One of my older wood-frame rentals has a hallway that squeaked so loudly the downstairs tenant could tell where his son was standing. I spent a Saturday under that floor with a dial gauge and found the centre joist dropping 4.2mm under my weight, well past the 3mm mark where I stop shimming and start adding real support. Across 15 years and 9 properties I have chased something like 30 of these squeaks, and the biggest lesson was simple: a floor squeak has three different causes, and each one needs a different repair. Guess wrong and you fix the same floor twice.
Three squeaks that sound almost identical
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To a tenant every floor noise is one complaint. Underneath, they are not. A dry, high creak that tracks each footstep is almost always a nail moving against a joist, what the Japanese trades call neda. A deeper groan you feel through your feet as the surface sinks a millimetre or two is joist deflection. A dull knock near a wall or a corner, often with a visible slope in the finish floor, is a settled post (tsuka) or girder (obiki). I once spent an entire afternoon driving screws into a floor that was actually dropping at the post, so now I always measure before I touch a tool. The measuring itself takes about 20 minutes once the hatch is open, and it has saved me far more time than that on almost every job.
Measuring deflection from the crawlspace
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Most of my houses have a floor hatch inside a closet or under the kitchen sink. I bring a headlamp, an offcut of 2 by 4, a dial gauge that cost me about 1,800 yen, and one helper who walks the floor above on my signal. I stand the 2 by 4 on a footing stone or the concrete, rest the gauge tip against the underside of the joist at mid-span, zero it, and have the helper step onto that exact spot. On a sound floor with the joists spanning 909mm between girders I read 0.5 to 1.5mm of movement. Between 2 and 3mm I write it down and re-check in a year. At 3mm or more the answer is a new support, not a wedge. I also press a screwdriver tip into the joist bottom while I am down there: if it sinks in more than a couple of millimetres, the wood is going soft and no amount of stiffening will hold.
The decision table I keep in my repair binder
After enough of these jobs I wrote the logic down so I would stop second-guessing it on my knees in the dark:
- Squeak only, no measurable drop, and the noise stops when the helper stands on the joist near the sound: nail squeak, fixed from below with a screw and a hardwood wedge.
- 2 to 3mm of deflection at mid-span and a wide, springy feel: the joist is undersized or has a knot crack, so sister a fresh joist against it.
- Over 3mm of deflection, or a gap of 3mm or more opening at the top of a post: add a steel adjustable post under the girder and re-level slowly.
- Finish floor sloping more than 6mm over a 3m span with a knock near the wall: the girder or a post has sunk into soft ground and needs a new footing pad first.
- Squeak plus a musty smell and dark, damp joists: stop, and look for a plumbing leak or ground moisture before anything else, because rot changes the entire repair.
Sistering a tired joist
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When the joist itself is the weak link I cut a matching length of 45 by 45mm or 45 by 60mm lumber to run the full 909mm between girders, brush on wood preservative, and clamp it hard against the old joist with their top edges dead level. Then I drive four 65mm screws in a staggered line and run a bead of construction adhesive down the mating face. Material runs about 600 yen per joist. On that hallway I sistered three joists in roughly two hours and the 4.2mm reading fell to 0.8mm. If the bay is too tight to swing a drill, I use structural screws driven at an angle and skip the clamp, holding the sister up with a bottle jack off a block instead.
Steel posts, settled girders, and when I call someone
For a sagging girder I set a steel adjustable post, called a kousei-zuka, which costs about 700 to 1,200 yen each, onto a concrete pad. Where there is no pad I dry-set a 300 by 300 by 60mm precast block on tamped gravel first. Then I raise the girder no more than 2mm a day over three or four days. Hauling a 30-year-old floor level in one pass pops nail heads and cracks the wall plaster in the room above, and I have the repair bills to prove it. I check the finish floor with a 1.8m level between each turn of the post, then lock the nut and add a dab of thread adhesive so vibration cannot back it off. If I find three or more posts hanging free of the girder, or the crawlspace soil is wet enough to hold a boot print, that is past a weekend job and I bring in a foundation specialist. It has happened on only 2 of my 9 houses, and both times catching it early kept the bill near 180,000 yen instead of a full floor teardown at several times that.
The gauge and the offcut of 2 by 4 cost me under 2,000 yen together, and that one measuring habit is why I now fix a squeak once instead of three times. That is the routine I run under every property I own.
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