changes. This story is testament to the fact that our deep understanding of how wood behaves is centuries old, and what a significant part timber has played in our buildings for such a long time.

Therefore, I’ve been surprised a few times when presented with the idea that ‘movement’ of wooden floors – expansion and shrinkage, is something unusual. Surely if medieval carpenters knew about this then we should know too!

Movement of building materials generally is not a new topic. Wood is not the only material that moves in some way or another. Substrate movement often needs to be considered too, because often there will be structural movement to the whole building.

We’re all used to seeing the expansion joints that run through tiled, stone and terrazzo floors for example. Plastics undergo thermal movement, including floor finishes such as vinyl, and this places stresses upon welded seams.

In the context of timber floors, we need to consider moisture movement; – the natural expansion and shrinkage that happens to all wood when it absorbs or loses moisture. This happens not only to wooden floors of course, but windows, doors and other joinery too.

I want to discuss movement in wood floors in general, how it manifests itself in the different floor types we use, and what manufacturers can do to control it. Having heard numerous urban myths and misunderstandings about ‘movement problems’ in wood floors I hope I can present here some of the basic ideas that underpin the reason why we have such a close relationship with wooden floors in our buildings for so many years.

From the outset I will say if you have a good quality, properly designed floor product, of any kind, accompanied by a reliable set of installation instructions, fitted correctly in the right site conditions it will not fail – whatever type of board you choose. I suggest that when you compare the movement characteristics of engineered and solid wood floors, they are certainly different – but at the ‘quality’ end of the market there should not be an inherent ‘movement problem’ with any of them.

In this scenario it’s more helpful to base your selection upon key issues such as environmental credentials, life span and longevity. You may find some significant differences between one product and another when you look into any performance claims and certification offered by the manufacturer.

I could give you lots of examples of ‘good’ and ‘bad’ products, but this would be based on many different criteria, and not simply on the basis of whether the product is a solid or an engineered one.

So, let’s look in a little more detail at what can influence how a wood floor moves with natural changes in humidity.

Manufacturing: the drying process
This stage of manufacture has a significant impact on the overall stability of the floor. The main aim is to limit movement of the wood and prevent splitting and warping.

Before the boards are assembled in the factory excess moisture needs to be removed to match more closely the environment in which the product will be installed. Kiln drying and vacuum kiln drying are widely used for processing hardwoods and softwoods and involves putting the wood into a dry, warm atmosphere to draw out the moisture.

This process can be done with varying degrees of accuracy depending upon how uniform you want the moisture content to be and how much resource the manufacture wants to commit to the process.

Manufacturers of more stable, higher quality products tend to use a longer drying process with quality checks at several stages, so every piece of wood has a very uniform and specific moisture content.

This means that when the floor expands and shrinks, it will generally move less, and any movement will be uniform.

With engineered floors there is less likelihood that the board will curl or delaminate if all the layers within the board have similar movement factors and are dried properly.

Other processes exist, including ‘press drying’. This was developed in the ‘60s by Junckers for its solid hardwood floors and is used for certain types of wood such as beech.

The process is very fast, produces little waste and makes the wood far more stable than can be achieved by traditional kiln drying. It also makes the wood stronger and improves the colour.

If the drying process isn’t carried out properly the boards may curl, engineered floors may delaminate, and large gaps can appear. Many years ago when drying of timber was generally a less precise process the floor would be left to acclimatise on-site for a week or two before installation. This would allow the moisture content to become more uniform, so when the floor was fitted it would expand or shrink less.

In theory this is fine, but it is rarely satisfactory today because it places an onus upon the contractor to decide what expansion gaps to leave – and this will be different depending upon whether the floor is being fitted in summer or winter. Also, on a modern building site the delay and inconvenience of laying out the wood floor for two weeks prior to fitting it is just not a practical proposition.

Board construction
The way the floorboards are designed is important. Achieving a uniformly dry board is a crucial factor in determining whether or not the product can be used with underfloor heating and other more demanding uses.

The design of a solid wood plank and strip floor is relatively simple with little to go wrong provided the boards have sufficient strength and the manufacturer has invested in accurate and controllable drying methods.

For multilayered floors things are a little more complicated because on their own the various layers that make up the board are relatively thin and lack stability and strength.

Laminating them together helps to makes a stronger board. It is also critical that the type and thickness of the layers are chosen correctly. To make a successful product the multiple layers must stabilise one another to prevent the board splitting, warping or delaminating.

Better quality boards will be designed in this way, but it is important to be sure that the layers are chosen to enhance the performance and integrity of the product rather than simply being the lowest cost option.

I’ve seen engineered floor producers simply swap a 3.5mm top layer for one 6mm thick. This produced a board with a disproportionally thick top layer, with relatively thin layers beneath.
If the board was originally stable with its 3.5mm top layer, if you simply replace this with one 6mm thick, you have to ask if this risks making the board less stable. Conversely, if a board with a 6mm thick top layer is stable when new, then sanding it over the years will make it thinner, and it will definitely not be as stable as when the top layer was 6mm thick.

For this reason, an engineered floor with a 6mm top layer will probably not have a lifespan as long as a solid hardwood floorboard, and its long-term stability is hard to guarantee.

A further example is an engineered squash court floor product with a 3.6mm top layer currently available in the UK, that the manufacturer will only guarantee if it is sealed and maintained with lacquer.

Squash courts in the UK are traditionally unsealed, so the requirement for lacquer is quite unusual, but without it the board loses its stability and has been found to split and delaminate.

Expansion allowances and laying instructions, site conditions
In order to address claims as to which boards are the most stable the manufacturer’s installation instructions can sometimes tell you a lot.

Wood expands and shrinks in accordance with changes in humidity and the manufacturer will specify the ideal building humidity range for their product. For a well-designed product the manufacturer’s recommended range will usually be somewhere close to what will be found in a typical occupied and heated building, ie 35% RH to 65% RH.

A range like this, ie 30% from lower to upper limits is quite normal and most manufacturers recommend something similar. There are however some products on the market where the manufacturer’s recommended humidity range is only 20% from top to bottom.

I’d avoid these floors because it may be difficult for the building owner to comply with this, and any guarantee may be invalidated.

Most of the expansion movement that occurs will be accommodated by clear gaps at the perimeter of the floor and all fixed points, and again, this is something that will feature more or less equally in all manufacturers’ instructions.

In fact, you will see similar recommendations from manufacturers of other timber-based floors made from plywood, chipboard, OSB and the like.

To examine the suggestion that some floors move more than others, it may be more useful to look at how they move. Some move more in one direction than another, and you can decide for yourself if this makes a floor ‘high movement’ or ‘low movement’.

I suggest though, such a blunt instrument is not always meaningful or indeed helpful as a basis for choosing or rejecting a particular type of floor. Here are a few examples of how some different floor types will expand and shrink.

Solid floor boards, plank or strip – expansion and shrinkage occurs perpendicular to the grain. There is almost no movement along the grain.

Engineered wood floors, plank, strip, herringbone and other patterns – there is a roughly equal rate of movement both along and across the grain (relative to grain direction is shown on the top surface layer, but it may vary in layers beneath)

Herringbone and basketweave solid wood floors – roughly equal rates of movement to the length and width of the floor.

With a solid timber floor, the boards will move more across the grain than along it, and small expansion gaps can be left between the boards during the installation process to suit the building environment.

This is a feature that makes solid boards very versatile for a wide range of site conditions and building types.

Of course, this is only one part of the process of making a final product selection and you may also want to compare costs, longevity, life cycle cost, the company’s environmental record, their corporate social responsibility policies (CSR), technical support and the like; some of these I have discussed in previous columns.

By understanding the importance of good product design and quality you can see that natural movement of the wood is nothing to be concerned about provided you make the right choices of product and manufacturer.

This will in turn result in a problem free installation for the contractor, a floor that performs well and a happy client.
www.junckers.co.uk
Richard Aylen is technical manager, Junckers