Floor joists are wooden beams that run across the framework of a house and support the floors. Once the flooring, such as hardwood, is installed, there is little reason to locate the joists unless you are fixing a squeak. Since any nails or screws you insert in the hardwood floor must anchor into a joist, locating them is the first step in any repair. Although the process can be tricky, once you're armed with a little knowledge about spacing, the task is easy enough to accomplish.
Wrap a clean cloth around the head of a hammer and secure it to the neck, using a rubber band twisted around it two or three times. The purpose of the cloth is to prevent damage to the hardwood floor while locating the floor joist.
Kneel down on the hardwood floor and gently tap the head of the hammer on it, paying special attention to the sound it makes. Move the hammer to the left or the right until the sound changes from hollow to solid. This indicates that you have located a floor joist.
Tear off a small piece of masking tape and place it on the hardwood floor to identify the joist.
Place the end of a tape measure directly on the center of the masking tape and stretch it to the left or to the right for 16 inches.
Mark the next location with another piece of masking tape. Tap the new location gently with the hammer to make sure it sounds solid and not hollow.
Repeat the process of measuring 16 inches and then tapping the floor with a hammer until you've identified all of the remaining floor joists under the hardwood floor.