The direction you choose to install hardwood flooring in a room has a visual impact and it can affect the overall design and placement of furniture. While the choice of direction is obvious in some rooms like hallways, it isn't in others. It depends a great deal on aesthetics. However, there are constraints dictated by the construction of the house. A floor is more stable if the boards run perpendicular to the joists. If design considerations prompt you to lay them parallel, you may have to add support to the subfloor.
Stand in the doorway to the room in which you are going to lay flooring and visualise the finished floor. Typically, the rule is to lay flooring parallel to the longer walls in a rectangular room, but this is not a hard and fast rule. If the room has a feature, such as a fireplace, it may make more sense to run it in the other direction to draw attention to the feature.
Open the curtains and let the sun shine in the room. The flooring will look better if the boards run in the same direction as the pattern the sunlight creates.
Find the direction of the flooring joists. This is easier to do if they aren't covered, but if there is a plywood subfloor, follow the lines made by the rows of screws. Flooring manufacturers recommend laying the boards perpendicular to the joists, even if there is a plywood subfloor. If you decide to run them parallel, plan on laying a 3/4-inch plywood subfloor, or adding 3/8 inch plywood to the existing one if it isn't thick enough.
Consider laying the boards diagonally or in a bordered or patterned installation, particularly if you plan on doing the installation yourself. It is more work and there will be more wasted wood, but the end result will be more striking than a conventional one.
If you have an older home with a plank subfloor that runs diagonally to the joists, you don't need any more support. You shouldn't run the flooring boards in the same direction as the planks, however, or they might sag or separate. In some cases, you may choose the direction based on ease of installation. For example, if you are continuing a floor from another room, it takes less effort to maintain the direction than it does to create a transition in the doorway.
If you choose to hire a contractor for a diagonal or patterned installation, be prepared to pay higher fees.