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Old 05-16-2010, 05:29 PM   #1
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Bathroom subfloor pier and beam foundation


My husband and I are preparing to renovate our bathroom. So far we have done most of the demo work and we are waiting for the plumber to install a new tub.

We plan on using ceramic tile on mesh as our flooring. We are not sure what to do about the subfloor. How do you know if you need new subfloor? Our house was built in 1961, so it makes sense to us to put down new subfloor. We went to lowes to buy our supplies and they told us OSB was the only option for subfloor. My experience is limited to watching DIY, HGTV and reading about the topic on the internet but I thought plywood was the best option for a bathroom?

I have heard to put down 3/4 inch plywood then 1/2 in plywood then cement board. I'm so confused now, and any help or advice would be greatly appreciated! This is our biggest DIY project so far and we already feel like we are in the "what have we gotten ourselves into stage!"

Thank you in advance for your time and advice!

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Old 05-17-2010, 09:37 PM   #2
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Bathroom subfloor pier and beam foundation


You shouldn't need a new subfloor. That would be a rare situation. Your home is of the vintage that it may have a particleboard subfloor and if that's the case then yes, get rid of the particleboard. No particleboard should be used in a ceramic tile installation.

As far as Lowe's....
That guy is a little goofy. OSB is the least costly subfloor material but certainly not the only or best option. But first let's define subfloor. Your subfloor is the sheathing material that is applied directly to your floor joists. Any sub-material you put over that is called underlayment.

If you now have a 3/4" thick tongue and groove subfloor that's all you need to add tile backer to. All of the cement board manufacturers I know of will warranty their 1/4" cement board over a 3/4" subfloor.

Critical information is still unknown however.
What size are your floor joists?
What is the spacing of your floor joists?
What is the free-span of your floor joists?
What exactly is your subfloor made of and how thick is it really?
Is it tongue and groove?

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Old 05-18-2010, 10:07 AM   #3
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Bathroom subfloor pier and beam foundation


Thank you for your response!

The answers to your questions to the best of my knowledge:

What size are your floor joists?

The width of the joist is 1 1/2 inch

What is the spacing of your floor joists?

14 1/2 inches apart

What is the free-span of your floor joists?

Total length 60 inches

What exactly is your subfloor made of and how thick is it really?

On top of the floor joist was a tar mat, wire mesh, and lots of concrete. I mesured from the floor joist to the top of the tile (they had our vanity placed directly on the floor joist so I had a clean edge to measure from) and it was 2 inches. We carefully took all of that out yesterday.

This is what it looks like currently:
Attached Thumbnails
Bathroom subfloor pier and beam foundation-preping-house-resale-317.jpg   Bathroom subfloor pier and beam foundation-preping-house-resale-316.jpg  

Last edited by Etomli; 05-18-2010 at 10:09 AM.
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Old 05-18-2010, 04:49 PM   #4
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Bathroom subfloor pier and beam foundation


Now I see why you say you need a new subfloor. I was misunderstanding your issues, I stand corrected. You are right - you do need a new subfloor.



Doesn't look too serious but first what is [the other] dimension of the floor joists? 1-1/2" X WHAT?

You should be able to add (install) new tongue and groove plywood (or OSB) on those joists. Glue it and screw it. Use 3/4" material.

After that you can install your cement board in a 100% spread of fresh thinset using a 1/4" notched trowel. Nail or screw the cement board according to the manufacturers installation reccommendations.

After that install your tile but don't forget to tape the seams as you go and fully imbed the seams with thinset.

Now...what size are the floor joists?
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Old 05-18-2010, 08:42 PM   #5
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Bathroom subfloor pier and beam foundation


Sorry, the floor joist are 1 1/2 by 5 1/4. Thank you for your help again! Your post makes perfect sense.

I want to make sure I understand the process correctly, I tend to get obsessed with the details sometimes. When you said to "After that install your tile but don't forget to tape the seams as you go and fully imbed the seams with thinset"

Do you mean to tape the cement board seams as I am applying them or as I am tiling? Also you imbed the thinset to the taped seams of the cement board not before you tape the cement board seams, correct.

I have another question about the plywood. In the corner and edges of the room there is some plywood on the floor joist that supports the walls. Since the plywood is covering the floor joist we can't place our 3/4 ply wood on the floor joist in the corners and along the walls. I'm not sure what to do to support the plywood.

I have some pictures that might help:
Attached Thumbnails
Bathroom subfloor pier and beam foundation-preping-house-resale-321.jpg   Bathroom subfloor pier and beam foundation-preping-house-resale-327.jpg   Bathroom subfloor pier and beam foundation-preping-house-resale-326.jpg   Bathroom subfloor pier and beam foundation-preping-house-resale-323.jpg  
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Old 05-19-2010, 09:41 AM   #6
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Bathroom subfloor pier and beam foundation


Quote:
Do you mean to tape the cement board seams as I am applying them or as I am tiling?
The tape has a sticky side. As you are installing the tile, stay ahead of the tile and apply the tape. Don't try to get too far ahead or you'll just wreck the tape. When you spread the thinset for the tile pay particular attention to (at the same time) embedding the tape 100% in thinset.

Quote:
In the corner and edges of the room there is some plywood on the floor joist that supports the walls. Since the plywood is covering the floor joist we can't place our 3/4 ply wood on the floor joist in the corners and along the walls. I'm not sure what to do to support the plywood.
I think I am seeing particleboard - not plywood, but I could be wrong. If it is particleboard it is easy enough to chisel-out of the path of the new plywood and leave under the wall. Please don't confuse plywood with OSB and with particleboard. Particleboard is bad news, you would want to get rid of all of it that you can.

Now the floor joists...
What you have is 2" X 6"'s. Not a good deal but in this small area you may get away with it. That size lumber was used during those years but is in no way acceptable for a ceramic tile installation. The small area and the short span may be what saves you. I would absolutely use plywood and not OSB in this case. In fact, more plywood would be better with those floor joists.
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Old 05-19-2010, 01:34 PM   #7
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Bathroom subfloor pier and beam foundation


Thanks for all of your time and advice! We had the plumber install the tub today and we are installing the subfloor. We went with 3/4 inch treated CDX plywood and we are using liquid nail subfloor adhesive to glue the boards to the joist with 1 5/8 inch deck screws.

We plan on putting 1/2 inch cement board down ( we were told to use 1/4 inch but due to the gap from the previous subfloor we went with the 1/2 inch instead) with 3/16 inch mortar coverage between the plywood and cement board then screwed in with 1 1/4 inch hardibacker screws.

We were concerned about movement over time associated with the pier and beam foundation so we went with smaller tiles. We picked the classic white octagon and black square pattern on mesh. We were told that the smaller tiles are more forgiving of the pier and beam foundation.

Hopefully it will work out somewhat as planned, as well as can be expected of these sort of things at least!
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Old 05-19-2010, 04:54 PM   #8
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Bathroom subfloor pier and beam foundation


Quote:
We were concerned about movement over time associated with the pier and beam foundation so we went with smaller tiles. We picked the classic white octagon and black square pattern on mesh. We were told that the smaller tiles are more forgiving of the pier and beam foundation.
Good! Looks like you're well on your way.
For the record I'll mention a few things for the benefit of those that may be lurking. Just to keep the record straight...

1. Two by six floor joists are not typically suitable for any ceramic tile installation. They would forbid the use of any stone tile installation.

2. CDX plywood is not suitable for a ceramic tile installation. I realize it is the cheapest of the cheap and that's why people find it desireable but the problem with it is it is full of voids between the laminations. Voids can cause movement in a substrate and movement will wreck a tile installation.

3. Cement board offers no structural benefit, doesn't matter what size it is. Half inch is no stronger than quarter inch. Using half inch to fill an area makes good sense however.

4. Small tiles are no more forgiving than larger tiles, that is just plain rediculous. What will generally give up first when there is movement (deflection) in a floor is the grout. The more frequently grout occurs the more damaged grout there will be. The smaller the tiles used, the more grout there is.

5. I think all cement board manufacturers recommend you use a 1/4" X 1/4" X1/4" notched trowel to spread the thinset that goes under their cement board. Not a 3/16" notch.

6. Treated lumber and plywood should NEVER be used in a ceramic tile installation. Most readily-available treated lumber and plywood materials have a very high moisture content. The products are permeated with the treating chemical and can contain anywhere from 40 to 80 percent moisture when they arrive at the marketplace. Over time these chemicals will dry and drying moisture in wood products spells: MOVEMENT. Treated wood products are never used as a component in any ceramic tile installation. Pre-dried treated wood products are available and are usually known as KD Treated. The KD stands for kiln dried.
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Old 05-21-2010, 02:38 PM   #9
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Bathroom subfloor pier and beam foundation


Also, you want to install the plywood so the grain on the outside layers runs perpendicular to the joists. Be sure to leave a 1/8" gap between plywood pieces and around outer edges. Any sheet of plywood should also span at least 3 joists minimum (which in a small bathroom, could be problematic if you're also trying to run perpendicular to the joists - I think the 3 joists minimum span is the more important thing to meet). See this thread for more info: Considering replacing playwood subfloor, many questions.

3/4" plywood these days may actually end up being 11/16" in actual thickness - the plywood I got was stamped as being 1/16" thinner than the actual dimensions. I don't know whether the guideline for 3/4" plywood takes into account the loss of 1/16" (some manufacturers only lose 1/32" - the actual thickness should be stamped on the plywood) or whether you should buy 7/8" plywood to be sure you have the minimal thickness.

You can use Schluter Ditra (www.schluter.com; available at Home Depot and certain tile/flooring shops) rather than the cement board if you don't want to add the height of the cement board. There are many posts on here & johnbridge.com, plus the schluter.com site has a good installation manual. Be absolutely certain to follow the instructions for what kinds of thinset to use under & over the Ditra - and use good quality thinset (many discussions on here, including a few started by me regarding these matters. Schluter Ditra is only 1/8" thick.

If you decide to use the cement board, read the relevant discussions on that. It seems to be fairly well agreed-upon that Durock is better than most other kinds (hardie-backer & wonder-board (and perhaps others) seem to be not recommended as strongly. I haven't used them, but that's what I gathered from various discussions).


Last edited by lazzlazz; 05-21-2010 at 02:44 PM.
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