Timber is broadly classified into two groups - softwoods and hardwoods. This can be confusing because the terms do not relate to the relative hardness of the wood but to the type of tree from which it comes. Softwood species come from evergreen coniferous trees, hardwoods from broadleaved trees. Within each group there are many different species of wood. Some are suitable for decking, some are not.
The key factor in selecting wood for use out-of-doors is durability - its ability to resist the conditions that give rise to decay (i.e. wood will start to decay when its moisture content is persistently above 22%).
Some species of wood have a natural ability to resist decay completely; others have varying degrees of natural durability and may require treatment with a wood preservative.
The TDCA recommends that only timber capable of providing a minimum service life of fifteen years should be used. This means selecting:
- a hardwood species that is classified as being naturally 'very durable' or 'durable' or in some cases 'moderately durable'; or
- a softwood species that has been industrially pressure treated with a wood preservative approved under UK Government regulations to the correct standard for its end use.
- a low durability softwood or hardwood species that has been modified with heat (thermal) or chemical treatment to improve its durability.
Softwood is used a lot more for decking because it tends to be less costly and easier to work with than hardwoods. However the rich attractive colours of some hardwoods add greatly to their appeal. Naturally durable hardwoods are usually higher in density than softwood and their impact and abrasion resistant properties are reasons why they are used on commercial projects that have a lot of heavy use.