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I want to run some component cables from one room to another by fishing down the wall into crawlspace and then back up the other wall. When I'm under the house there's no way to tell where the inside of the wall is. I've read about the trick where you pull up the edge of carpet and drill a pilot hole but in my case I just have tile so that doesn't really work for my application. Whats a good way to tell where to drill from my crawl space?

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If it's a framed wall it should have been anchored to the floor with nails, which you may be able to find protruding through the bottom of the floor. They can sometimes be hard to see even when you know where to look, so bring a good light down there. (You could also try dragging a straight edge across the surface until it catches on them.) Try to find some nails on each side of where you want to drill, assume they put most of the nails through the center of the base plate and draw yourself a centerline to drill on.

A second thing you could try if the floor isn't too thick is a stud finder (one with a deep scan mode may be useful here).

A third option if there are no protruding nails would be to get a handful of rare-earth magnets and slide them around until they stick to embedded nails (that aren't visible). If you can get a few of them in place in a line where you were expecting your wall, that can also give you a centerline. (I haven't tried this through floors, but have used it to locate joists in the ceiling through drywall by getting the magnets to stick to the concealed drywall screws. The popcorn texture made it impossible to get a good reading with a stud finder.)

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After 6 and a half months of getting the idea to do this, I finally did it. As it turned out, the first interior wall didn't have any nails but I had already cut out some dry wall in a closet to drill down from above so that wasn't an issue. The other wall did have the nails which helped a ton. Of course my measurement to be in between the correct studs was off so I had to do twice the drilling. At the end of the (long) day I got the job done. –  Dean MacGregor Oct 13 '13 at 4:39
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One way to locate the wall base location in the crawl space is to go ahead and make the cutout for your component cable wall box and cover plates. Such wall plate may look similar to this:

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Then using a long flexy drill bit of this type:

enter image description here

You can then drill down through the wall base into the crawl space. Leave the drill bit in the hole so you can see it in the crawl space.

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