If you have a metal detector or can borrow one, it will help you locate where the floor joists are. You will be looking for where the previous nails were sunk into the flooring. You’ll need to locate them as accurately as possible and mark them (ie; placing something on the spot). When you get several, you should see it forming a line. That line should be where the joist is. Spacing between joists is 12inches or 16inches, depending on size of joist. One has to realize though, that this may not be very successful in locating where the joist is because the squeak may be caused by the fact that the contractor missed the joist when nailing the floor. Normally they use ribbed nails when doing this. These nails don’t back out and they are a royal @#$%@! to pull out if you have to. Stud finders use ultrasound, which the carpet will absorb. The ‘hammer’ technique mentioned below – kind of works through carpet.
[quote MANmom]You simply drive the screw into the subfloor, then using a tool like a screwdriver, you break off the mushroom head of the screw, leaving the first head in the subfloor and under the carpet and you can’t see it…simple fix! [/quote]
The problem with this is;
Makes it harder to pull up the mat under the carpet if the carpet is going to be replaced
Leaves potentially sharp ends under the carpet, that you might step on
May leave the appearance of dimples in the carpet because the screw may end up compressing the mat under the carpet
If it is important, I would look at pulling up the carpet. In fact, if you have old carpet – replacing it will help you get a better price on the house. When the carpet is up, you will be able to see where the nails were placed. Stud finders should then work as well. You can also rap(not drive) the floor with the hammer. It will sound different when you are on top of a joist and the hammer will want to bounce back a bit.