Knots, probably the most recognised growth characteristic of wood, are also the most commonly occurring visual feature in wood after the grain itself. Not surprisingly therefore knots also appear in every timber grading rule, most often classified by size, frequency and type.
In structural grading rules the position of the knot in relation to the edge of the piece of timber and grain distortion around knots are considered. The allowances for each grade have been determined by testing large numbers of pieces to destruction. In end-use specific grades, knots will be limited to those that do not compromise performance in service. In appearance grades, knot allowance has been a progressive development over time, based on the raw material available, mill production and consumer comment. Knots are permitted in most commercially available grades - even in the 'clear' grades, some grading rules permit 'pin' knots to a limited extent.
Knots are often excluded from specifications but the amount of clear wood available from each tree is limited and clear grades are also the most costly. If you must have clear wood, confirm clear wood availability and check species suitability before specifying.
Ubiquitous knotty pine furniture has been a high street feature for many years, but the knotty look has not been a prominent architectural specification. Popular in North America are the 'rustic' grades in both softwood and hardwood. Softwood rustic grades, including only sound tight knots, are ideally suited to exterior use. Hardwoods, with colour and grain variability around knots, provide a unique appearance, for example in flooring. If you wish to be environmentally proactive you must specify your wood accordingly, 'beauty spots' and all!
In the European Standards for round and sawn timber the terminology standards contain definitions for 13 'types' of knot!
John Park runs the Wood Bureau - which is dedicated to increasing the understanding and use of timber.