A knot is a defect in a piece of wood caused by the presence of a branch. In grading lumber knots are classified for their size, form, soundness, and the firmness which they are held in place. Knots are not only unattractive looking they also can weaken the board strength and often act as the starting points for checking. Higher grades of lumber will have fewer and smaller knots.
Dead or Loose Knot
– During the development of a tree lower branches usually die off and are surrounded by subsequent growth around it. Dead branches produce knots in this way that are not attached to the wood and are more likely to fall out.
Tight Knot or Closed Knot
– Tight knots do not affect the woods strength and has a flat face free of openings on the board’s surface. These knots were produced from a living branch that was incorporated into the tree as it grew.
– Spike knots are the result of a branch that runs into the edge of a piece of lumber, growing larger as it nears the edge. These can become raised on a deck surface and can be very sharp and dangerous to step on with bare feet. Do your best to cut these sections out as they can pose the worst problems.
– A knot that is less than ½". Smaller knots are usually sounder and are less visible. They may be present on some higher grades of wood.