Subfloor/Underlayment Advice Needed

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  #1  
Old 07-11-07, 03:23 PM
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Subfloor/Underlayment Advice Needed

Hello - I've searched this site/web and I haven't quite found a thread like my situation so please forgive me if these questions are redundant.

I am planning to put ceramic tile in my 50 sq. foot bathroom but first I would like to know if my underlayment planning is adequate.

SPECS ON FLOOR JOISTS:

Type = Doug. Fir (? House built in 1920)

Height = 7.5"

Width = 2"

Spacing = 16" o.c.

Span = 11'

Deflection rating = L/377

SUBFLOOR = 1x6" planking (diagonal)

TILE STYLE = Dal Tile / Octagon & Dot - Mosaic Sheet

SIDENOTE: Tile will lay under a clawfoot tub. Where the tub will sit I have doubled the floor joists.


I HAVE TWO PROBLEMS TO OVERCOME:

1) There is spacing between the 1x6" subfloor for motar to fall through.

2) The very top of the cast iron toilet flange is 1" off of the subfloor (the bottom of the flange is 3/4" from subfloor). If I use this plan my finished floor will be taller than the flange.

I'm guessing I'll need to put something stable on the floor to eliminate the motar from falling through. I plan to screw down 1/4" plywood to block these gaps.

FLOOR WILL BE LAID DOWN AS SUCH: plywood 1/4" + thinset 1/4" + cement board 5/16" + 3/16" V notched mortar + 3/16" mosaic tile = 1 1/4" in total height

When added together the tile and needed underlayments my finished floor will be around 1/4" above the top of the toilet flange.

I'm wondering if the thinset and motar will "squash" a bit and lessen the height?

QUESTONS:

1) Does this plan sound adequate? Should I use 1/4" plywood or thicker? Will it be a sturdy enough for ceramic tile?

2) Is there anything thing else I can to that may lessen the total height in the floor? If not then:

3) Can I increase the height of the cast iron toilet flange without disrupting it? Will a second wax ring work or are there adaptors to raise a toilet flange? (this question may not apply to this section of the forum)

I hope I've included all needed info. for your evaluation.

Thanks very much in advance for you time and attention!

R-DYI'r
 
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  #2  
Old 07-11-07, 06:39 PM
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You want your subfloor to be at least 1"-1 1/8" thick. I would put down a sheet of 5/8" ply at least then the CBU. You might even want to put down 1/4" sheet of ply on top of the 5/8.

Tile around the toilet flange and double up on the wax ring, that would be sufficient.

Hang it there so the pro's can have their input.
 
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Old 07-11-07, 08:05 PM
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Hi Reluctant,

If your 1x6 subfloor is in good condition you can get by with an additional 1/2" underlayment grade plywood. Thicker is always better. Do not buy cheap CDX grade. Do not use 1/4 plywood as it does not add any structural strength.

Your calculations on total thickness is a bit off. You added too much for the thinset. You can save another 1/8" if you use Ditra instead of 1/4" CBU.

Jaz
 
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Old 07-11-07, 08:50 PM
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Jaz & Mark - Thanks for the quick responses!

Sorry if I'm a bit slow, but please tell me if I'm wrong.

My plank subfloor is 3/4" thick. If I add 3/8" plywood that would bump the subfloor up to the recomended 1 1/8".

Is that not enough or are you both saying that it's better to go beyond the subfloor recomendation of 1 1/8"?

Also - Jaz you mentioned that the thinset for the cementboard is too thick. Are you saying 1/4" application is too much or that the cementboard will "squash" the thinset to less than 1/4", thus throwing off my calculations?

(I am not familiar with Ditra so I'll probably skip that because I don't know how I'd layout the tile to it.)

One other question if I may?

Do I have the correct thinset mortar thickness of 3/16" V notch for the mosaic tile? I read that in a DYI book and just want to be sure because I purchased the trowel tonight and those 3/16" notches look small.

Thanks very much again!

~RDIY'r
 
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Old 07-12-07, 06:36 AM
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You need at least 1/2" plywood, and thicker would be better. Forget about the 1 1/8" in your situation. Planking is different than plywood. Each board moves independently with seasonal moisture conditions and as such it cannot support a tile installation like plywood which is more stable. You need to isolate that movement from the tile and thats why you need at least 1/2" plywood. Use exterior grade plwood, bc grade or better.

Use a 1/4" notch trowel for the cement board. The notches will get nocked down to a little less than 1/8" when the cement board is mechanically fastened with screws or roofing nails. If your mosaic tile is 2" x 2" or smaller then ditra is not an option.

I use 3/16" x 1/4" V notch trowel for small mosaics. You can play with the angle of the trowel to get the thickness right without the thinset purging too much from the joints.
 
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Old 07-12-07, 05:36 PM
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Good post Johnny.

Let me add a few words about recommendations that you should have at least 1 1/8" of plywood as a subfloor before we consider ceramic floor tiles. This of course is NOT true, unless you're going to install the tiles directly onto the plywood. That would not be such a good idea. We always recommend either a tile backer board or an isolation memtrane such as Ditra under all floor tile installations on a suspended floor. (I also recommend Ditra or ? over slab floors). As for the subfloor thickness, we have no problem with 3/4" plywood or OSB, topped with the above mentioned options. It is even possible to get by with 5/8" plywood, but I would not try it myself.

Jaz
 
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Old 07-12-07, 07:25 PM
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I agree whole heartedly with johhny and jazman, BUT, in the interests of fair play, I don't believe Hot in OKC was recommending 1 1/8 inch plywood. I re-read his post because that was what I had thought he said also, but such does not appear to be the case. It looks like the term "subfloor" may have been used a bit incorrectly because he seems to be lumping the ply wood and backer board in that general term to include all these items in "subfloor".
 
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Old 07-12-07, 07:39 PM
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I agree Smokey. I wasn't referring to Hot in OKC this time, but it is often told by even some that should know better. Having that thick a subfloor is good, but it would discourage many to install tile. Fact is that when properly fastened a single layer of 3/4" ext. grade plywood topped with Ditra or a CBU will give good results. Don't forget to check the joists system too.

You're right about using the wrong terms. like 'subfloor'. Subfloor to me means the original decking that was installed over the joists BEFORE the walls were erected. Any other layer over it is an 'underlayment' Underlayments are usually installed to prepare for the finish flooring.

Jaz
 
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Old 07-12-07, 08:21 PM
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No argument there. That's a very good description of "sub floor".
 
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Old 07-13-07, 05:50 AM
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I really appreciate the explanations and attention given to my questions.

Thanks to all who have pitched in.

I now understand why so much plywood is needed on top of my plank flooring.

So if I put 3/4" OSB on top of my existing plank, add the thinset, concrete board, and tile I get a height of 1 5/8".

I have to meet up with the hall flooring that is only 3/8" in height (off the original planking) and that also puts me 5/8" above my toilet flange.

If push comes to shove I can cut and reset my flange but can I find a floor transition for the 1 1/8" difference?

Also it was mentioned that 3/4" would be fine if fastened properly. What exactly does that mean? Lots of deck screws and should I avoid screwing to the joists?

What is it meant by "check the joist system"? With that additional weight will I need to double the joists?

And can someone please tell me if it's ok to use galvanized deck screws for the concrete board? I have a DIY book that shows them using these screws but I haven't read it anywhere else.

Thanks again everyone. I appreciate your patience to this simple questions but I just want to do this once!

~RDIYr
 
  #11  
Old 07-13-07, 07:54 AM
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You can use 1/2" exterior grade plywood, you dont need 3/4" osb.

What are you transitioning to in the hallway, carpet, hardwood?

I think Jaz was talking about 3/4" plywood on its own as the subfloor. You already have planking as a subfloor so this doesnt apply to you.

The issue is not adding the weight of plywood etc. The issue is if your floor is strong enough to support a tile installation in the first place. If you can tell us what size joists you have, what there on center spacing is and what the span of the joists are from support to support from below, then we can tell you if you are ok.

They make special screws for cement board. You'll see them at the big box stores where they keep the cement board. Buy the square drive ones, as they are easier to set the heads flush with the top of the board. The phillips heads are sometimes difficult to sink flush. You can also used galvinized roofing nails.
 
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Old 07-13-07, 08:10 AM
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I think the recommended thickness for installing directly to plywood is actually 1 1/4", not 1 1/8". I havent looked at the 07 handbook to see if it changed, but my recollection was minimum 5/8" subfloor and minimum 5/8" underlayment for a total of 1 1/4".
 
  #13  
Old 07-13-07, 08:20 AM
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You can save yourself some additional height by using ditra or another uncoupling membrane in place of the cement board. Ditra will save you approx 1/8" or a little more. Noble CIS will save you even more height. Keep in mind that most tile installation will have minor height transition issues, its just the nature of tile.
 
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Old 07-14-07, 04:44 AM
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As for membranes, since it's a bath, I'd say Kerdi would be fine, or
Noble CIS. I suggest those membranes because the Dal Oct w/dots, those dots are what...1/2" or 5/8" square??? and the minimum tile size on Ditra is 2" to help spread the compressive load on a tile over multiple areas of the ditra grid.

Since you questioned how to lay out tile over something like Ditra, it all lays out the same, no matter what the underlayment. If you were figuring on using Hardi with the grid for layout, forget it. Just the slightest skuing of one shet to the next will throw things off and the tile won't actually size up to the grid lines anyway. Membranes have no joints to tape. A big plus with such small mosaics and small notch when keeping everything in plane. BTW, when setting mosaic sheets, I always use a grout float to tamp them into the thinset. Keeps everything at the same level real easily.

For your plywood, resecure the planking to the joists and then add your plywood which needs to be as mentioned, 1/2" exterior (exposure 1) rated and graded "cc plugged" or better. Nothing listed as CDX or "sheathing rated". Fasten every 6" around the perimeter and every 8" within the field. and secure it only to the planking. Deck screws work great for this. Work in straight lines to remove any bowing in the plywood. 1/8" gap between sheets and 1/4" perimeter gap, stagger rows so 4 corners don't come together in one spot. Lastly, face grain of the plywood crosses the joists.

Given the age of the joists, I also suggest you give consideration to sistering the joists. Many of the houses here are old plank floors over 2x8 joists spanned at 11.5' and while the span tables say it should be ok, they all have noticable bounce. A 2x6 slapped against the middle 2/3 of each one makes for a really tight flooing system.
 
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