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Old 07-27-2017, 08:57 PM   #1
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Subfloor for tile


I'm redoing my master bathroom (on the second floor) and have removed everything i think I could.

I did find that some of the subfloor is damaged so I'm going to replace it. It's currently 5/8in OSB. My plan is to eventually lay tile down so I will have the 1/4 cement board on top of the subfloor.

Is OSB an okay subfloor to use when installing tiles in a bathroom, or should I be looking to replace it with plywood?

Also, is the 5/8in and 1/4 cement board enough or does the whole system need to be thicker?
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Old 07-28-2017, 02:12 AM   #2
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Re: Subfloor for tile


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Originally Posted by mercurybubbles8 View Post
I'm redoing my master bathroom (on the second floor) and have removed everything i think I could.

I did find that some of the subfloor is damaged so I'm going to replace it. It's currently 5/8in OSB. My plan is to eventually lay tile down so I will have the 1/4 cement board on top of the subfloor.

Is OSB an okay subfloor to use when installing tiles in a bathroom, or should I be looking to replace it with plywood?

Also, is the 5/8in and 1/4 cement board enough or does the whole system need to be thicker?
Adventiech looks like OSB in many ways is better than plywood but I have never seen 5/8. Sounds a little light.
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Old 07-28-2017, 05:21 AM   #3
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Re: Subfloor for tile


Being on the second floor you may not have the resources to find out critical information needed. You need to know the size of your joists and the total unsupported span of the joists. 5/8" is too light for tile. You really need another 1/2" of subflooring on top, then your 1/4" cbu. Remember, cbu does not offer any strength to the subflooring. When applying the second layer of subflooring, first screw the first layer down to the joists with deck screws (2 3/8"), then apply the second layer with no glue and screwing it to the first layer, but not into the joists. (You'll get differing opinions on that method, but opinions are like belly buttons.....that's my opinion)
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Old 07-28-2017, 09:22 AM   #4
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Re: Subfloor for tile


Please let us know the spacing of the floor joists as well - 16" OC is standard but not always what is found.
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Old 07-28-2017, 06:07 PM   #5
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Re: Subfloor for tile


From the holes in my subfloor currently, it looks like the joists are at different distances. Once I cut up some subfloor, I should have a better idea. I'll try to post a picture too. I do believe on other things, the joists might be 14ft long.

I'm also concerned on how high the total subfloor will be compared to what It would match up to to the bedroom, which is currently the 5/8 OSB and 3/4" hardwood flooring.
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Old 07-29-2017, 02:22 PM   #6
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Re: Subfloor for tile


So I took up most of the subfloor to get a better look at the joists. Basically, the joists are spaced differently. One set is 18", another is 10" and another is 16". Is tile still possible? The plan would the 3/4" plywood and 1/2" backerboard, then tile.
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Old 07-29-2017, 03:56 PM   #7
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Re: Subfloor for tile


Without us guessing what are the joist sizes? Have you confirmed 14' open span? 2x6's cannot span over 11'8" depending on the species. Remember I am asking for the "unsupported" span. If you have a wall below, it will offer support. I still recommend a beefier than 3/4" subflooring for tile. You will only use 1/4" cbu. Remember it does not add strength to the subflooring, so 1/2" is not used.
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Old 07-29-2017, 05:14 PM   #8
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Re: Subfloor for tile


I forgo the that part.

The joists are 2x8. There is one joist that has another one glued/nailed with another 2x8 joist. I'm thinking the span is about 14ft by inserting a measuring tape back until it stopped something and then measuring from there to the end of the exterior wall, since the bathroom is on an exterior wall. There is no wall below either. The only thing different is that 2 feet in, there are some 2x8 installed perpendicular (in the picture), but those are there more for soffits on the first floor

My plan for the subfloor part is to replace the damaged 5/8"then adding another 1/2" and the the 1/4" like suggested, but now I think the span will be more the issue.
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Old 07-29-2017, 06:38 PM   #9
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Re: Subfloor for tile


Check this floor deflection calculator.

In general a deflection less than L/360 is required for ceramic tile (and twice that for natural stone). Assuming 16" centers and SYP in good condition, I get L/200, which is a problem.

How feasible is it to sister additional floor joists? If its not, I hate to say this but you might want to consider other flooring options.

(This all assumes you are correct in having a 14' unsupported span.)
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Old 07-29-2017, 07:11 PM   #10
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Re: Subfloor for tile


An engineer explained one fix BUT I have never tried it.
He said to put a 2x4 on the flat under one joist length way and put one screw in the middle. With a post and jack put a little up pressure, about a 1/4" deflection and screw it every 6" and then do the rest the same. He said he has been doing it for years.
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Old 07-29-2017, 08:03 PM   #11
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Re: Subfloor for tile


The bathroom is on the second floor and the 2x8 meets the ceiling on the other side so I can't do much with a jack.

If I sistered the floor joists, would I have to do it the entire length of the joist? The joist length is past the length of the bathroom so I wouldn't be able to get to anything without cutting holes in my first floor ceiling.
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Old 07-30-2017, 09:24 PM   #12
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Re: Subfloor for tile


Sistering the floor joist for only part of their length will stiffen them, and decrease your deflection, but not as much as doing it for their entire length. Unfortunately, I don't know how to find deflection values when only partial sistering is done.

It looks like you need to decrease you deflection rating significantly, which doesn't look for for an easy fix.
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Old 07-31-2017, 12:41 AM   #13
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Re: Subfloor for tile


How do you like vinyl planks?
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Old 07-31-2017, 08:59 AM   #14
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Re: Subfloor for tile


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I'm also concerned on how high the total subfloor will be compared to what It would match up to to the bedroom
This is the last thing you should be worried about in a tile installation.
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Old 07-31-2017, 04:47 PM   #15
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Re: Subfloor for tile


I agree with Jim, you will inevitably have to install a transition at some point. It is important to have a solid floor. I make all my own transitions, so I can configure them to be as bold or narrow as needed for the traffic. Being stuck with the box store transitions is second best.
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