So I have an old house built in the 30's and the hardwood floors creak and crack and I have tenants on the first floor and I'm above them.  I bought the kit with the snap off screws and I'm going to give it a try but is there a stud finder that will work through hardwood or should I just try to screw down the boards with whatever is under them?  Somebody out there must have done this before?

You will have a subfloor to screw into
Finding the rafters from above will be basically impossible

its just carpet. small drill bit and see if you hit a joist. miss, move over, you'll never know. unless you know which way they run you could be there for a while.

Those screws need to go into the joist…not the subfloor. I saw a video on it where the guy used string to lay out the lines of the joist after he found it. Ill look for it.

Here it is

there are no joists under your hardwood floor they are called sleepers.  If you were to rip up the hardwood you will find a wood sub floor that is either face to face or tongue and grove. That sub floor is then nailed into the floor joices below. On top of that sub floor is a 2x2 which is nailed down and the hardwood floor is laid on top of that. Many times the electrical and gas lines are set into this void area.

If you look at the image bellow you can see a modern example of this, just think of the cement as the sub floor.




Now how to fix this. If you can find the sleeper and screw down you will secure any loose hardwood flooring to what it was originally fastened to, This is all good but what if the sleepers are loose? I rehabbed a kitchen in which I ripped out the old hardwood floor and sleepers. I cleaned up the sub floor from all the construction debris and reused the old sleepers because they matched the height of the old flooring the the adjacent room. I made sure to screw down the sleepers in new spots so it would be super secure.... guess what it still squeaks.  If I had to do it over again I would have glued it down and used new wood that I planed down, Live and learn.

I am in the same boat as you because my rental unit above my place has squeaky floors and its annoying but you get used to it. The methods I have seen all point to the snap off screw method but that is for new hardwood flooring which is laid flat on a sub floor of plywood. The method I plan on using to fix it this spring will be to locate the sleepers under the floor and first using the snap screw and see if that will work. How I will locate the sleeper I have no idea, I am going to take my stud finder and make a mock floor in my workshop to test if it will work. If I can locate the sleepers and the snap screws don’t work My next plan of attack is to drill a pilot hole through the floor then counter sink down just enough to hide the head of the screw. After that I will take a long drywall screw and screw down hoping to get into the sub floor pulling everything together. The only problem is the 1 in a million chance that I hit right between the joint of the sub floor, sometimes they can have big gaps. To fill the screw hole I would use a plug cutter and cut out a plug from some old scrap wood I saved or used the sawdust and wood glue to fill the hole. You need to get it down low enough to hide it and make it safe to sand it in the future but not to low that it will not hold the floor down strong.

Its a super annoying problem, I know your pain, old houses have nails and everything squeaks.

I don't now about that as old as the OPs house I bet it is nail right to the sub floor. I have never seen sleepers between the sub floor and the hard wood unless the subfloor was concrete.

The snap off screw kit comes with a "stud finder" kind of long shaft with a screw end.  You are supposed to screw that down into the floor with a drill and then see if it moves or not.  If you go through and can move it up and down you are not in the "stud".  If you can't move it you hit the stud.  I'm going to try that.  So far the screws I've sunk in and snapped off into the "sub floor", if there is one, have not done anything to stop the squeeks.  Since I have the stuff I'll keep trying for a while but I don't have much confidence this kit will help me.  I appreciate the feedback.

....So update this afternoon I managed to get up one of the pieces of hardwood and there is nothing but plywood under it, no joists to speak of.  So I forgot to drill a pilot hole with this snap off screw kit and then realized they hold much better without the pilot hole which was the right diameter as I bought a bit at the hardware store just for this so the hole isn't too big.  Anywho, I drive the screws directly into the wood and snap them off by hand, not using the drill to snap them off and now those areas of the floor are quiet.  I need to order a few hundred more screws.

That is awesome that you had success with it! Did you use a regular drill or a impact driver?

do u have access from below? if so screw from the bottom with screws that are too SHORT to go all the way through the subfloor and hardwood. Like if the total is 2 inch with the subfloor and hardwood use screws that are 1.5inchs.

J-