Timber movement in a garden shed

Yes. it does move. Wood has within in it a lot of water; This is an integral part of the living tree as without it, it would die. When it is chopped down the water starts to evaporate out and, as a proportion of the tree is no longer there, it shrinks.

There is, as ever, a little bit more to it and understanding the process that is taking place enables us to manage the process much better. The tree in its growing form (green) is up to 40% moisture.

This water is held in a tree in two ways:

• In pores within the tree

• As part of the cell structure making up the tree fibre

The first water to evaporate is that held within the pores. When gone this is replaced by air and so the size of the timber remains the same. This initial drying out generally finishes at about 30% moisture, at which point the evaporation continues but now from the cell structure, meaning that the fibre of the tree shrinks.

The structure is vertical up the tree and so any reduction in its size affects the width, not the length (i.e. a 4inch wide x 10ft long board will shrink over the 4 inches but not noticeably over the 10ft). The main movement takes place in the early days of drying i.e. between 30% and 20%. Whilst timber will in time continue drying in external conditions, it normally stabilises at about 17% moisture as at this point it matches the moisture levels in the air.

The movement of timber is quite pronounced, particularly as it initially dries below that 30% level, it will move about 1mm for every 2% of moisture loss. As the shrinkage is along the grain, which does not run in a straight line, this shrinkage will also mean the timber moves around and warps.

If the wood is being machined, not just sawn, such as in the case of a tongue and groove profile, it needs to be at about 20% moisture otherwise the machining is difficult. So at the timber mills, after it has been initially cut into sections, it is kiln dried before machining. External use timber is then generally pressure treated and water is used as the solvent for the copper sulphate, which is the main treatment chemical, and so this process puts water back in which then needs to air dry back out again.

Companies making timber-based products would always be aware of the implications of movement and will be ensuring that timber is at a certain moisture level prior to using.

Each piece of timber used will be moving as described above. Due to the grain variance in each piece the movement will not be uniform. Sheds walls are not generally made of single pieces but lots of pieces of a cladding material fitted together. Generally, the cladding comes in one of two profiles

Overlap - The same process happens to a less obvious degree on lap jointed timber. Lap is normally cut in a triangular profile and referred to as featheredge, each piece is fitted with an overlap with the one below. These pieces would generally be hung horizontally.

The door should be built at a moisture content of 18 – 20% which will mean that the movement is minimal. It is always worth asking what the moisture content is when buying the shed. A local DIY store or garden centre will most probably not know but the manufacturer will. In the event the door is sticking it will be during wetter weather and the answer is simply to remove the bit that is sticking by sanding it down. Do not take off more than is needed as it will shrink again, and you may then have the reverse problem. It is a bit of a pain but be reassured that it is simply a natural process and will calm down once bedded in and should give you years of good service.