One of the characteristics of wood that causes great confusion in practical use is moisture movement, or the shrinkage and swelling that occurs when wood takes up water or dries out.
It leads to the problems people sometimes see with windows or doors ‘sticking’ in some seasons yet having clear gaps at other times of year.
If poorly understood, it can also lead to huge forces and failures in larger projects - if no gaps are left for expansion between cladding boards, for example.
Ancient man knew the power of wood swelling in the wet and harnessed it to break large stones using only drills and wooden dowels. Yet modern designers often overlook it, thinking: ‘How much stress can it cause? It’s only wood, isn’t it?!’
Wood has three principal directions, or axes, and each has distinctive properties in strength, but also in response to moisture. This is a result of the wood grain. Long fibres, or cells, are aligned with the length of the tree trunk. In the other two dimensions, they are much shorter and nearly rectangular in shape. The moisture movement in the long axis of the cell, or the grain direction of the wood, is minimal as the stiff cellulose in the cell wall prevents much movement. But laterally, there are regions of other polymers between the cellulose fibrils, which are better able to take up water. When the wood gets wet, this causes expansion of the cells in both directions of the ‘rectangle’ that I just described, causing the whole piece of wood to expand width-wise.
But before we forget, the tree trunk was initially round in section, even if we are working with neatly squared-off planks of wood cut from it.
So the tree has laid down growth rings within the plank, caused by growth conditions in springtime and autumn.
This gives very different properties in what was originally the radius of the tree (or radial direction) and the circumference of the tree (or tangential direction).
For some species, you can clearly see these curved features in the plank, where latewood is perhaps darker than the earlywood.
The radial direction is the line that most directly joins these concentric circles, extending towards the core of the tree (even if that core is in a different plank, after the cutting and processing).
Studies have shown that the swelling due to moisture in the radial direction is in the region of half of that seen in the tangential direction. So in a plank, it is often worth looking to see whether the growth rings are aligned nearly parallel to the face of the plank, or perpendicular to the face.
The swelling for the first case will be about double the swelling seen for the second. I have a door in my house that someone in the past fixed an extra strip onto, to make it fit in the frame. In the summer, it becomes obvious that the extra strip has the opposite alignment to the main door, so the strip is a good 3mm thinner than the rest of the door. In autumn and winter as the wood swells, the difference disappears but the door also begins to stick in its frame.
LIGNIA is a small movement timber
In future I could consider a wood such as LIGNIA to reduce this problem, as its moisture movement is smaller than normal timber.
Why does LIGNIA work? This comes back to the polymers I mentioned that are between the cellulose fibrils in the cell wall. There are infinitesimally small pores amongst these polymers that the water gets into, to cause the swelling for normal wood.
The LIGNIA process impregnates these spaces and cures the treatment agent in situ. This holds the wood in a swollen state, which means there is less opportunity for swelling when the LIGNIA wood becomes wet in future.
Dr Morwenna Spear FIMMM is a Research Scientist at the BioComposites Centre, Bangor