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Locating Floor Joists under Carpet

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Picture of Locating Floor Joists under Carpet
I want to know where the floor joists are inside a small closet because I may need to make an additional trap door to access the crawl space nearby under our kitchen. We have had a frozen waterpipe there and I want to check for damage to the pipe, as well as do what is necessary to keep it from freezing again. I am trying to gather information before I make a decision on whether to add an access door to this part of the crawl space.. Pictured is the trap door in another closet at the other end of the house. I cannot get to the kitchen from the door in the photo because there are two low supporting foundations for load bearing walls located between this door and the kitchen. I would need shoulders less than 14 inches wide to squeeze between the short studs on top of these low support wall foundations.

This Instructable will show how to locate the joists under the subfloor without pulling up the carpet and without damaging the carpet.
 
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Step 1: Tools and materials

Picture of Tools and materials
I tried our electronic stud finder, but it provided no differentiation in readings through the carpet and fairly thick subfloor. I tried pounding on the floor with a hammer and tried to detect any change in the vibrations from the hammer blows or their sound. I even went to the trap door shown in the first photo and pounded on the floor. I know where the joists are there, but I still could discern no difference in the sound when pounding near a joist or between joists.

Materials
  • 1/8 inch steel rod
Tools
  • Electric drill
  • Measure
  • Hammer

Step 2: Drilling to locate joists

Picture of Drilling to locate joists
I wanted to drive some long finish nails to locate the joists, but my longest finish nails are too short to pull out of the carpet again. I decided to drill, but not with a twist drill. Twist drills often pull fibers and leave damaged spots in the carpet. I had an idea for making a better bit.
bfk7 months ago
Great idea. Would drilling up from the crawl space, next to the joist save you the trouble of measuring, guessing, making test holes and additional passes through the carpet?
Phil B (author)  bfk7 months ago
My problem is that I cannot yet get into the crawl space, unless I find the location of the floor joists and make an additional door through the floor.

I did learn from the contractor that my one door at the other side of the house is supposed to serve as my access to the whole house, but I would need to crawl through an opening under the floor that is 11 x 17 inches. The bones in my shoulders are wider than 17 inches.
Good idea !
Phil B (author)  spiderstones1 year ago
Thanks. I hope you can use it someday.
Bill WW1 year ago
Thank, Phil.
"Twist drills often pull fibers and leave damaged spots in the carpet." So you learned that lesson also? When I tried it, I had wound up quite a ball of yarn before I knew what was happening. Will use your tool next time!
Phil B (author)  Bill WW1 year ago
Winding up a ball of yarn with a twist drill in a carpet leaves a bad bald spot that looks like a neighbor with no experience volunteered to cut your hair. My tool did tug a piece of yarn loose, but that was because the rod I used for my bit had a clump of rough, hard paint on it; and the paint clump caught the yarn. The lesson to me is to use a very smooth piece of rod next time.
pfred2 Phil B1 year ago
Does a spade bit wind up carpeting too? I should drill a piece here to find out. Not a piece in my floor mind you, I have some scraps of carpeting lying around.
Phil B (author)  pfred21 year ago
I used 1/8 inch rod for my special bit because it would leave a very small hole. I think a spade bit will leave a bigger hole that ought to be plugged.
pfred2 Phil B1 year ago
The smallest spade bit I have in a set is a quarter of an inch one. Although the bits are made much smaller. The smallest bits made in the world are of the spade style that I know of. At some diminishing point it just doesn't pay to try to put a twist in the shank of a drill. People drill with nails to start them so that makes me wonder if you couldn't have just used a common nail? You said your subfloor is pretty thick though.
rimar20001 year ago
Maybe if you put in the hole a curved steel wire and you turn it around, it is easier to detect the position of the joist, at least tentatively.
Phil B (author)  rimar20001 year ago
That is a good idea. I was trying to keep the hole very small and the wood under the floor is about 35mm thick, so it would be difficult to get enough of an "L" shape to feel the joist.
Neolp1 year ago
Great Idea, this method seems very easy, although I would advise caution if you are uncertain how your wiring and plumbing are run under the home.
Phil B (author)  Neolp1 year ago
Those are good points. I was checking for joist location in a corner of the house where the wires are in the walls and the water pipes would be a few feet away under the next room. I am thinking and hoping no furnace ducts are where I would like to make an access opening to the crawl space.
Jim_dJ1 year ago
Good stuff, Phil. Thanks for sharing. Years ago I tried unsuccessfully to fix a squeaky floor by screwing the subfloor to the joist.
Another idea: Instead of creating a smooth-side bit, I wonder how using a straw as a sheath around a regular bit would work. I'm imagining a soft drink type straw from McDonalds, trimmed to length and then crushed as the bit goes in.
I don't have a grinding wheel, so I'll give that a try sometime and let you know how it works. Now that I think about it, maybe even a single layer of masking tape would help with the fiber binding...
Phil B (author)  Jim_dJ1 year ago
Thanks. You have an interesting idea. Hobby shops and hardware stores sell brass tubing in quite a variety of diameters. The right size tube on the right drill might work.
yopauly1 year ago
There are electronic stud finders the have settings for a standard thickness and a thicker wall. It worked for the shared wall (1.5"?) of the duplex I live in. I believe I payed 15 bucks for it and has come in very handy. I also finds live electrical wires. 8-O
Phil B (author)  yopauly1 year ago
The stud finder I used claims to have a deep reading (as well as the normal setting) for penetrating thicker material, but it did not give me any indications I could use. It is a Sears Craftsman and we have had it for quite a few years.
iceng1 year ago
That is a very good bit idea !
IF you don't keep construction pictures :-)
Phil B (author)  iceng1 year ago
What the customer does not know will not hurt here. ;-)
Phil B (author)  Phil B1 year ago
will not hurt her, not here
iceng Phil B1 year ago
Slowly :)
dejure1 year ago
Presuming you cut the carpet in the same pattern and size as your trap door, will you be using double sided carpet tape to hold the carpet in place?

Of course, you may find it helpful to seal the wood [with varnish, shellac or some other finish] to get a good surface adhesion for the tape.

As usual, good job
Phil B (author)  dejure1 year ago
Thank you. I looked at the existing trap door. The subfloor is about an inch and a half of chipboard. The edge was not finished. I do not know what was used to fasten the carpet to that trap door, perhaps a glue and/or tacks. It puzzles me that the underside of the whole house is not accessible from the one access door. There is also no way to access the underside of the house from the outside. Making an opening from the outside is not an option, either.
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