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Unread 03-10-2021, 02:57 PM   #1
KSimon
1923 Bungalow Baths
 
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1923 Bungalow

Big fan of John's books and this site, starting a new project after a DIY bathroom reno in 2014, have some questions.

My Case: 1923 Bungalow, 2 Upstairs baths.
Joists are 9" tall, 1.5" wide, and 16" on center. Deflecto is L/805 but doesn't take the diagonal subfloor into account. I think I'm good, esp. since I'm not doing stone.

Original subfloor is 3/4 thick tongue and groove planks. They have either shrunk over 97 years or were laid with space between them, unsure which. (See second photo showing them inside an eave area next to bathroom).

Once I remove the 13/16" wood flooring in these rooms, I plan to leave the existing subfloor, repair any spots that squeak or have any bounce, and then lay an additional subfloor over them.

Here is the plan:

1.) Add a 1/2 inch subfloor. OSB or Plywood. Any preference?
2.) Regular ditra. I would have preferred the XL but the thickness concerns me in this situation. I am in an earthquake zone but can't think of another good reason to add a 1/4" to my floors.
3.) With regular ditra, I will have about 1/4" higher floor than previous. Easily resolved with a threshhold.

Any concerns or thoughts on these decisions before I move forward? Thanks in advance folks.
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Unread 03-10-2021, 03:01 PM   #2
KSimon
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1923 Bungalow addendum

Shoot I forget to mention size of the tile floors in these 2 baths, sorry.

Master: 80" x 115"
Guest: 90" by 60"
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Unread 03-10-2021, 11:07 PM   #3
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Welcome, Kayle.

Your joist design deflection, if the 10-foot unsupported span is accurate, would calculate to L/805 only if you're sure you have a very good grade and species of lumber in good condition. Likely you're fine even with a lesser species and maybe a lesser grade, I'm just trying to verify that you've gathered all the correct information. And our Deflectometer has never heard of a subfloor and never considers that in its calculations.

The diagonal subflooring looks as though it was not kiln dried and has done a lot of shrinking over the years, which would be expected. Definitely makes a difference in the between joist deflection as does the diagonal orientation. Perhaps the boards under the flooring is in better condition than what we see in the photo, but you'll need to remove the wood flooring to determine that. And why ever would you wanna remove all the character of that wood flooring?

1. The tile industry standards consider a board subfloor to be nominal 1/6 T&G and presume them to be oriented perpendicular to the joists. And a layer of nominal 1/2" plywood over that is satisfactory for nearly any substrate and ceramic tile. Will be up to you whether your board subfloor is gonna be suitable for that type of installation. And I would always want the top layer to be plywood rather than OSB.

2. Not even Herr Schluter would pretend that either substrate will protect your tile installation from an earthquake. I recommend you use whichever you prefer and make the transition to your adjacent wood flooring as necessary. I'd stay with the regular Ditra.

3. See #2.

My opinion; worth price charged.
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Unread 03-11-2021, 12:34 AM   #4
KSimon
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1923 Bungalow

Thank you so much for your thoughts.

1.) If the existing subfloor is not in any better shape, would you consider it wise to pull it off and replace it entirely? With 1.25 thick subfloor plywood, I guess?

2.) With respect to the deflection, 10' span, I should add that every time I've opened a ceiling in this house I have found blocking between joists, including those under this bathroom. So I'm feeling pretty good about that.

3.) As to the existing floors. Yes they are beautiful, esp. refinished, as shown in a 1st floor room image below. But they are fir, albiet old growth fir. While common in the Pacific Northwest, fir is quite a soft wood. I'm new here, but have been told that in a "working bathroom" fir is just plain dumb.

I could probably save the fir floors in the master, it just needs refinishing, but I'll see what's under the existing tub. The floor in the guest squeaks badly and is "soft" in a spot or two, and it's unreachable from under without massive problems. So perhaps I could change that room to tile but try to keep the master wood.

Something to sleep on. It's lasted 97 years...although frankly I doubt these were bathrooms for many of those. In 1923 they had one bathroom, I imagine, and probably on the first floor.

Thank you again.
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Unread 03-11-2021, 09:24 AM   #5
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1. I would never suggest using inch-and-a-quarter plywood unless someone had seriously irritated me and I wanted to get even. If you want a subfloor that thick, which would be fine, albeit a bit of overkill, for a ceramic tile floor, I would recommend you do it in two layers. Not only a better subfloor, but far easier on the people installing it. A first layer of nominal 3/4" plywood or OSB and a second layer of nominal 1/2" plywood makes for a very good subfloor.

2.
Quote:
Originally Posted by Kayle
...every time I've opened a ceiling in this house I have found blocking between joists, including those under this bathroom.
That doesn't change the design deflection of the joists at all. Might make the floor feel a little stiffer, but with only a ten-foot span I doubt that would even be the case.

3. For those folks who think it "just plain dumb" to have old growth fir flooring in their bathrooms I would suggest they not use it in their bathrooms. When it comes to your bathroom, I say if you like it and are willing to do the maintenance, use it. And tell them other folks to have a Coke and a smile.........

My opinion; worth price charged.
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Unread 03-11-2021, 06:07 PM   #6
KSimon
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1923 Bungalow

Thanks again for the response.

If I remove the subfloor, which is currently 3/4" thick, I would be thrilled to replace that with 1/2" OSB; that would get rid of my height difference at the end.

But I read somewhere in an article someone here linked to, that subfloor under tile should always be a minimum of 1.25". For such a smallish bathroom, with ceramic not stone, and with 16" centers not 24" as some folks report...I'm probably okay with the combined 1"?
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Unread 03-11-2021, 10:09 PM   #7
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There is no ceramic tile industry requirement for a 1 1/4" subfloor.

Your board subflooring does not technically meet the requirements in the ANSI standards for a sawn board subfloor where an additional minimum 1/2" plywood second layer is indicated, but if your board floor is sound I would personally not hesitate to add the 1/2" plywood, a tile substrate and ceramic tile. See my warranty information below.

If you're not comfortable with that, you might increase to a nominal 5/8ths" plywood. Still not technically complying with any industry standards, but most substrate manufacturers would allow you to tile over the 5/8ths" plywood alone, which I would never recommend you do.

I would not use OSB as the second layer.

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Unread 03-24-2021, 01:47 PM   #8
KSimon
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1923 Bungalow, next round

So I have removed my wood floors from this bathroom I want to do, and found the subfloor planks in quite terrible shape. So now I'm in here hunting for best combination of items to use after I remove the existing planks entire.

Some people talk about OSB then Ply, some mention hardiboard, then Ply. I would like the full thickness of whatever I use to end up being around 1" to 1.25". So maybe .75 and .25.

I know about what quality of Ply I need, (exterior, no D sides), but could use some advice about what should go under it.
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Unread 03-24-2021, 02:01 PM   #9
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With 16" on center joist spacing you can, technically, use a single layer of nominal 3/4" plywood or OSB as your subfloor, with a 1/4" CBU or a thinner tile installation membrane over that. A second layer of nominal 1/2" plywood makes for a much better subfloor if you have room for it.

With CBU it makes no difference whether you use plywood or OSB as your subfloor. If you'll be bonding a membrane to it, I would want plywood as the bonding layer.

My opinion; worth price charged.
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Unread 03-24-2021, 02:02 PM   #10
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Kayle,

If you are going to remove the plank subfloor altogether, leaving nothing but the joists exposed, the first thing you would want to install is a new subfloor of 3/4" B/C Exp 1 tongue and groove plywood, with the plywood's long edge perpendicular to your floor joists. The 3/4" ply will be suitable for ceramic tile, and you'd simply install a tile underlayment of your choice, Like Ditra or 1/4" cement board/Hardie board. If you intend to install some kind of stone, you would need an additional 1/2" layer of plywood on toppa the 3/4", follow by an underlayment.
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Unread 04-10-2021, 08:59 PM   #11
KSimon
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Shower subfloor

Okay moving along on my 2 bathroom Reno and have a new question. In small bath I kept my 98 year old subfloor, replaced a few sections that had cracks, and then screwed down a 3/4” AC ext plywood layer as directed here (though someday I want to understand why one should miss all joists with those screws...but I did so). That floor is now rock hard and ready for decoupling underpayment and tile. In the master bath I removed the wood floors only from the 36x48 shower. So I’m down to the old subfloor there. I’d like to avoid a build up that makes stepping into the shower feel like a step up, but would also prefer a mud pan. Do I need to use 3/4 ply under a mud pan? Doing so brings me up to the floor level outside the shower; can we skip that under mud or use something thinner?
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Unread 04-10-2021, 10:04 PM   #12
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Kayle, your photo is of the shower in question? I fear some confusion of advice might creep in with your doing two bathrooms in one thread and we don't want any such confusion.

Showers are usually addressed separately from floors in the ANSI standards, but I generally consider the requirement for subflooring under a mortar bed to be the same in either case. In your case, that would require a reinforced mortar bed of a minimum of 1 1/4-inch thickness over your existing board subfloor (although the diagonal orientation still makes the application a bit questionable. If you were to add a nominal 1/2" exterior glue plywood over what you've got you could just do a mortar bed over a cleavage membrane and expanded metal lath. The minimum thickness there would be 3/4s of an inch.

I would recommend the plywood and mortar bed.

My opinion; worth price charged.
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