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Unread 07-28-2022, 10:26 AM   #1
K3093
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Tile in a 1939 bathroom floor

I’m new here, so thanks in advance. My house was built in 1939. I would like to tile my bathroom with 6”x36” wood grain porcelain tile planks. I’ve used the site’s deflection calculator and get thumbs down for tile and stone because of .578” of deflection or L/290. It does state that some things can influence the rating and to ask for help. The area I want to tile is 38” wide and 8’ long. The joists run across the 38” width or short side to short side. I am unsure of the type of wood used for my floor joists, but it is dimensional lumber 1.5”x 9.5” from the 1930’s and are in excellent condition. They are on 16” centers My bathroom is located directly above a 5.75” wood “girder” spanning the center line of my home and supporting the 14’ joists from each side of the foundation to the girder. The tiled area would extend out about 24” from the side of the girder, the rest overlapping the girder. Also directly under my bathroom is a 6”x6” wooden post stepped on a pier poured with the original basement floor, which is supporting the girder, along with others. It wasn’t something stuck in there to fix a sagging floor. The subfloor is unknown dimensional lumber and is 7/8”x9.5” tongue and groove planks, again of excellent condition. The area to be tiled will be transitioning from original hardwood floors at the bathroom door and the hardwood floor is 7/8” thick. I’m trying to match as close as possible in height. My intentions were to drive wood screws through the subfloor into the joists below. Use Liquid Nails under 1/2” plywood and again screw the plywood down. Use the Schluter Ditra decoupling sheet set in Schluter All Set and set the 3/8” tile on top of that. I think I will be just a little higher than the neighboring hardwood floor and would consider 1/4” plywood if it made a more even transition without compromising strength. I know 1/2” provides more rigidity, but I’m looking for input whether going to 1/4” is totally out of the question as well as am I totally out of luck for being able to install tile? Thanks for your help.
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Unread 07-28-2022, 01:23 PM   #2
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Welcome, Ken.

Common for folks to mention that a support beam is directly under part of their area to be tiled, thinking that must make the floor more rigid and better for the tile installation. Not necessarily true, and more frequently the opposite of reality.

Much depends upon how the junction of the joists on top of the support were constructed. Frequently you end up with a pivot point on top of the support, creating a rather severe uplift condition when a heavy load is applied near the center of the unsupported span on either side. Without more knowledge of your actual overall situation, it's not reasonable to try to guess what might go on in your tile area.

If you could determine the species of your nominal 2x10 joists, and if they are 9.5" tall, chances are good that your floor structures would meet the required L/360 deflection requirement. But you'd still have the uplift consideration and the treatment of the joists over the center beam would still be very important.

Your subfloor plan would be OK except for the Liquid Nails under the plywood. Please don't do that. You could end up creating a problem that you don't now have nor need. The nominal 1/2" exterior glue plywood is the minimum you must install over your sawn board subfloor and you want to use mechanical fasteners only and only fasten to the existing subfloor, not to the joist structure. Best also to re-fasten the boards to the joists before installing the plywood in most cases.

The plywood is not to provide rigidity, it's there to isolate your tile installation from the vicissitudes of the sawn boards, which tend to be rather dimensionally unstable.

Out of luck? Maybe, maybe not. Need to evaluate your joist species, the treatment over the support beam, and your risk tolerance.

My opinion; worth price charged.
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Unread 07-28-2022, 06:20 PM   #3
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Thank you CX for giving your time to answer my question. I appreciate it. I live in Eastern Washington state and am blessed with a full basement, so I stuck my head up between the joists and was disappointed to find the junction of the joists was simply a single nail driben between the two joists, where they lay over the beam. I completely understand the pivot point you describe as a result of this junction. Don’t I feel dumb feeling rock solid standing on my built in hinge.

Were I to drive at least a screw further away from the single nail would I be decreasing the potential pivoting problem you pointed out?

Regarding the overall situation, we have no ACME safes on the span of joists as a heavy load. There are no other beams supporting these joists between the center and outside wall of my basement. Topside, there is the bathroom wall about 32” from the edge of the center beam supporting the joists. From there to the outside wall of our foundation is my bedroom, which I wouldn’t describe as burdened with furniture. The load only changes a couple hundred pounds nightly. The same goes for the other direction from the center beam, which is another spartan bedroom.

As for species of the joists, I’ll have to continue my research. I have four 14’ cedar joists sitting in my basement waiting to replace some of my garage’s flat roof joists. These joists look very different from those in my house, so I don’t believe my joists are cedar. There are no lumber marks that I can find anywhere. There is absolutely no pine pitch anywhere, so I also don’t believe them to be pine. Construction on my house was begun in 1938 and is in Eastern Washington. I read a previous response of yours to another gentleman about species identification and you said that can sometimes be helpful. I don’t have any other examples to compare to in order to eliminate species, but I will try to look deeper into this.

If I do proceed, I will certainly refrain from using the Liquid Nails under the plywood. I understand your point and reasoning for not driving the screws through the plywood and into the joists. I will be sure to only drive into the subfloor.

Thanks again for your time.
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Unread 07-28-2022, 06:49 PM   #4
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Ken, if you don't add that geographic location to your User Profile, the information will be lost before we leave this page, and it's quite often helpful in responding to your questions.

As for what might help with your situation over the support beam, much will depend upon how much overlap there is between the two adjacent joist ends. A photo or two may help.

Photos of your joists might also allow someone here to help identify the species of your joists.
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The load only changes a couple hundred pounds nightly.
Not only could that be a lot in the center of your unsupported span, but you must consider that one or more full-size MILs might be on that span at some point, or several large moving company gorillas, or similar. And it takes only one occurrence to crack your tiles.

Consider that when we're talking about moving in a tile installation, we're not talking the tiles packing their little ceramic bags and going to another state, we're frequently talking only about thousandths of an inch and when that movement is in the direction perpendicular to the tile surface, it can do damage. And when you have a potential arm of seven feet on one side of the fulcrum, and only maybe six inches on the other, and you load the seven-foot arm, the force generated on the six-inch arm can be pretty dramatic, even though it doesn't move very far.

Or it may not be a problem at all in your situation. I wouldn't wanna count on that, depending upon the joist treatment at the beam, but it's all about risk tolerance.

My opinion; worth price charged.
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Unread 07-28-2022, 08:40 PM   #5
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I’m sorry, the overlap of the opposing joists is 5-6”. They aren’t uniform in how much they overlap. There are 6 joists spanning the area I’d like to tile. I also forgot to mention in my previous post, that the joists vary between 9.25” and 9.5”.

I’m attaching a few of pictures in hopes someone can help identify the wood species of the joists. The wood appears a little dark because most everything in the basement had a coat of coal dust from the good old days.

It also looks like some of the subfloor was used as forms for the concrete foundation, so what may look like mold in the subfloor and joists is 83 year old concrete mess that never got cleaned up. From my pictures, I apparently need to do some high dusting in my basement too.

What if I post a maximum weight limit on the bedroom doors? Sorry…

Thanks
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Unread 07-28-2022, 09:27 PM   #6
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Well, not worst case, at least. If the joist ends were at the minimum of 1 1/2" bearing surface, there would be minimal uplift with loading in the unsupported span. Conversely, if the joist ends were overlapped a foot or more past the support beam and solidly fastened together, there is minimal uplift. I would say there is nothing you can do about what you currently have, but someone else might have a different take. We can't guarantee failure any more than we can guarantee success here. Might be fine. Might not.

Type of wood? My first guess would have been cedar, had you not said it was not. And from what little I see, not a particularly good grade (knots close together and near the edge), but I've not had any grading training for about thirty years and the standards have changed twice in that time. And you really need to see a lot more of the board (all of it, both sides) to make an accurate evaluation. Perhaps some of the younger eyes will have a different guess as to the species.

I see nothing wrong with placarding the doors! I've even recommended it for bathroom doors here on the forums more than once. And we do it on airplanes alla time. Enforcement would likely be the issue.

Not at all unusual for owner-built houses to have the concrete form lumber used in the framing. Prudent use of materials and no downside I can think of.
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Unread 07-29-2022, 11:23 AM   #7
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I’m glad to hear this isn’t worst case at least. I’m going to hope all the good things said about Ditra are true. I know there are bad things said too, but I’ll focus on the positive here. My wife would really like a tile floor in the bathroom instead of a painted wood floor. I did want to double check my situation by coming here to this site and getting a reality check, which you have very kindly and honestly provided.

I was always impressed by the lumber in my basement. Apparently, I’m no judge. I’m just glad they aren’t TJI’s. There are much better looking parts of the joists, where it’s just beautiful straight grain, but I thought showing the area with characteristics might help with the identification of species. I thought the reuse of the wood as forms was a smart choice too.

My last question for you, if you don’t mind, is in regards to the plywood underlayment installation. I will use a lot of screws to secure the wood subfloor planks to the joists. I won’t glue the plywood down and I will be careful not to drive screws through the plywood into the joists. Will I be better served in using as large a single piece of plywood as possible to isolate the tile as a unit from the subfloor, or would it be better to use multiple smaller pieces, like 3’x2’? I will be using Ditra uncoupling membrane over the plywood.

Thanks again and your advice has been worth at least 5x the asking price!
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Unread 07-29-2022, 11:35 AM   #8
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Well, I certainly hope we get some other input here, because:
Quote:
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I’m just glad they aren’t TJI’s.
I couldn't disagree more.
Quote:
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I’m going to hope all the good things said about Ditra are true.
What I think you'll be dealing with here is mostly a potential vertical movement, which no manufacturer of tile substrate products will warrant against. Not at all. None of them, including Schluter. My personal opinion is that you'd likely be better off using a 1/4" CBU, but that provides no guarantee either.

As for the plywood, I always prefer the largest pieces possible, but the far more important consideration is that the plywood be oriented with the strength axis perpendicular to the joist structure.

My opinion; worth price charged.
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Unread 07-29-2022, 11:43 AM   #9
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Hi Ken,

I grew up in the better part of Washington State, Seattle.

I would guess you've got Douglas Fir joists. Could also be Hem/fir. Nobody knows what species are in that, but all of them are pretty stiff framing lumber.
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Unread 07-29-2022, 01:23 PM   #10
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CX

My trepidation over TJI’s is more my experience with them as a professional firefighter with a couple decades experience in both legacy and modern fuel structure fires. Crawling over a TJI floor above a basement fire, wearing an extra hundred pounds, trying to find the stairs/chimney into the basement, in zero visibility, before the saw dust and accelerant constructed floor and joists give out, isn’t my idea of a good time. Give me sawn lumber any day of the week. We frequently save the lumber houses. The modern houses don’t fare as well despite our best efforts. I understand “unburnt” TJI’s can be stiffer then sawn lumber though. I’m biased I guess, like Grandpa not wanting to buy German or Japanese cars, even though they are high quality products, but I digress.

I’ve been reading as much as I can about plywood vs. CBU as an underlayment to make an educated decision, but it seems many favor plywood. Much of the written material is a few years old and most of the arguments against has been because plywood is cheaper. Speed forward to today’s market and two pieces of 1/2” CBU 3’x5’ is half the price and larger sq. ft. than exterior use 1/2” plywood 4’x8’s, at least here locally. I already bought some to replace the backing of the tile bath/shower, which led to replacing the discolored 25 year old tub, which led to considering upgrading the floor in the first place. I will attach a picture later of a tile engineering feat (read failure) you may appreciate, which started this adventure.

If you think CBU would be a better choice, it would be in line with the advice of others, with their caveat that it’s more expensive no longer being the case.

My understanding of CBU is that there is no inherent strength axis to consider? Choosing them over plywood would be because they are more likely to crumble themselves than transfer the vertical lift, like plywood?

As always, thank you
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Unread 07-29-2022, 01:37 PM   #11
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Hi John,

Better part of Washington? I'll give you greener for sure, albeit we have had the wettest year in a very long time, but now it’s all dry and it’s over 100 degrees. My tile floor for a cool ocean breeze.

Thanks for your input about the species of my floor joists. That was my suspicion after looking at hundreds of photos of board grain. That’s encouraging., thanks!

Thanks for the site and the reference material too.
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Unread 07-29-2022, 01:52 PM   #12
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You can see here in the phoyo that there were four pieces of 4” tile with their four center corners centered above this hole in the wall. One tile with barely an inch on two sides for adhesion. What was probably the saving grace in why it lasted these last 25 years was because it had a ceramic soap dish siliconed over the middle of the four tiles concerned. The grout must have finally given out and the drywall got soaked leading to the other tiles falling off. The original owner bought the ten year vacant house to flip, so everything was done as cheap as possible. They ripped out the lathe and plaster behind the shower walls and replaced with drywall and no waterproofing. Please ignore the current blue tarp decorating our shower wall. We look forward to the new bathroom as a result.
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Unread 07-29-2022, 02:14 PM   #13
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Quote:
Originally Posted by Ken
I’ve been reading as much as I can about plywood vs. CBU as an underlayment
Misunderstanding, Ken.

You don't have any choice about the plywood. You must install the plywood over your sawn board subfloor. That's part of your subfloor, not your tiling substrate/underlayment. You had planned to use Ditra as your tile substrate. I'm suggesting that a 1/4" CBU might be a better choice than the Ditra on top of the plywood in your application.

As for the burning dimension lumber as opposed to burning TJI floors, my recommendation there is not to burn the floor at all. The ceramic tile industry has no recommendations where fire is considered except when it comes to actually making the tiles, eh?

My opinion; worth price charged.
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Unread 07-29-2022, 02:19 PM   #14
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Ohhh, I understand now. Thanks
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Unread 07-29-2022, 02:33 PM   #15
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I'd never heard the concern for engineered wood products where fire is concerned. Probably well known in the fireman circles, though, eh?

Proof once again that the thing we do best on this site is learn stuff. Those who've been here longest tend to learn the most.
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