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Old 07-06-2013, 03:46 PM   #1
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Replacing subfloor for bathroom tile


Hi again. The wife has me on a new project. I have yet to rip any flooring up as I wanted to get some opinions first

I want to replace linoleum in a 2nd floor bathroom on a house built in 2000. The joists appear to be prefab 19"oc (9.5" tall, 2.5" wide, with the plywood style centers.. not solid wood). I don't know the span.
The subfloor layer is 3/4" chipboard
There is a 3/8" MDF sheet over that.

I am aware I will have to tear the MDF out and put down plywood due to water concerns, so I am trying to figure out how thick this should be first. Ideally I will be changing adjacent flooring to hardwood in the future, so if I can have as little of a transition as possible, that would be best. With a ~3/4" hardwood, that would give me 3/4 to play with in the bathroom for tiling. If the tile/cement is aprox 3/8 thick, that leaves me with 3/8" for a top layer of plywood.

Do you think 3/4" chipboard + 3/8" plywood (1-1/8 total thickness) is sufficient for ceramic tiles (I haven't even considered tile size yet).

Any input is appreciated.

THANKS!
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Old 07-06-2013, 04:11 PM   #2
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We will need that joist length---

Adding 3/8 or 1/2 inch BC ply will be enough for your 19" centers---
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Old 07-06-2013, 04:18 PM   #3
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I'm not well versed in structural support. How could I determine this, short of tearing up the subfloor or tearing out my main floor ceilings? Is "joist length" the distance between supporting beams? I have an unfinished basement where I can see all the supporting beams, but I don't know if that translates all the way up into the 2nd story.
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Old 07-06-2013, 05:38 PM   #4
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On a second floor --measure the ceiling down stairs from outside wall to the next supporting wall---
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Old 07-07-2013, 01:10 AM   #5
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It looks to be 15' from outside wall to the next supporting wall, directly under the bathroom on the main floor.
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Old 07-13-2013, 04:01 PM   #6
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Quote:
Originally Posted by Drizzt View Post
It looks to be 15' from outside wall to the next supporting wall, directly under the bathroom on the main floor.
Since no one else replied.
Based on your description (9 1/2" I-joist with 2 1/2" flange spanning 15 Ft @ 19.5 oc and ceiling below) your floor will not exceed L/720 and T/480 deflection required for tile. Even though 5/8" subflooring can be rated to span 19.5, I would go with 3/4" under tile.

There are a lot better qualified people to advise you on building the floor on the structure.

Last edited by goosebarry; 07-13-2013 at 04:12 PM.
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Old 07-13-2013, 04:09 PM   #7
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Quote:
Originally Posted by goosebarry View Post
Since no one else replied.
Based on your description (9 1/2" I-joist with 2 1/2" flange spanning 15 Ft @ 19.5 oc and ceiling below) your floor should not exceed L/720 and T/480 deflection required for tile. Even though 5/8" subflooring can be rated to span 19.5, I would go with 3/4" under tile.

There are a lot better qualified people to advise you on building the floor on the structure.
Thanks for the reply. So my 2 layers of ply over the joists (3/4" and 3/8") totaling 1-1/8" should be enough. Unless I'm misunderstanding you, and you say that I should have (3/4" and 3/4") totaling 1-1/2"
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Old 07-13-2013, 04:50 PM   #8
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I wouldn't do 3/8 I would do 1/2" if your joist is that far apart. Both layers should be perpendicular to joist and seams offset a foot on 2nd from 1st layer.
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Old 07-13-2013, 09:27 PM   #9
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Quote:
Originally Posted by Drizzt View Post
Thanks for the reply. So my 2 layers of ply over the joists (3/4" and 3/8") totaling 1-1/8" should be enough. Unless I'm misunderstanding you, and you say that I should have (3/4" and 3/4") totaling 1-1/2"
3/4 for the bottom is for structural reasons. I'll defer on the top layer to the tile experts.
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Old 07-13-2013, 09:41 PM   #10
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Quote:
Originally Posted by goosebarry

3/4 for the bottom is for structural reasons. I'll defer on the top layer to the tile experts.
Both layers of plywood are for structure.

Then it's either backer board which should be 1/4" or a ditra membrane.
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