The Sagging Floor That Started With a Drain Line

Why this matters

A bouncy floor and a cracked tile usually get blamed on the tile setter or the house settling. In this case the cause was a notch cut by a plumber, from the same shop now answering the call, nine years earlier. The diagnosis came almost entirely from the shop's own job file: what it said, what it showed by accident in a photo, and what it never recorded. This case is told backwards from that file, because the record is where it was solved and where it could have been prevented.

The call

A homeowner calls about the first-floor hall bathroom. The floor "gives" between the tub and the vanity, and a crack has run across three floor tiles in a straight line over the winter. The bathroom sits over the basement, which had open joists until the owner finished it with a drywall ceiling in 2019. The tile is about eight years old. The office pulls the address and finds two prior jobs: a 2017 bathroom remodel the shop did, and a 2021 callback.

The tech who takes the call is not the plumber who did the remodel. That matters to the story only because nobody on site in 2026 remembers what was done in 2017. What exists is what was written down.

What the file said, read in order

JOB 17-0412  Bath remodel, hall bath, 1st floor
  Relocated vanity drain approx 4 ft toward
  window. New 2 in PVC run in bsmt ceiling
  (open joists) to existing stack. Tied in w/ no-hub.
  Labor 4.5 hr. Photos: 3 (attached)
  Inspection: passed (plumbing rough)

JOB 21-0930  Callback, floor bouncy near tub
  Checked supply and drain under tub at
  access panel, no leaks, no staining. Advised
  customer normal for older house.
  Labor 0.75 hr. Photos: none

Read the first entry as a structural record and it says nothing. "New 2 in PVC run in bsmt ceiling" is a plumbing description: it says where the pipe went, not what the pipe went through. The basement ceiling is the underside of the bathroom's 2x10 floor joists, and since 2019 it has been closed in with drywall. A drain relocated 4 ft across a joist bay has to cross at least one joist, and the entry is silent on how.

The second entry is a correct answer to the wrong question. The tech in 2021 looked for water because "bouncy floor near a tub" sounds like rot from a leak. Finding no leak, the tech closed the call. Nobody asked what was holding the floor up.

The photo that answered the question nobody asked

The three 2017 photos were taken to show the drain fittings to the inspector. Photo 2 is a close shot of the no-hub coupling. In the upper left of the frame, out of focus, the pipe passes through the bottom edge of a joist. The joist has been notched from below, a rectangular bite with square corners, and the pipe sits in it with the joist's remaining wood above.

That photo was never looked at as evidence of anything. It was filed under plumbing. It is the reason the 2026 tech knew where to cut an inspection hole in the basement ceiling instead of opening the whole finished basement ceiling.

Reading the framing through a 12 in inspection hole

The tech cut a 12 in square inspection hole in the basement ceiling, under the photo's position, after scanning for wiring and confirming no duct ran through that bay. Through it, with a light and a tape:

• The members. 2x10 sawn lumber joists, 9-1/4 in actual depth, at 16 in on center, spanning 14 ft 0 in from the exterior wall to a center bearing wall. Sawn lumber, so the sawn-lumber limits apply. See related: Notching and Boring Limits in Wood Framing, which owns them.
• The notch. Bottom edge, 2-3/4 in deep and 3 in long, its center 7 ft 2 in from the exterior wall bearing.
• A split. From the notch's inner corner, a crack running along the grain toward the center bearing, 14 in long.
• The pipe. 2 in PVC, about 2-3/8 in outside diameter, sitting in the notch and strapped to the joist's side beyond it.

The number that mattered, and the one that did not

Run the notch against the limits the IRC sets for sawn lumber (R502.8.1 in the 2021 edition, as adopted by your jurisdiction):

• Location. Notches are prohibited in the middle third of the span. The middle third of 14 ft runs from 4 ft 8 in to 9 ft 4 in. The notch at 7 ft 2 in is close to dead center. Fails.
• Depth. A notch may not exceed one-sixth of the member depth: 9-1/4 divided by 6 is about 1.54 in. The notch is 2-3/4 in. Fails.
• Length. A notch may not exceed one-third of the depth: about 3.08 in. The notch is 3 in. Passes, by a hair.

Two of three fail, and the location failure is the one that explains everything. At mid-span of a uniformly loaded joist, the bending moment is at its maximum and the bottom edge carries the highest tension. That is where this notch removed wood. The remaining depth is 9-1/4 minus 2-3/4, which leaves 6-1/2 in. A rectangular section's bending strength goes with depth squared, so (6.5 / 9.25) squared is about 0.49: the joist at the notch has roughly half its original strength in bending. Its stiffness at that section, which goes with depth cubed, is (6.5 / 9.25) cubed, about 0.35 of original.

The square inside corner of a notch on the tension edge is also where wood splits, because the stress concentrates there. The 14 in split is that mechanism in progress, and it was making the joist weaker each year.

The number that did not matter was the house's age. "Normal for an older house" was the 2021 conclusion. The neighboring joists, same age, same lumber, were fine.

Measuring the sag so the repair had a target

A string line needs the joists exposed bearing to bearing, which a 12 in hole cannot give, and the repair would need the same access, so a 40 in strip of basement ceiling was opened along the notched joist and both neighbors. The tech ran a string along the bottom edge of each joist, using equal 3/4 in blocks at each bearing end so the string sat parallel to where a straight joist would be. At mid-span, string-to-joist distance minus the 3/4 in block gives the sag:

The notched joist sits 3/4 in low against 1/8 and 3/16 in for its neighbors, a difference of at least 9/16 in across 16 in. Tile on a subfloor bridging two joists that far apart cracks along the line between them, which is exactly where the crack ran: parallel to the joists, directly over the notched one.

For context only: the IRC's live-load deflection limit for floors is L/360 (Table R301.7 in the 2021 edition), which at 168 in is about 0.47 in. That limit is for full design live load on a healthy joist, a different condition from a permanent sag in an empty bathroom, so it does not make the joist "code-failing by 0.28 in". What it does say is that the notched joist has moved further sitting idle than a sound one is allowed to move under full load.

The repair, in the order the engineer wrote it

The shop stopped and ran it through its structural approval procedure (see related: The Structural Cut Approval SOP), because a repair to a damaged joist is a structural design, not a plumbing fix. A registered design professional the customer engaged, through the shop, issued a written detail:

1. Move the drain first. Reroute the 2 in drain to run below the joists on hangers inside a boxed soffit in the basement ceiling, keeping 1/4 in per foot of fall, so the reinforced joist carries no pipe. A drain cannot be reinforced around; it had to leave the joist.
2. Jack the joist back toward straight, slowly. A temporary 4x6 beam across three joists under mid-span, on a screw jack on a pad on the basement slab. The detail limited the lift to 1/8 in per day and stopped it with 1/8 in of sag remaining, so the floor finished close to its neighbors rather than forced to them.
3. Sister a full-length 2x10. Bearing on both supports, glued and fastened to the existing joist on the schedule the detail gave, with the split clamped closed and included in the fastening zone.
4. Leave the jack until the sister is fastened. Then release.

The hazard during the repair. A screw jack under a damaged joist is stored load. Nobody stood under the beam while the jack was turned or while it carried the floor; the jack sat on a solid pad, never on a block on its side; and the bathroom above stayed out of use during the five days of lifting, because lifting a floor stresses the toilet flange and the supply connections. Each morning before turning the jack, someone looked at the bathroom for new cracking and checked under the toilet for water. When the drain was rerouted, it was water-tested before the old section came out of the notch, and the supply valves to that bathroom were opened only after the plumber looked at every connection for movement with the lift complete.

How they confirmed it

After the sister was fastened and the jack released, the string line was rerun: the repaired joist read 1/4 in sag, 1/16 in more than the south neighbor's 3/16 in, and stayed there at a check one month later. The bounce was gone when someone walked the bath. The cracked tiles were replaced only after that month, by the tile installer, so new tile did not go down on a floor still moving. The opened ceiling strip and the soffit were closed with the same thickness of drywall.

What the record should have held, and now does

The 2017 job was not a bad plumbing job. It passed inspection, because a plumbing rough inspection looks at the drain, not the joist it sits in. It was a missing line: one sentence saying the pipe crossed a joist and how. Had the record said "notched joist 2-3/4 in at mid-span", the 2021 callback would have gone to the framing in a quarter of an hour. The shop's penetration record now asks, for every pipe that crosses framing, which member, how (bored or notched), the size, and the distance from the nearest bearing. Whether the 2017 notch is a warranty or a liability question is one for the shop's insurer and attorney; the framing question was settled by the file.

References

• International Residential Code, 2021 edition, R502.8 and R502.8.1 (drilling and notching of sawn lumber joists) and Table R301.7 (deflection), as adopted by your jurisdiction
• See related: Notching and Boring Limits in Wood Framing; The Structural Cut Approval SOP; The Penetration Record SOP; How to Patch Drywall After an Access Cut