How should I handle replacing the damaged subfloor where the floor joist was cut for the toilet drain pipe in my new home?
Asked by Minerva Maestas 
5 months ago
Last Updated: June 24, 2024
What’s the best way to go about replacing this part of the subfloor? I just bought a house and found out after the home inspection that the plumbers had removed part of the floor joist to install the toilet drain pipe (I’m guessing this was done during construction). I’m planning to reinforce the joist and had a plumber move the drain so there’s room to add a new joist, but I’m not sure what to do about the subfloor or if I should cut out more where the water damage is.
Locate a joist, measure 16 inches on each side, and around a foot out. Trim the new plywood to fit and secure it onto the joist. Thumbs up Sister the removed piece to the 4×4 for added strength.
Square off the cut right up to the wall and introduce a new section
Wait a second, what’s the deal with that floor joist?
Make sure to cut it out in a neat and square manner to avoid causing any damage and extend it halfway to the adjacent section. You might need to reinforce a joist for additional support near the wall. The joist is likely positioned beneath the wall.
I’d recommend cutting it out all the way to the edge of the stain just to be safe. It’s better to discover any potential rot in the floor now than later.
Put in a fresh piece
You want the ends of the subfloor to align with a joist so you can nail it down securely. You might have to remove more of the subfloor to make that happen. It’s hard to determine the location of the joists from the photos.
Maybe use dead wood in an out-of-the-way area with low traffic
To avoid that mess in the future, once you have the new plywood piece cut and the drain hole ready, apply a coat of fiberglass resin on top and on all edges of the plywood.
Replace the old piece with a new one. Remove as large a piece as possible. It’s pretty straightforward.
Use a plunge cutter (Fein Multimaster or similar) to remove the damaged area. Cut it so it aligns with the center of the floor joists. Then cut a new section of subfloor, apply glue, and screw it down. Consider adding bridging under the edges of the new subfloor that doesn’t rest on top of the joists.
Yeah, just cut it out cleanly, leave an edge, and drop the new board in.
Wait, did you mention the plumbing is finished or at least roughed in? What’s the distance from the back wall to the center of the 3 or 4 inch drain pipe? It should be at least 12 inches but no more than 15 inches (which also includes buying a special toilet) from the back wall. Looks like this one is around 20 inches.
I’m always doing this. Find the joist, measure how big the cut will be, then cut the plywood and make the hole. Set the piece where you want it, trace it, cut it out, add nailers, and install the new piece.
Alternatively, I could suggest, “Consider hiring a Professional.” Are you prepared to address the mold underneath? What impact has the water damage had on the floor joists and the underfloor insulation in what appears to be a crawlspace? I’m interested in hearing the story about the individual who damaged that engineered joist, believing it would remain stable structurally. Thank you for ensuring that true professionals like us stay in business.
My main concern would be the state of the floor joists rather than replacing the subfloor that requires joist repair.
Looking at this situation, I am amazed. The main issue here is the presence of a leak. How did the inspector know about the altered floor joist without x-ray vision? To truly comprehend how engineered joists function, it’s important to realize that the compressive component is carried by the top of the joist, while the tension component is carried by the bottom. The engineered joist is connected by a thin waferboard web that serves minimal purpose. If the weight was solely supported by the top and bottom of the joist, the top component would need to be larger due to wood being weaker in compression than tension. So why is the top component not larger? This is because when the subfloor is LUED and nailed to the top of the joist, it teams up with the top of the joist to handle the compressive stress on the floor joist. Understanding how this type of engineered structure functions reveals that cutting part of the joist was not as critical as you or an unqualified “inspector” may have thought, as the subfloor was still intact to bear the compressive stresses until it was removed.
As a retired home inspector, I used to place the toilet between my knees and rock it gently to check if the floor support was compromised.
Bower there could be a broken flange, loose or broken toilet bolts. It’s a bit of a stretch to make so many conclusions based on one symptom.
You’re sistering the joist and are having trouble deciding what to do with the subfloor? Seems like this might be a bit above your level of expertise.
Might as well consider replacing a 4’x4’area