Effective Wood Joints

by Ron Genge

When joining woods together it is very important to consider the alignment of the annual grain rings in the wood. All woods move with atmospheric changes in humidity, but not at the same rate. Each species have their own rate of movement but more importantly this rate depends on the alignment of the annual growth rings in the specific pieces being joined. The three types of cuts are as follows:

FLAT GRAIN: This type of wood has the annual rings along the width of the board at from 0 to 30 degrees to the major width of the board.
DIAGONAL GRAIN: This type of wood has the annual rings along the width of the board at from 30 to 60 degrees to the major width of the board.
EDGE GRAIN: This type of wood has the annual rings along the width of the board at from 60 to 90 degrees to the major width of the board.

Shrinkage, or expansion, occurs in wood at a different rate in each of these three types of cuts. Flat grain boards may move to the factor of 8 compared to diagonal grain with a factor of 6, and edge grain with a factor of 4. All these figures relate to the relative movement across the wider dimension. This factor rating denotes the relative degree of movement for that species of wood comparative to the length and width. An example of this might be .1 inch to the foot in length compared to .8 inch for flat grain, .6 inch for diagonal, and .4 inch for edge grain cuts. Therefore, to join an edge and a flat grain piece together, in the wide dimension, would create a joint, which would be very likely to fail. Wherever possible, it is advisable to match the pieces to reduce the risk of joint failure.

However, matching wood grain types alone is not an assurance that a problem will not result. When wood loses moisture it does not do it uniformly throughout the whole piece. The outer layers have more moisture so greater movement will occur in this area. Sap wood, the outer annual rings of a tree, carry the sap to feed the tree. As the tree grows, these layers move into the central areas and eventually become heart wood which carries no sap. Consequently, because the outer annual rings are longer and contain more moisture, they will shrink at a greater ratio than the shorter more inner layers of grain. This causes a cup in the width of the board.

The solid lines denote moist wood, the dotted line denotes a drier state. Therefore, to ensure that the glue line will not fail, it is necessary to consider the alignment of the annual rings. The first sketch shows the right method while the second one shows the wrong method. Arrows are used to indicate the stress on the glue line; one assists while the other detracts from a good glue joint.

Ideally this type of joining of two pieces to obtain a larger dimension should be done by matching the three types of cuts to obtain a more stable joint, as above and below. It is worthy to note that wood towards the outside of the tree will tend to straighten the curvature of the annual rings. This is more noticeable in flat grain wood and decreases as the grain lines move toward the edge grain type.