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Old 11-25-2010, 10:09 AM   #1
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Very Uneven Subfloor


The subfloor in my bathroom is extremely uneven at the joint between two pieces of plywood. It looks as if one sheet is thicker than the other, but I'm not totally sure. Anyway, it's so severe that the vinyl tile that originally was there was very obviously lumpy and snapping in half. It will be impossible to put down ceramic tile there if it's not fixed.

I thought about ripping it out and putting new plywood, but the problem is that the walls are sitting on top of it. It's probably OK to cut it flush with the wall on walls that are perpendicular to the floor joists, but what about those that are parallel?

But there are two other reasons to get rid of the subfloor:
  • There is a powder room below that has an incredibly noisy builder-grade bath fan in it, and tearing out the subfloor would be a good chance to replace it since you need access from above.
  • It's so nasty and gummy from the self-adhesive vinyl tiles that were there before that it's nasty to touch and I'm not sure if mortar would even stick well.

Do you have any suggestions?
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Very Uneven Subfloor-dscf0486.jpg   Very Uneven Subfloor-dscf0488.jpg  


Last edited by mtk369; 11-25-2010 at 10:12 AM.
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Old 11-25-2010, 09:44 PM   #2
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Very Uneven Subfloor


Is sanding "smooth" and putting new flooring over top an option? Even 1/4" whatever is required for tile would fix it; glue it and staple and/or screw it down. If so, cut a hole and replace that fan (any way to make it accessible from below while you're in there?) beforehand. Also, how is the fan's exhaust piping? Sealed? Panasonic Whisper Green fans are very quite, and somewhat programmable.

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Old 11-26-2010, 08:09 AM   #3
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Very Uneven Subfloor


What do you mean by sanding, just sand it with sandpaper? That's a lot of material to remove, about ⅛", and over an area of 10-15 square feet.

Also, on the worst parts, there is some cement patch there that I put a long time ago and it didn't help because you would basically have to cover the whole floor with it to work. It might interfere with fixing this.
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Old 11-26-2010, 12:02 PM   #4
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Very Uneven Subfloor


I would suggest replacing the floor. Take the old one out, glue and screw a new floor down so it can support tile nicely if thats what you want. If the difference between the 2 sheets is significant and in a large area putting plywood over the problem won't really help. An issue you can have is when you take the floor up there could be an issue with the floor joists. The height difference could be with the joists and thats why its showing on on the sub floor. Hopefully thats not the case. If you take the floor up and there are more issue post some pictures. I had an issue with my living room floor. Joists were all uneven leaving a hump in the middle of the floor.
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Old 11-26-2010, 12:09 PM   #5
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Very Uneven Subfloor


But what about the fact that the walls are ON TOP of the subfloor? What about a situation like in the attached image, will the wall just hang from the ceiling or what?
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Old 11-26-2010, 12:17 PM   #6
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Very Uneven Subfloor


Are the walls parallel with the joists?

When removing the existing floor you would leave the plywood under the walls. So the walls would be sitting on the old floor which would be supported by the joists. Now you might need to add support to an existing joist when putting the new floor down so it has support under it. You want to make sure you don't skimp on support so the floor isn't compromised. That all depends on where the joists line up and where you remove the floor according to the joist lay out.
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Old 11-26-2010, 12:19 PM   #7
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Very Uneven Subfloor


Rent, borrow or buy a belt sander with 60 grit sandpaper. Should take 5 minutes to taper or that edge back far enought that anything you put over it won't show.

Also as mentioned above. Cut out a hole to access that fan and make it accessable from below.
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Old 11-26-2010, 01:24 PM   #8
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Very Uneven Subfloor


Quote:
Originally Posted by 17Irish17 View Post
Are the walls parallel with the joists? So the walls would be sitting on the old floor which would be supported by the joists. Now you might need to add support to an existing joist when putting the new floor down so it has support under it.
What do you mean exactly? Aren't the walls what needs support, not the joists?

Anyway, in the below diagram, the black lines are the walls and the gray lines represent the direction of the joists.
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Old 11-26-2010, 05:48 PM   #9
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Very Uneven Subfloor


Quote:
Originally Posted by Shamus View Post
Rent, borrow or buy a belt sander with 60 grit sandpaper. Should take 5 minutes to taper or that edge back far enought that anything you put over it won't show.

Also as mentioned above. Cut out a hole to access that fan and make it accessable from below.
That is what I was getting to. Just taper the abrupt edge; there is probably an aberrant joist there (crowned) to make the abrupt height change, or chit under it, or whatever. Once you taper that, new plywood should sit fairly flat. However, you are there looking at it, so decide. If the whole floor is a mess, take it out. If a wall is parallel to joists, it should have some blocking under it (between two joists) to take any load on the wall to the adjacent joists, unless the wall is on top of a joist. Whatever happens, the walls have to direct their load to a joist. A wall perpendicular to joists and passing the last one by 10" or so won't mean stink, as the load is on the rest of the wall which is supported by joists. (Barring extreme spot loading on the end of the wall.)
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Old 11-27-2010, 01:44 PM   #10
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Very Uneven Subfloor


I had the same problem. Feather the edge with a sander or hand planer (beware of nail heads along the joint!). Then add a layer of new subfloor. Worked for me...

Remember to consider the total subflooring thickness when you add to the subfloor since you are doing ceramic tile. I've always read (and tried to follow) the advice that 1 1/4" plywood really is necessary to prevent grout cracking. (Maybe a pro can comment on this...esp if DITRA is used). While adding a layer of thin plywood or tile backer board will smooth out the floor, it will not add any rigidity against deflection.

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