How Much Subfloor 'dip' Is Acceptable? - Flooring - DIY Chatroom Home Improvement Forum

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Old 04-30-2009, 04:59 PM   #1
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How much subfloor 'dip' is acceptable?

Sorry if this has been covered in other threads -- I can't find it.

In my bathroom, currently under DIY remodel, I've got new 3/4" plywood on top of 16" o.c. 2x10 joists, with the longest unsupported joist span only 5 feet. (Not sure if any of that is relevant to my question but I thought I'd throw it in there.)

Running a long level over the finished subfloor I see there are a couple of spots where it dips 1/8". I'm wondering if that's enough to worry about. I'm planning to put in 12x12 ceramic tile.

My plumber has offered to do the tile floor and said he'd do a 'mud job', which I gather would obviate any need to worry about the dips.

But I'm also thinking of trying it myself, with 1/2" Durock as underlayment. (I know, everyone likes Kerdi, but I need the height and I've used it before, though only on walls.) If I go that route, are 1/8" dips enough of a problem that I'd have to get out the self-leveling compound and fix it before putting in the underlayment, or would the mortar in which I (gather I'm supposed to) embed the underlayment deal with that amount of 'dip' adequately?

I'm probably being exessively anal but I want to make sure. After all this work I'll kill myself if the tile cracks because of voids under the underlayment.

Thanks for help.


Last edited by RDS; 04-30-2009 at 05:03 PM. Reason: Left out crucial info re. finished flooring
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Old 04-30-2009, 05:21 PM   #2
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You shouldn't have any problem as the long as the joists have good integrity.


Josh Jaros Remodeling in The Woodlands, Texas
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Old 04-30-2009, 07:45 PM   #3
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The industry spec for in-plane floors is: "Within 1/4" in 10 ft. AND 1/16" in 12" of the required plane. Since you know you have an 1/8" valley, why not fix it?

Your subfloor framing is very good and 3/4" ply is good enough for tiles. Remember 1/2" Durock does nothing extra for the floor's strength. You could end up at close to the same height, but have a better floor if you add 3/8" ply, then 1/4" cement backer. Or if you could find Ditra, use it instead of the board and have an even better floor.

TILE GUY - retired- TROY, MI - Method & Product suitability consulting.

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Old 05-01-2009, 07:40 AM   #4
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Thank you both, that's helpful.

I think my joists are fine. The wood is hard as rocks. The old mud floor that I removed only had one crack, between joists, where the old subfloor had been compromised from below. Otherwise pristine after 68 years. That suggests to me that the joists are solid.

You're right, JazMan, I will fix those dips with SLC.

I will give some more thought to the underlayment. If I were to put down more ply, as you suggest, am I correct to think I should avoid having seams in the same places as the seams of the 3/4"? Should my fasteners for the second layer penetrate all the way to the joists, or only through the existing layer of plywood?

Honestly I think I would go the Ditra route if I could find someone around here who'd install it. Maybe I'm asking the wrong people but it doesn't seem to be widely used around here.

Thank you again.
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