Oliver Rackham, The History of the Countryside
(London: J.M. Dent & Sons Ltd, 1986), p. 67:
The wood therefore yields two products, timber from the trunks of the timber trees, and wood from coppice stools or suckers (plus the branches of felled timber trees). Timber and wood had different uses and are not to be confused; we talk of 'timber' buildings and 'wood' fires. Wood is rods, poles, and logs, used for fencing, wattlework, and many specialized purposes but in large quantities for fuel. Timber is the stuff of beams and planks and is too valuable (and too big) to burn.
In Latin there seems to be a similar distinction between materia
(timber) and lignum
(wood). See Pliny, Natural History
16.76.206 (tr. John Bostock and H.T. Riley):
Next after them comes the cornel, although it can hardly be looked upon as timber [materies], in consequence of its remarkable slimness; the wood [lignum] of it, in fact, is used for hardly any other purpose than the spokes of wheels, or else for making wedges for splitting wood, and pins or bolts, which have all the hardness of those of iron.
ab iis proxima est cornus, quamquam non potest videri materies propter exilitatem, sed lignum non alio paene quam ad radios rotarum utile aut si quid cuneandum sit in ligno clavisve figendum ceu ferreis.
and Justinian, Digest
32.1.55 (Ulpianus, On Sabinus
, Book XXV, tr. S. P. Scott):
The term "wood" is a general one, and is divided into building material [materia] and ordinary wood [lignum]. Building material consists of what is necessary in the construction and support of houses; ordinary wood is anything which is intended for fuel.
Ligni appellatio nomen generale est, sed sic separatur, ut sit aliquid materia, aliquid lignum. materia est, quae ad aedificandum fulciendum necessaria est, lignum, quidquid conburendi causa paratum est.