It is preferred to run the planks parallel with the incoming light. However, if the room is considerably longer than it is wide, or vice versa, installers usually run the planks parallel to the longest wall. Another thing to consider is that you should run the planks perpendicular to the flooring joists to avoid creating "valleys and hills" in the event that the joists are slightly uneven.

The general rule of thumb is A. Install perpendicular to the flooring joists for added support. B Try to install it parallel to the longest wall. C Install parallel to incoming light.