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Unread 01-11-2022, 10:04 AM   #1
hepcat72
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What kind of plywood should I install?

Hi,

I'm doing a bathroom remodel and I would like to know the answer to 2 questions.

Details:

- Home built in 1920
- 9'x8' room
- Tile: 1" hexogonal marble that comes attached to 1'x'1 mesh
- Joists: 12' long
- 2x10" Joists (actual: 1&7/8" by 10")
- Joists are old growth very dense wood (through I don't know how to identify it)
- Most joists were sistered by previous owner with modern 8x10"s to level the ceiling below from end to end
- Joists are 16" on center (one joist space was 19" and another was 17", but I added 16" braces and another sister)

The floor isn't "level". I shimmed the joists to make it planar. I didn't want a noticeably angled threshold into the bathroom and it would have been a significant height increase to achieve "level".

1. Should I install a 3/4" or 1/2" (of 5/8") plywood subfloor? Host floor prep for tile info I have found starts with the plywood already down.

2. Although I shimmed to make the floor planar, what do I do (or do I even need to with such small tiles) to address any low spots I find when I run a long level across after the plywood is laid down?

I ran a deflection calculator (via tile geeks app) and if I select doulgas fir, I get an L/360 of 723 (sufficient for stone tile). If I select "unknown wood in good condition", I get 523 (not suitable for stone). I should note that the floor in various rooms of the second floor of this house have sagged, but I feel confident that the sisters in the bathroom I'm remodeling, with the shimming are effectively "good condition", and given my experience with this wood, it is definitely dense and strong. People I've hired in the past, like the one who installed my garage door were impressed with the wood & construction. He said it was a pleasure to work with even though it's difficult to get a screw through it.

If 1/2" plywood is sufficient to prevent cracks in the tile/grout, given my bracing/sisters, I would go with that to reduce the size of the lip going into the bathroom. The previous subfloor was 3/4" (diagonal tongue-in-groove) plywood. Though a previous remodel in the 70s was poorly done which left some give due to some big gaps that were only covered by flooring, which resulted in some give in spots, especially in that 19" joist space.

Also, we're installing a clawfoot tub and I re-enforced with 2x8" braces where the feet will be.
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Unread 01-11-2022, 06:03 PM   #2
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Welcome rob.
Quote:
Originally Posted by Rob
Most joists were sistered by previous owner with modern 8x10"s to level the ceiling below from end to end
I'll need some help with that, Rob. 8 by 10s? 8 by 10 what? Surely not wood?
Quote:
Originally Posted by Rob
(one joist space was 19" and another was 17", but I added 16" braces and another sister)
Please explain "16" braces." And tell us more about the sistering. How was it done? With what?

Your tiles don't care a whit about level, they care only about flat. And your small mosaics don't really care much about flat, as they will happily follow the contours of your substrate, but that will also be very obvious in the finished tile installation. I recommend a very flat substrate.

I'm not familiar with the Tile Geeks app. Is it a link to our Deflectometer, perhaps? I would need to know a good deal more about the existing sistering of your joists to comment much on the design deflection.

I'm also concerned about the "shimming" of which you speak. Tell us about that.

And no matter how many times I've read your post, I cannot find what the existing subfloor consists of. I see reference to a "previous subfloor" consisting of diagonally installed plywood? But what have you now for a subfloor? Can you help me a little there?
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Unread 01-17-2022, 11:41 AM   #3
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Thanks for your reply! Sorry for the slow response. I did not receive an email notifying me of a reply. Incidentally, I never got the account confirmation email either. I'm guessing that the forum's email capabilities are broken somehow - at least for me...?

Anyway, sorry, the "8 x 10" was a typo. I meant "2 x 10". This is a second floor bathroom. The original joists are 12 feet long 2x10s. All but two of them have 2x8 sisters that the previous owner installed to level the ceiling below in the kitchen. The remaining two joists without full sisters are the ones that are further from their neighbors than the rest. One gap is as much as 19 inches and the other is 17 inches. I took a 10 foot 2 x 6 and cut it into segments to perpendicularly span that gap every 16 inches to deal with a possible deflection issue for a tile floor. For the other smaller gap, I added a sister that runs along the top.

Both of those joists which I am concerned about have previously installed partial sisters. For example, one has a 2 x 4 that spans the entire length along its bottom and the other has a 2 x 6 that goes approximately 4/5 of the 12' distance. That joist is the one that is closest to the far wall and is the one I added a sister to to the top.

If you use a different deflection calculator, could you provide me a link so I can try it as well? However, given my efforts to reinforce the floor, I'm somewhat less worried about deflection issues with regard to the gaps (except for two spots where there is too much in the way for me to add a brace, but I'm not looking for a solution to that ATM.)

As of yesterday, I now feel like I am completely ready to buy the plywood and I cleared out all my tools and debris, so I'll post a few pictures.

So keep in mind my immediate concern is deciding whether to get three-quarter inch or half inch plywood. The deflection calculator I used surprisingly did not take the thickness of plywood into account. I would think that that would have an effect on deflection.

I will post this response now and then follow up with some pictures in a bit.
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Last edited by hepcat72; 01-17-2022 at 11:46 AM. Reason: Geez. I did it again and said 8x10
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Unread 01-17-2022, 12:12 PM   #4
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Rob, you will not receive an email with new posts to your thread unless you "subscribe" to it and specify email notification. See the Thread Tools to the upper right.

The Deflectometer, and other span tables, consider only the design deflection of the joists. No consideration is given to the subflooring. There is also a concern for the deflection of the subfloor between the joists, but that is a matter for the subfloor material and thickness. The only time the subflooring is considered in joist deflection is when using engineered joist structures, which have subflooring and installation instructions from the manufacturer. No application in your situation.

The blocking you installed does not improve the joist design deflection at all. It will help some in your subfloor deflection in those joist bays.

Can't comment on your proposed subfloor addition without knowing what you have for your existing subfloor.

And I'm still concerned with the shimming you spoke of.

My opinion; worth price charged.
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Unread 01-17-2022, 12:24 PM   #5
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Oh yeah, you asked about the shimming I did. There was a dip in the center of the floor that was not due (primarily) to bowing of the joists. The joists already had sisters installed by the previous owner, but those sisters are at the bottom of the joists to level the ceiling of the kitchen below. I had considered adding additional sisters to level the top, but instead decided to shim the tops to address the dip. The joists at either end of the room from the door to the far wall are high and the joists, as as you get closer to the center of the room are lower and lower. The joist in the middle of the room is about an inch and a half lower than the joists at the ends. Since there does not seem to be any movement in the floor, probably due to the strength added by the previously installed sisters, I decided to shim the tops of the joists to address that dip instead of adding additional sisters. Does that make sense?

And note, I didn't level the floor. I made it planar. Had I leveled, the floor would have increased in height a good bit and I wanted to avoid an awkward lip at the entry door that would be at a different angle from the hall floor. Honestly, the hallway floor has the same angle as the new bathroom floor will have when I'm done, and I didn't even notice that the hallway floor was angled before I started this job. What I did notice was the dip in the middle of the floor as you walk perpendicularly to the joists from the entry door to the far wall. I suspect that this dip may have been due to "settling". Regardless, I have done my best to make the floor planar. My main concern now is dealing with deflections saw that no cracks develop between the tiles of my new floor, yet to be finished.

Here are a couple of pictures I just took and annotated to hopefully clear up any confusion from my poorly worded explanations above.
Attached Images
  
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Unread 01-17-2022, 12:43 PM   #6
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Oh, and when I was deciding whether or not to level the floor or make it planar, I put my level on top of each joist at different spots along its length, and except for a bit more of an angle right up against the exterior wall on the left side (relative to the entry door), that angle was the same for the length of each joist (perhaps due to the efforts of the previous owner when he sistered from below?), so when I did the shimming, I was mainly just adding wood to the top of the joist that was not cut to any sort of angle or anything, except for the last foot or two on the left side.

Originally, when I started out with the floor, I was shooting for making the floor level and was having a really tough time figuring out how much wood I needed on top of each joist to make it level in both directions. I had tried using strings and such, but every adjustment necessitated tweaks to previous adjustments and it was driving me crazy until I finally decided to go planar instead of level. So I only added an amount of wood to the top of each joist that made them roughly level in the direction perpendicular to the joists. (Though technically, the height of The far joist was slightly higher than the height of the joist nearest the door. So I ended up establishing a very slight angle in the direction perpendicular to the joists. Basically, when I put my level between each pair of joists, I got it so that the bubble is right up against the line closer to the far wall.)
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Unread 01-17-2022, 12:44 PM   #7
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Your "Orig diag ply" appears to actually be sawn wood boards. Is that not correct?

Do you plan to remove all the original subflooring and start over?

If your shimming is solid across the entire joist, that would be less worrisome than spaced shims, but still of concern. Did you glue the shims to the existing joist tops, or just use mechanical fasteners?

Making the floor planar, rather than level, is fine. Your tile installation cares only about flat. The larger the tiles, the more they care.

My opinion; worth price charged.
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Unread 01-17-2022, 12:52 PM   #8
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Ah, I see you replied above already while I was replying to my own comment. Regarding subscriptions, when I created my account I selected to subscribe to any of my posts or comments by default. So when I look at the thread tools menu you suggested above, I see that I am already subscribed and the only option is to unsubscribe, so like I said I think there may be an issue with the forum's email capabilities, perhaps just with respect to my email. I don't have any emails from this for him in my spam box either. I don't know why I did not receive an email about your response to my original post.
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Unread 01-17-2022, 01:02 PM   #9
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]Do you plan to remove all the original subflooring and start over?
Except for the closet where the original boards are seen, yes I did remove all of that original subflooring as much as I could.
If your shimming is solid across the entire joist, that would be less worrisome than spaced shims, but still of concern. Did you glue the shims to the existing joist tops, or just use mechanical fasteners?
I glued and nailed to each one. There was one however that I forgot to glue.

What is the nature of your remaining concern?
Making the floor planar, rather than level, is fine. Your tile installation cares only about flat. The larger the tiles, the more they care.
After I get the new subfloor down, if I find any dips that I decide that I want to address, I can't use leveling compound since the floor is not level. Do you have any suggestions about filling in any minor dips?

Also, there is maybe 10 inches of original subfloor from the first joist from the entry door to the entry door. I did not remove that because I could not reach the next joist under the hall floor. If I use three-quarter inch plywood, there will be a slight height difference between the new subfloor and that 10 inch area of original subfloor. Do you have any recommendation on how to "shim" that 10 inch span to make it "level" with the new subfloor? There will be about a quarter inch difference in height, but that depends on whether or not I get half inch plywood or 3/4 inch plywood.
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Last edited by hepcat72; 01-17-2022 at 01:09 PM.
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Unread 01-17-2022, 01:06 PM   #10
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Missed this in my copy:
Your "Orig diag ply" appears to actually be sawn wood boards. Is that not correct?
They are tongue and groove. There are three-quarter inch thick. My understanding is that it is plywood. I have worked with it quite a bit now. It is really strong and the cross-section looks layered like plywood. Originally, there was flooring added on top of it.
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Unread 01-17-2022, 01:18 PM   #11
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Incidentally, I just triple checked my email address on my account and it is correct. So I have no idea why I don't receive any emails from this forum. The only email I have received from John Bridge is the response from my using the contact form to request that my account be activated because I never received the confirmation email.
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Unread 01-17-2022, 01:25 PM   #12
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Oh, I should add one other thing with regard to the original plywood boards in the closet. Given the shimming, I will be adding whatever plywood I get on top of that diagonal plywood that you can see in the photo. That remaining original plywood is just about even with the level after my shimming.
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Unread 01-17-2022, 05:38 PM   #13
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I think we're dancing around the issue, Bob. Let's put it this way, if you're putting down the first layer of subflooring over the joists, I would use nothing lighter than nominal 3/4" plywood. If it's a second layer of subflooring over a layer on top of the joists, I'd use nominal 1/2" plywood. All must be exterior glue plywood with no face of grade lower than C.

On the shims again. My concern with what you have now is that you might be fastening only into the shim when you install your initial layer of subflooring. That's why I asked if the shims were glued or only mechanically fastened. I would recommend you use fasteners for your subfloor long enough to go into the actual joists in either case, but it would be even more important if the shims are not glued.

My opinion; worth price charged.
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Unread 01-18-2022, 08:06 AM   #14
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Thanks so much. I think you're totally right! Those are good things to keep in mind. And I will go with the 3/4" ply.

I have a couple other detail questions regarding cement backer boards.

I have a number of nail plates - enough that I think chances are high I will hit a few with the cement backer board screws. The shortest ones sold at Home Depot are 1&1/4". With the 3/4" ply and 1/4" backer, those screws will poke out at least a quarter inch.

I saw that there exist 1" general purpose (but similar) screws. With screwing them in flush, would those also poke through? Should I go shorter? 7/8? 3/4?

Second, a subfloor prep video recommended a drill with a clutch? What does that mean and what does it do? How can I tell if my old drill (from my dad) has a clutch? My guess is that a clutch might stop screwing when it hits some degree of resistance? Is that right?
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Unread 01-18-2022, 10:25 AM   #15
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The bigger problem with the nail plates is the installation of the subflooring. As they're currently installed, they will interfere with the intimate contact you want between the subflooring and the joist tops. What you really want to do is recess the plates into the joist tops just enough to make the top surface flat. Not complicated and only a little tedious with a sharp chisel. You really want to use construction adhesive as well as mechanical fasteners to install the subfloor.

As for hitting the plates when installing the CBU, that requires only moving your fastener a little to miss the plate if you do hit one.

I recommend you stay with one of the "backer-on" type fasteners for your CBU. They are specially made with ridges under the head to aid in sinking them flush to the CBU surface. Some types of CBU may install OK without that feature, but some will not.

I suspect your old corded drill is not clutched. If it is, it will have an adjustment ring immediately behind the chuck. And regular drills are not the best tools for the job at hand. Might be time for you to own a nice impact driver, preferably cordless, that will work much, much better at driving the screws for both the subflooring and the CBU. Your drill might do the subfloor screws, but I think you'll have problems with the CBU screws.

My opinion; worth price charged.
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