In many cases construction instructions will state;
make sure the crown of the board is facing up, or possibly another direction.
All straight length boards have a crown. Lumber crowning is a natural occurrence. In some cases the crown is very obvious in other cases you have to look very hard to see it.
What is the crown of a board?
A board crown is the deflection from a flat plane on the edge of a piece of lumber, as shown in Figure 1.
Note: The drawings are exaggerated.
Figure 1 - Board showing crown
Why is it important to know which edge of the board has the crown?
All lumber deflects when a load is placed on it. The amount of deflection is relative to the thickness, depth, length, and species of lumber of the board.
When the board is used in a horizontal position, the builder, in all cases, wants to place the board in a position so that the load is on the crown as shown in Figures 2 & 3.
This applies equally to floors, ceilings, roofs, lintels, girders, beams, cantilevers and rafters.
Figure 2 - Correct position of load relative to the board crown
With a load applied to the crown edge of a board, the board will most likely return itself to a relatively flat plane.
Figure 3 - Incorrect position of load relative to the board crown
If the load is applied to the opposite edge, the board will most likely crown or sag even more.
In vertical positions, such as wall studs, the position of the crown is not as critical. However, it is generally easier to place finish material, such as drywall, on studs when the crowns on the boards all face the same direction.