How To: Purchase Timber at a Merchant
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A couple of things to be aware of:
Timber is sold "sawn ex", which means that a declared sectional size of 38×100mm in the sawn state is that size (to a +1mm -3mm tolerance usually), a planed all round piece of the same size may well be 35×96mm as the planing removed some material after sawing into boards.
Timber is sold in discreet lengths, if you need 3 off 600mm lengths and the merchant has only 2.4m lengths in that size, they will (if they offer a cut to size service) sell you a whole 2.4m length cut into 3 600mm bits and an offcut, but *won't* sell you 1.8m and not charge for the remainder.
If you're getting timber cut to size, give them a cutting list of the sectional sizes and lengths you need, they will pick the least wasteful stock to cut it from. If you specify 2.4m cut into 4 pieces, you may well get 4×598mm pieces, due to the kerf of the blade.
Timber is prone to deform due to changing internal stresses when it's moisture content alters or it is cut, selection of straight grained boards with no obvious tensions guards against this but inevitably it happens, so if you need a critical dimension for a piece of wood, work on the principle that you're going to cut it to say 50mm longer than intended, let it settle in the environment you're using it in then plane down to your final thickness and width, before trimming to exact length.
Also, with the equipment the hackspace has, we can reduce thickness (ideally of pieces less than 260mm wide) and cut pieces lengthways to width, but we can't Re-Saw (cut lengthways through the thickest dimension to give two thinner boards) anything but the smallest of sizes.
Bigger and/or specialist merchants (especially those who deal in hardwood) will Re-Saw and plane timber to your specifications, usually at a cost of £35-60hr labour, which when you want quite unusual sizes or you're buying all your materials in one or two big pieces can be sensible.