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Unread 06-20-2022, 01:44 PM   #1
moneypit2018
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hump in subfloor advice

We have torn out our master bath floor in preparation for new 12x24 tile. After cleaning up the subfloor and putting a straightedge and level on it I have found the 3/4" plywood subfloor has about a 3/8" hump over a steel beam just past the joint in the attached photos. This is obviously a problem, but I'm unsure of the best solution. The joists run perpendicular to the joint of course so this isn't a simple case of planing down a joist or 2, I would have to tear out significant subfloor and plane down both joists that meet over this beam. I am fine on deflection at L/667 but I think the amount of work and complication of the tub makes it untenable. I will be removing the vanity and replacing.

My first thought was SLC, but that would add 1/2" to the overall installation and give us close to a 1/2" step up where be abut carpet on both sides of the bathroom by the time I install ditra and tile. Any other suggestions I may not be thinking of?
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Unread 06-20-2022, 03:40 PM   #2
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Common but annoying problem you’ve got. Best solution is to remove the subfloor in that section and cut down the joist top over the beam to bring it into plane.

If you have access from below you could cut a bit off the bottom of the joist over the beam and then pull it down into alignment with the subfloor still attached.
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Unread 06-20-2022, 05:15 PM   #3
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Unfortunately it's over a finished basement. Wouldn't I likely have to plane the whole length of the joist for every joist?
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Unread 06-20-2022, 07:58 PM   #4
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In my experience, it’s often just a single high joist that causes the humps like you’ve described. Sometimes you’ll have a high joist next to a low joist making the situation worse. You may be able to determine how many are impacted by marking the joists to see if the hump aligns to one or two joists.

If it’s a low one, you can sister a 2x4 to bring it up. High ones more than 1/4” I’ll often cut with a circular saw versus a power planer.

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Unread 06-20-2022, 10:53 PM   #5
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If you are sufficiently motivated, trimming the tops of those joists will give you the lowest possible transition into the adjoining rooms.

Can you split the difference to add some height to the door and some height to the opposite side? Or would you feel better correcting the hump with the former suggestion?

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Unread 06-21-2022, 12:26 PM   #6
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Ok, so if I went with trimming the joists, thoughts on managing the tub? I cannot remove the tub to remove subfloor there, so wouldn't I end up with a cross slope where the lowered floor area blends into the higher subfloor between the vanity and tub? I sketched up the bathroom layout since I guess you can't really see it in the photo. And while I suppose it could be a single joist or two, I suspect the issue is what I have drawn below(greatly exaggerated) in thet all joists fall off from the beam. The hump seems to be consistent in grade across the floor to the ridge, not just localized to one part, if that makes sense.
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Unread 06-21-2022, 08:31 PM   #7
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Nice job with the drawing, very helpful. I’d tackle this from below and notch every joist where it sits on beam to bring all down into plane. Understand this will require pulling the drywall ceiling down but it will be less of a mess and more effective that trying to do from above with the obstructions you’ve noted.

You can just imagine how happy my lovely wife was when I took out the ceiling of our dining room to tackle similar problems. But it all worked out in the end.

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Unread 06-22-2022, 07:27 AM   #8
moneypit2018
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PC, not possible...the area below is a fully finished basement with a soffit enclosing ductwork.

I shot the floor last night with a laser and found the high spot. I have a floor that is largely flat with a small area that is 1/8" higher than the rest (about 2'x1') right above the beam. The edges at the doorways and exterior plate fall off to a total of 1/4" and 3/8" total. So I'm leaving towards planing down the high spot 1/8" and using SLC on the floor. As the high spot is right above the beam and has a slightly reduced span due to the doubled up joists I suspect it will be okay?

If I was more confident in my tile laying ability with LFT I'd probably call the floor good and feather in the trouble spots, but I think the SLC over the entire floor will help the finished product.
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Unread 06-22-2022, 09:03 AM   #9
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Sounds like a plan! Make sure your plane the end tails of the joists down if they extend past the beam as the end tend to lift like a teeter-totter when the floor is loaded.

I've no experience with SLC but others here certainly do and I'm sure will chime in if you have questions.

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Unread 06-23-2022, 08:52 AM   #10
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If you were to put a level on your straight edge which way is mostly level? When you push the edge down towards the vanity or when you push the edge down in the doorway?

I'm wondering if the entrance is the low spot in the bathroom.
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Unread 06-24-2022, 01:18 PM   #11
moneypit2018
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Tiger,

See my recent update. The doorway to the bedroom is the lowest spot, but the doorway into the closet is close.

Got self leveller arriving tomorrow, will sand down the highest 2sf and go from there...
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