How to Determine Floor Joist Width

Determining the width of floor joists is not at all challenging if you have a basement or a crawl space that provides access to the underside of the floor. However, for homes built on a slab, with carpeting or those with multiple stories, locating the joists can be perplexing. Accomplishing the task requires that you create test holes in the floor and then carefully measure out to the standard widths used when constructing the joists.

Hammer
Cloth (optional)
Drill
Joist finding screw
Masking tape
Tape measure
Pry bar (optional)

Instructions

- 1
  Tap the floor gently with a hammer, listening closely to the sound the hammer makes. When you hear the sound change from hollow to a dull thud, place a piece of tape there since it is most likely a floor joist. If you have hardwood floors, wrap a cloth around the head of the hammer to prevent damage to the floor.
- 2
  Insert a joist finding screw into a drill just as you would a drill bit. A joist finding screw is about 5 to 6 inches long and has a smooth shaft.
- 3
  Place the tip of the screw at the location where you believe a joist is and squeeze the trigger. If you feel the screw slide after approximately 3 inches, there is no joist there. If no joist is located, remove the screw and move it 1 inch to the side and try again until you feel resistance.
- 4
  Place a piece of masking tape on the floor at the location of the drill and then remove the drill from the floor.
- 5
  Place a tape measure at the location of the tape and measure out to either side for exactly 16 inches, which is the distance of standard floor joists. Place a piece of tape in the location.
- 6
  Continue measuring 16 inches from each piece of tape to locate the widths of all remaining joists across the entire floor.

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