The extent to which knots affect the strength of a beam depends upon their position, size, number, direction of fibre, and condition.

A knot on the upper side is compressed, while one on the lower side is subjected to tension.
The knot, especially (as is often the case) if there is a season check in it, offers little resistance to this tensile stress.
Small, knots, however, may be so located in a beam along the neutral plane as actually to increase the strength by tending to prevent longitudinal shearing.

Knots in a board or plank are least injurious when they extend through it at right angles to its broadest surface.

Knots which occur near the ends of a beam do not weaken it.

Sound knots which occur in the central portion one-fourth the height of the beam from either edge are not serious defects.
Extensive experiments by the U.S.Forest Service[33] indicate the following effects of knots on structural timbers: [Footnote 33: Bul.