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My floor is getting a little creaky in my bathroom. Unfortunately, it has ceramic tile over it (along with mortar and backer board). What I need to do is drill a clearance hole through the grout and then put a screw through the subfloor and into the 2x8 boards beneath it, but I need to locate them first. What are my options in locating these 2x8s?

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Do you have access to the joists from below (either as the ceiling on another level or from a basement or crawl space)? –  auujay Oct 18 '11 at 21:32
    
You could try a metal detector, you should be able to detect the screws/nails holding the sub floor to the joists. Do a few sweeps and mark any "hits" with tape, then look for patterns. –  Tester101 Oct 19 '11 at 1:02
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Depending on construction, it could be the cross-braces. I put in a new bathroom floor not long ago, and must have put in at least a couple hundred screws all about 8" apart. It helped, but there are still a couple squeaks which I'm fairly certain are the cross braces between the joists. Just a heads up -- would hate for you to destroy all your grout and not even fix the problem. –  gregmac Oct 19 '11 at 4:10
    
What if the grout lines don't line up with the floor joists? –  aphoria Oct 19 '11 at 12:00
    
@gregmac: true, but you still may be able to see a pattern well enough to determine where the joists are. I agree it's a long shot, but it might work in some situations especially if you know the joist spacing. –  Tester101 Oct 19 '11 at 12:36

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Assuming you do not have access from below, (if you do we have a completely different solution) you need to drill holes in the perpendicular grout line apx 16 inches from the parallel wall to the joists to start with. You may have to drill a couple of holes until you find a joist. Then calculate every 16 inches on center and drive screws. When driving the screws, they should bite and countersink themselves if you have hit a joist. Since you will have to re-grout the seam anyway, strip the grout and find the joists. Even doing this, I am not totally convinced a few screws will solve your problem. The best solution will be securing the subfloors from the bottom.

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Unfortunately, I don't have access to the floor from beneath as this is the bottom floor. I believe I have it narrowed down to where the squeaky nail is, so a few screws might do the trick. I will give your suggestion a try and let you know how it goes in a few days. –  oscilatingcretin Oct 18 '11 at 23:58
    
@oscilatingcretin- Even on the bottom floor if there is any way to get to the underside it would probably work better. Do you at least have a crawl space or is this on a slab? –  auujay Oct 19 '11 at 14:24
    
I am just now getting back to this. Shirlock, does it help that the suspected area is in the corner of the bathroom? Not only would be easier to find the joists, but a diagonally-driven screw would easily hit the frame, right? Edit: A little more about my house: When you walk in the front door, you're in an entry way whose floor is raised off the slab by joists. The bathroom is built on top of this. Walking into the living room, you step down onto the concrete slab. –  oscilatingcretin Dec 23 '11 at 18:26

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