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

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

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

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

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?

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.