Introduction: Locating Floor Joists Under Carpet

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.

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.

Step 3: A Custom Bit

Picture of A Custom Bit

I decided to use some 1/8 inch rod I have to make my own bit. The first photo shows the profile of the bit. It does not cut fast, but it also does not usually grab carpet fibers and twist them around the bit. The second photo shows how I held the steel rod on a grinding wheel to make the profile. It was very simple. I used the rounded corner of the grinding wheel to make cupped indentations on opposite sides of my bit.

It may sound like a bad idea to drill holes in the floor, but these test holes are relatively small. For all practical purposes, the carpet fibers close over them. If I do make a second trap door access to our crawl space, I could always fill the test holes with putty.

Step 4: Drilling

Picture of Drilling

I drilled test holes through the carpet and subfloor. I was looking for the bit not to break through the subfloor, but to continue cutting after it normally should have broken through.  I did some preliminary measuring to determine where a joist might be, knowing that the joists are supposed to be 16 inches on center. My best guesses were very wrong.

The tape measure presses against the baseboard trim on the inside closet wall and provides a reference point for future work. I had to drill a series of test holes at one inch intervals. When I was drilling one hole the bit stopped cutting and I heard the sound of my bit rotating against the head of a nail. Finally, I had found the center of a joist.

If you try this process, begin with a very slow drill speed until the bit breaks through the carpet. That is very easy to detect. Slow rotation speed reduces the chance the bit will grab and twist carpet fiber strands. Then drill at a faster rotation speed. This bit cuts more slowly than a twist drill, but its smooth sides do not grab and twist carpet fibers.

The steel rod I used is much softer than tool steel and may well bend if too much pressure is used when drilling. If the bit bends, just straighten it as much as possible by hand. It will still work fine.

Step 5: Which Way Do the Joists Run?

Picture of Which Way Do the Joists Run?

My theory was that all joists under the subfloor run in the same direction in all parts of the house. But, such assumptions can be dangerous and false. My test holes had all been parallel to the yellow tape measure. When I located the center of a joist, I made another test hole at a right angle to the measure tape. The bit did not break through as it should have if it were between joists. The joists in this part of the house run at a right angle to the yellow measure tape, which is consistent with the direction they run in the closet on the other side of the house.

Look closely at the carpet near the yellow tape measure. A number of test hole locations are within the photo, but they are not visible.

Now I need to convince my wife that we need an extra door into the crawl space so I can check the plumbing for freeze protection or for cracked PCV piping. If she agrees, I will need to pull up the carpet in this corner of the room. Then I will need to saw a rectangle out of the subfloor that aligns with the joists. I will need to put the carpet down again and cut around the new door. Then I will need to fasten that piece of carpet to the new door. This all seems like a lot of work, but home repair sites frequently recommend making a new door in the floor to access parts of the crawl space that are otherwise inaccessible.

Comments

OldRaider76 (author)2017-02-15

Heat the drill but with a lighter just before drilling through the carpet. Get it plenty hot. It will melt the carpet fibers and you can drill throu without the carpet ravelling. Leaves an almost undetectable hole. I saw this trick on an episode of This Old House.

Phil B (author)OldRaider762017-02-15

Thank you.

OldRaider76 (author)Phil B2017-02-16

I cut out an access hole today in our old farmhouse built back around 1920 or earlier. We needed an access near the bathroom and main sewer line. I just pulled up the carpet in that closet, We had a nice oak floor under the carpet. Then I marked the outside dimensions of the hole and started drilling small holes with a very long bit about every 1 to 1 1/2 inch apart until I found the floor joist. I was able to cut the hole with one side overlapping a joist. That is kind of tricky because you want to cut through the wood flooring and subfloor without cutting into the floor joist. I cut most of the hole with a Skill saw and finished the corners with a reciprocating saw. I was careful not to cut too deep where I suspected the floor joists were and came back and finished cutting over the top of them with the reciprocating saw laid down as much as possible. The only problem was the subfloor was nailed to the floor joist with some very large nails and they did not come out very easily. I had to remove a few of the tongue-in-groove oak top boards to get the hole out but will put them back and use a finishing nail gun to hold them in place. All in all it went pretty well and now we have a large access hole withing 4 feet of all the major plumbing which is due to be replaced next month. Thanks to the tips I read from others on this forum this project was not too difficult.

bfk (author)2013-09-10

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?

moosetooth (author)bfk2015-10-20

That's how I do it. The straight part of a wire coat hanger chucked into a drill makes a great bit and leaves no visible damage.

Phil B (author)bfk2013-09-10

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.

spiderstones (author)2013-03-17

Good idea !

Phil B (author)spiderstones2013-03-17

Thanks. I hope you can use it someday.

Bill WW (author)2013-01-22

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 WW2013-01-22

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 (author)Phil B2013-02-06

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)pfred22013-02-08

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 (author)Phil B2013-02-09

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.

rimar2000 (author)2013-01-22

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)rimar20002013-01-22

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.

Neolp (author)2013-01-22

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)Neolp2013-01-22

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_dJ (author)2013-01-22

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_dJ2013-01-22

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.

yopauly (author)2013-01-22

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)yopauly2013-01-22

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.

iceng (author)2013-01-21

That is a very good bit idea !
IF you don't keep construction pictures :-)

Phil B (author)iceng2013-01-21

What the customer does not know will not hurt here. ;-)

Phil B (author)Phil B2013-01-22

will not hurt her, not here

iceng (author)Phil B2013-01-22

Slowly :)

KellyCraig (author)2013-01-22

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)KellyCraig2013-01-22

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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Bio: I miss the days when magazines like Popular Mechanics had all sorts of DIY projects for making and repairing just about everything. I am enjoying ... More »
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