Design

• Most homeowners choose to run the flooring planks in the same direction as the longest wall in a room, but there are many exceptions. If the room is almost square, it may make more sense from a visual point of view to run the boards from the doorway to the opposite wall, or to run them toward a prominent feature, such as a bay window. Moreover, in an installation that covers the entire house, maintaining the same flooring direction in all the rooms can integrate the floor plan. In long rooms and hallways, though, it's usually best to run the boards along their length rather than across their width.
  
  

Stability

• Flooring boards should run perpendicular to the floor joists for stability. This is self-evident if there isn't a subfloor, because there isn't any other way to install them, but this orientation is recommended even when there is one. Plywood can buckle or sag between the joists, and if the floor is running in the same direction and is attached only to the plywood, it will suffer the same fate. Running the boards perpendicular, or at least diagonal, to the joists and nailing them to the joists ensures that they have a solid backing, and will reduce the likelihood of squeaks, separation and other defects.
  
  

Special Subfloors

• Stability is not a concern when the subfloor is concrete. Similarly, older houses that have 1-inch planks running diagonal to the joists can support planks running either parallel or perpendicular to the joists. In either case, design considerations prevail over those of stability with one exception. You shouldn't run the boards parallel to diagonal planks, not because there is any danger of sagging or separation, but because the nails that hold down the boards may penetrate a gap between the boards instead of wood. Running them diagonally in the opposite direction, however, should pose no problem.
  
  

Considerations

• If you want to lay a floor parallel to the joists on a plywood subfloor, you should add an extra layer of plywood. It doesn't have to be as thick as the underlying layer -- 3/8-inch plywood will do -- but the sheets should run perpendicular to the ones underneath, and they should be glued down. This will raise the level of the floor, so you'll need to allow for this in doorways or other places where the floor transitions. Also remember that some flooring orientations, particularly diagonal ones, force you to waste more wood during the installation, so plan accordingly.