Locating the Joists

• Because you’ll have to attach the loose subfloor to the joists beneath, the first step is to determine the location of the joists. If you have a good ear, you can lightly tap the floor with a hammer and listen for a “dull” sound that indicates a joist is just beneath. A stud finder simplifies the process. When you move the stud finder over the floor, it will beep as it passes over a joist. For locating joists, deep-range stud finders work well.
  
  

Nails vs. Screws

• Screws are better than nails for tightening the subfloor to the joists, but using them on a carpeted floor is risky. During insertion, the screw can snag the carpet fibers, causing them to unravel. The safest choice is a casing nail, which has a very small head. Separate as much of the carpet fiber as possible with your fingers before driving the nail in with a standard hammer. Typically, you’ll insert additional casing nails, about 6 inches apart, along the entire length of the joist. You may have to repeat the process on the adjacent joists, if the subfloor is loose there, as well.
  
  

Nail Set

• After each nail is in place, use a hammer and nail set to countersink the nail head. A nail that extends, even a little, above the floor surface can be painful if stepped on with bare feet. Once you countersink the nail head, give the carpet a little tug to pull it up and away from the nail so the fibers lie flat once again.
  
  

Tightening from Beneath

• If you have access to the underside of the floor, you might be able to tighten the boards from a basement or crawl space. A drill or a screw gun allows you to insert wood screws through wood blocks, installed along the side of the joist and to the subfloor from beneath. Add some wood glue to the blocks before attaching them to the joist and to the subfloor. As you tighten the screws with the drill or screw gun, they will pull the subfloor down.
  
  

Carpet Replacement

• The best time to tighten a loose floor is when you re-carpet. At that time, you can use a drill or screw gun and insert wood screws from the top of the subfloor directly into the joists.