Lumber Strength, Dimensions, Orientation, and Grain Direction

An explanation of factors that influence the strength of a wood floor joist or rafter. October 1, 2009

The debate is: A 4x6 standing 4 wide and 6 high... Is this stronger with 6" being flat grain or vertical grain?

Forum Responses
(Sawing and Drying Forum)
From contributor Z:
I have always thought that having the 6" side vertical would provide more rigidity and less sag, although I may be wrong.

From contributor G:
With all things being equal, meaning knot locations and sizes, a 4x6 is stronger with the 6" side being vertical due to what is called "moment of inertia". In similar terms this is a measure of objects resistance to rotate.

From contributor A:
Unless it comes a ways from the pith of the log it will be stronger if it comes from a "boxed heart" timber. A good 2x8 will support as much as a 4x6.

From Professor Gene Wengert, forum technical advisor:
The grain direction (flat or vertical) has essentially no effect on the strength. As mentioned, a piece that is 4" high with the load vertically (a 4x6 where the width is 6") is not anywhere as strong as a piece with the 6" being vertical (a 6x4 with the width being 4"). However, when sawing a piece so that the 6" face is vertical grain (or quartersawn), you are likely to get close to or into the pith, compared to sawing the piece so that the 6" faces are flatsawn. Hence, the flatsawn grain, due to the location and shape of the knots (round is stronger than spike and spike comes when quartersawing or vertical grain sawing) will be stronger (in general) if the tree is not real large. For example, a 2x10 will be about 50% stronger when edge loaded than a 2x8.

From contributor S:
I'd think the flat grain being vertical would be more rigid than if it were horizontal. Cut a piece of 3/4" plywood 3/4" wide and see which way it bends. It may not be stronger but it's certainly more rigid. The widest width will give more strength in the direction of load/strain. Thus rafters are always up and down, not flat.

From Professor Gene Wengert, forum technical advisor:
There seems to be some confusion on terms. The grain of a piece of lumber refers to, among other things, how the growth rings are oriented. That is, if the rings (when viewed from the end) run from edge to edge, the piece is flat grain or flatsawn. If the rings run from face to face, then the piece is vertical grain or quartersawn.

So, we can have a piece of lumber that is installed on edge, which is how rafters or trusses would be installed, but the grain within the piece could be either flat or vertical. Vertical or flat grain does not make any significant difference in strength. However, if the lengthwise grain (which now refers to the direction that a split would go lengthwise or from end to end) is not parallel to the sides of a piece, then substantial strength loss can occur. The term used for this grain angle is slope of grain (SOG). You can have strength reducing SOG with either flat or vertical grain patterns. The confusion is that the word "grain" has several different meanings.