It's nearly impossible to build a room's walls absolutely square, level and straight. There's usually a margin of error, which will affect later work such as laying a hardwood floor. Usually, the builder tries to ease these errors out at the edges of the room or in inconspicuous areas. Some homes also may have walls set at an angle, posing further problems positioning the linear direction of hardwood floors. Some methods are available to solve these problems. The floor installation technique doesn't change, but the line or angle of the floor sometimes does.
Measure the dimensions of the room and transfer these measurements onto 1/4-inch grid graph paper. Sketch out enough of the adjacent rooms to understand their relationship. Note where the flooring changes from tile or carpet to hardwood. Tape down your floorplan on a table.
Draw parallel lines on a piece of tracing paper the approximate width of the flooring planks. Draw the lines so that you can place the tracing paper over your floorplan at any angle and still have flooring.
Place the tracing paper following the line of the longest wall and perpendicular to the floor joists. This is the traditional way to orient hardwood flooring. Keep the lines parallel, and examine each additional wall to see what happens to the flooring at each point. If the room is slightly out of plumb, the deviation will probably be less than a half inch over the distance of the room. This can be dealt with by absorbing the deviation in the board closest to the wall.
Draw a cut line reflecting the deviation at the wall if the rest of the hardwood is kept square to the majority features of the room. Try rotating the tracing paper so that the ends of the hardwood are against the unsquare wall. This option allows you to see if the opposing orientation looks better. The deviation in this option becomes invisible among the dozens of end cuts.
Rotate the tracing paper to align with each wall in rooms that have intentionally angled walls. Often, one alignment will look much better than the others, making deciding your hardwood alignment easy. If no choice seems to work, position the tracing paper at 90 degrees to each wall. This will create different lines, and may make the choice easier.
Install 15# asphalt felt with a staple gun and staple every 12 to 24 inches. Overlap the seams by 3 inches. Snap a chalk line over your felt in the direction and alignment you have decided will work best for your installation. Snap a parallel chalk line 1/2 inch from the wall. You will keep your planks 1/2 inch from the wall to allow for expansion. Mark the positions of the floor joists on the wall.
Rip (cut lengthwise) the groove side of your first row, using a table saw. This means your tongue side will face toward the center of the room. This is for a room where you are absorbing the deviation in the plank closest to the wall. Use a chop saw with a 40-tooth blade to cut your board lengths. No board end should be within 6 inches of another board end. Use your longest boards along the wall.
Drill pilot holes in oak floors close to the wall edge of the planks every 10 inches. Face-nail with 1 and 1/2 inch finish nails and a hammer using a nail set to recess the nail heads into the wood. Blind nail the tongues of the first two rows by drilling pilot holes and nailing with finish nails. Use a piece of scrap flooring and a mallet to tap planks into place. Do not allow a joint between the end of one plank and the beginning of another to fall directly over a seam in the subfloor.
Nail with a wood floor nailer after the third row. Slip the nailer over the tongue of the plank and hit the plunger with a rubber mallet. Drive a 2-inch nail or staple through the tongue into the subfloor. Wedge the last boards into place with a block and prybar and drill and face nail the boards into place.
Allow hardwood flooring to adjust to the humidity at the installation location for up to two weeks for best installation results.