Squeaky floors can be incredibly annoying. Fortunately, they're also relatively easy and inexpensive to fix. Most of the time floors squeak because the plywood subfloor is no longer resting on the joists in some spots, so every time you step on a loose spot, the plywood then rubs against a popped up nail. If you're working on a squeaky floor that's on the second floor, you have to find a way to silence it from above. Below are some suggestions for how to accomplish that task.
The easiest way to silence a squeaky floor on the second story is to drive a 1 5/8” self-tapping screw into the joists where the squeaking plywood is. This might require you lifting up any carpeting or removing any other layers of flooring that are in the way.
To do this, you'll first have to find the location of the squeak by walking around on the floor until you hear it. Then you will have to find the joists, which is easier than you may think. Turn on a stud finder, put it on the floor, and then move it slowly in one direction. The stud finder will either beep or flash once it locates the joist. Once you've located the joists, you can then drive the screws into the joists on one side of the squeaky area. Repeat this on the other side of the squeaky area.
Hardwood floors are a little easier to fix, as you can put the nails in as needed until the squeaking stops and then putty the holes with a putty stick.
If you have access to the floors from underneath, this makes your job even easier. Have someone walk around upstairs until you hear a squeak. If the problem ends up being a gap between the joist and the subfloor, you can insert a shim into the gap. Once you've wedged it in and the squeak is gone, apply some wood glue and secure the shim until it's snug. This will keep the floor from bouncing when somebody steps on it, and therefore it won't squeak any more. If the gap is a lot longer than just one small spot, you can add a bunch of shims along the entire joist and then run a bead of construction adhesive. This adhesive should be worked into the gap to stop the squeak.
Other times, the joist might actually be warped. Joists can twist or deteriorate over time, resulting in a space that's opened up between the joist and the subfloor. An easy fix for this problem is to nail a block of wood alongside the warped joist. Then apply a bead of construction adhesive along the top so it's snug against the subfloor. Once that's done, you can nail or screw the board right into the joist.
When wooden floorboards are squeaky, you can add a lubricant to that area. Powdered soapstone, talcum powder, and powdered graphite can be poured between the floorboards. To really work the lubricant in, place a cloth over the boards and walk on it. Once that's done, vacuum up any remaining powder. This will help reduce squeaks caused by wood-on-wood rubbing between planks.
Another easy solution to eliminate squeaks from above is a kit that includes a tripod tool, bit, stud finder, and screws. These screws are wax-coated to drive through carpet without snagging it. The tripod is used to drive the screw through the floor covering and subfloor into the joist.
These kits can be used for carpet, vinyl, linoleum, and wooden floors. While it's easy to hide through carpet, wooden floors will need to be filled with wood filler to hide the holes. Linoleum and vinyl will actually expand to partially cover the hole, but it won't be completely hidden. This means you have to decide if you prefer to live with squeaks or a tiny hole.