Why Small Movement Timber is Important in Joinery

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?!’

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).
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