Ruh Roh-- bath floor not flat. Now what?? [Archive] - Ceramic Tile Advice Forums - John Bridge Ceramic Tile


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Setter Wannabe
08-06-2004, 12:44 AM
Hi all,

My wife got me going today on a bath floor tile project she's been wanting me to start. It has (had) some nasty carpet in there and she took it out today. I just removed the toilet and checked the OSB floor for flatness-- Oh no, it is very not flat along the front of the tub. If you can tell from the picture, there is a high spot along the joint of the two floor panels, which is on the right side of the tub, about 1/3 of the tub width from the wall. You can see how big the gap is between the subfloor and the tub against the right wall, and at the high spot there's hardly any gap at all. The subfloor slopes off again toward the left after that high spot.

What now?? Do I need to take the subfloor out? I believe the OSB is 3/4", and from the screws in the floor it looks like I have two joists spaced at 16" and another couple at 19". Not sure what the span is. Any suggestions on how to figure that out? This is a 2nd floor room, about 5 year old construction in So. California. I was hoping to be able to use ditra to keep the floor height small. Thanks for the help.


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08-06-2004, 05:25 AM
Bennett, look for first floor walls that run perpendicular to your joists, somewhere near the middle of the house. Then look in the basement for support piers. The walls that line up with the piers are load bearing. If you are slab on grade, then they will be, regardless.

You need a leveling product to fill the low spots before you use Ditra. I can't recommend one, but I'm sure the pros will be able to.

Setter Wannabe
08-06-2004, 03:03 PM
Thanks for the hints about figuring the span, that will help a lot.

About the floor leveling, I think what I have is a high spot rather than a low spot. I'll have to check but I'm pretty sure that spot is higher than the floor at the door opening. In that case if I put self leveling stuff on it I'll wind up with it running out the door, right? Is there a way to flatten this without leveling it? Since the high spot is along the joint of two OSB floor boards next to the tub, would a likely cause of this be that water got into there from the tub and expanded the OSB? Should I be worried that the joists themselves aren't flat relative to each other? Help, pros, what would you do here???

Thanks very much. By the way, at least I won't have to think about this for the next 24 hours or so. I'm off to go deep sea tuna fishing-- hopefully they'll be hitting hard and I'll feel so good when I get back I won't really mind about this floor....

Thanks again.


08-06-2004, 03:19 PM
Hi Bennett. If you catch Charlie, got to release him you hear? :D

If all you got is a puffy seam where the OSB has swelled up an 1/8" or so from moisture, hit it with a belt sander using 40 grit. The pichure's not too clear but from your description, sound's like an easy fix.

Now back to fishing. Where are you Bennett, West Coast? This time of year off Washington and Oregon, the Japanese Current swings in close to shore (50 miles or so) and tuna fishing very popular, mostly overnight trips. That what you do?

Setter Wannabe
08-06-2004, 03:41 PM
Unfortunately no it's not just a puffy seam. It's more like a bow over the top of that joist. It is most severe next to the tub but is still there a few feet away from the tub, just a lot less pronounced.

Going fishing out of San Diego, Mission Bay really. I hear we may wind up throwing our lines out in Mexican waters. And yes, this is an overnighter trip, fishing for albacore tuna. The guy running the trip gave me some last year that he caught, and it was mighty tasty off the BBQ. This'll be my first deep sea fishing trip, and they're telling me that the REAL fishermen eat the heart of the first tuna they catch... :sick: Don't know how much of a real fisherman I wanna be. Rather set tile. :cool:

Thanks for all the help guys.

08-06-2004, 03:55 PM
Never heard of eating the heart Bennett...have seen making people kiss their first one though. Whatever you do, don't bring any bananas on board. Mucho Grande bad luck. Most Captains get very upset with bananas onboard (which is putting it mildly and in keeping with the decorum of this Forum ;) ).

Oh, and least we not forget about your bump. Can you give us some sense as to how high it might be? Best guess is OK. And what size tile do you plan to put in there?

Setter Wannabe
08-06-2004, 04:56 PM
My best guess (since I'm not at home to measure right now) is somewhere between 1/4" to 1/2" high. I know that's a pretty wide variation, I can eye it up in person and make a better guess after I get back from fishin'.

If you look close at that photo you can see a gap under the tub at the far right hand side. The floor slopes up coming from that side towards the left. About 2 feet from that right wall there's a place where there's a reddish smudge on the side of the tub right next to the floor. That's where the highest point is, you'll notice you can't see any gap under the tub there. Then continuing on to the left, the floor sort of slopes down again to the left wall. Also, that high point seems to slope very slightly away from the tub, so it seems that the "bump" is really more of a ridge that runs straight out at a 90 degree angle from the edge of the tub, following the joist there. The highest point of the 'ridge' is next to the tub but it does go out several feet.

OK, so I went ahead and drew a quick and sloppy little diagram to show what is going on. I hope this helps somewhat....


08-06-2004, 07:40 PM
well that makes things easier. in order to lay tile on the floor said floor must be FLAT not LEVEL. i would float the high spot back to the cabinet and behind the toilet thus making it slope away from the cabinet and the toilet. than with the ditra and than the tile and whala your done.

08-06-2004, 08:09 PM
I agree ifloor - but what will bond to OSB. That's the part I'm not sure of. Somebody out there knows and will cough it up. :shades:

08-06-2004, 08:44 PM
Mmmmm. Dunno, Bennett. Can't see much about the OSB condition from that photo, but it does look like it's been wet. What covering was on the floor before it got tore out? Carpet, maybe?

I'm one of them's as is not opposed to the use of OSB - if it's kept dry. But once seriously wetted, stuff's not worth diddly.

That hump, even if only a quarter inch, is significant in so short a span. I can see where the floor dips down along the tub toward that wall on the right. In a five year old structure, a fella's gotta think that was just piss-poor construction. So the joists run the direction of the dotted line in your drawing? Is the other end of the tub raised above the subfloor, too?

Along with the span of the joists, we're gonna need to know what they are, too. I'm thinking if you've got some spacing of 19.2 inches, they aughta be engineered joists of some sort, in which case they shouldn't be havin' any such crown difference as we appear to be seeing there.

And find out about the subfloor thickness for sure. Drill you a hole if you need to. Two holes, actually; one where it looks like it has been wet, and one where it has for sure always been dry. See do they both indicate the same thickness.

I think there are SLCs that say you can go over OSB with the correct primer. I don't think that's gonna be a problem. But first I'd wanna know what's the deal with the floor structure.

My opinion; worth price charged.

Mmmmmm. This here post was made by CX, no matter what it says over there. And I can't make right with all these new moderator tools, even. But it is moi. :wave:

Peggy Donahue
08-06-2004, 09:08 PM
Hey - "worth price charged". Get yourself re-registered so everyone else knows who you are!!!!!!!!!!


08-06-2004, 09:17 PM
Oh, quit naggin', woman! :mad:

Besides, I didn't even charge him nothin'. :D

Peggy Donahue
08-06-2004, 11:16 PM
Hey "moi" - when you gonna get yourself an avatar that "fits" you? I'll take an Abe Lincoln over the scary guy your using.

Or maybe i'll ....:x:

Still looking for something to tile.

John Bridge
08-07-2004, 08:13 AM
I would remove the puffy OSB. OSB is fine when it's tight, but it loses just about all of its stength when it's swelled like that.

08-07-2004, 09:33 AM
ArdekK-15 can go over wood, including osb (min 3/4"). It requires two things, and maybe a third, depending on thickness. You must use their primer and the elasticizer addative. Depending on the thickness, you may need metal lath. Note that they require either 3.2 or 3.4 lath (most places only carry the lighter 2.5). Min thickness 1/8", max 1.5". Their max spec is higher than most.

08-07-2004, 09:54 AM
Hi jadnashua: :)

Yeah but.....and this becomes an academic question of course if Bennett replaces the OSB I'll pose it as a general technical question: What 'leveling compound' will bond to OSB AND take a feather edge?

08-07-2004, 10:10 AM
I'd have to go read the Ardek specs again, but I thought the minimum was 1/8", it MAY support a feather edge.

08-07-2004, 10:14 AM
I just double-checked the Ardex website - the K-15 can be spread to a feather edge; the min is NOT 1/8" as I thought. Hey, I"ve never used it, but have some in the basement waiting to be put down when I ever get home from shooting drones out of the sky at the missile range.

william keller
08-07-2004, 11:56 AM
i had problems like you had and i took everything out leveled the floor joists and then two layers of sturdi floor. i did this thinking it is cheap compared to what it would look like if i did not do it. half way measures do not really work as well as tearing it up and doing it right. just my way. lots of luck.

08-07-2004, 03:42 PM
Peg, that avatar suits CX just fine. ;)

Jadnashua, Ardex tends to be ultra conservative in their specifications. 2.5 will work fine in that application, there will be no heavy traffic there. While K-15 can be feathered you can't feather over mesh and there must be mesh wherever the K-15 is.

I would take out the OSB.

Setter Wannabe
08-09-2004, 09:34 AM
Wow, thanks for all of the advice and insights. To answer a bunch of questions, the joists do run the direction of the dotted line in the drawing. Now that I've measured, the high spot is 1/2" above the lowest spot at the right side of the tub. The left end of the tub is also raised above the subfloor, about 1/4". The trusses are engineered, at least some of the other ones are. I haven't had a peek at the one that seems to be high, but saw some a few feet over from where this one is when I was working on a can light in the kitchen (on the 1st floor below this 2nd floor bath). They were engineered trusses. There wouldn't be a reason for this one to be different, would there?

There was carpet in that bath previously and it does look like the OSB has been wet, not sure how much. I will drill the holes and see if and how much the OSB has been puffed by the water, and report back a bit later.

By the way the fishing was great, despite my having a little bit of a queasy stomach early in the morning. We had 22 people on the boat and caught about 70 albacore and 15 yellowtail between the bunch. It was my first time and I got 2 of the yellowtail and none of the albacore. Some generous others gave me a couple of albacore so I got to bring home a couple of each. Tasty off the BBQ, if any of you make it out this way and stop by I'd gladly BBQ some up for you.

Thanks for all of the help!!

Setter Wannabe
08-10-2004, 11:31 AM
OK, I inspected a bit closer. As I said, the OSB seems to have been wet but it looks like only surface wetting. I drilled and measured, and got 3/4" in the spot that looked like it had been most wet. I also got 3/4" where the floor had not been wet. With my closer look, I also notice that there appears to be some kind of plaster or something under both ends of the tub to hold it level over that 'teeter-totter' joist.

Since it looks like the OSB has not puffed up and I am not too anxious to get into whatever reason there is for that joist to be high, I am feeling pretty safe to float behind the toilet and under the cabinet toe kick as some suggested. I am interested in the Ardex 2.5 that was mentioned above-- can you give me any pointers about it since I know nothing as of yet. I'm sure I'm about to learn plenty! :) Thanks.


08-10-2004, 04:39 PM
Hi Ben, what I said was you could get away with 2.5 mesh. You can look over Ardex products at

08-10-2004, 06:52 PM
I'm nearly in the same boat - I'm about to use Ardex stuff for the first time. I've read some about it, and read the info on their web site. The stuff has an expiration date - makesure that the stuff you buy has not expired! Also, if you read their installation instructions, you need to put down a primer. This stuff is a 2-part mixture. The timing about when you put it down and when you start pouring the concrete is fairly specific, as well. You have from when it first dries to about 24 hours, so you can't get distracted and not finish! They also specify that you use an addative when you mix it when using it over a wood floor. This stuff is also not cheap. While you probably don't have anything to worry about if you use the lighter mesh lath, their specs do specify the heavier stuff. While I wasn't able to buy it at Home Depot, I did buy it at a local stone supplier. Experienced(?) people say you don't need the heavier stuff. They're probably right, but I talked to the manufacturer, and they said they hadn't tested the lighter stuff and wouldn't warranty it used that way. It meets their specs using the heavier stuff (which is impressive). You probably dont' need that much strength, but why take chances if you can get it locally without hassle? I wouldn't worry about it either way, but I would call around and see if anyone locally carries it first before using the lighter stuff.

08-11-2004, 04:05 PM
Hang on guys you are mixing up products.

Jad, you can use Custom's self leveling cement available from HD almost half the price and use the 2.5 mesh. At one time Ardex used to spec 2.5 but they do a lot of commercial work with heavy foot traffic so they upped the spec to 3.5. We are talking about a small residential bathroom here.

If you're going to put down a SLC take a look in the Liberry for installation procedures.

The Ardex product we suggested was for a trowel on application.

08-11-2004, 04:28 PM
Do not pour over the OSB.Make sure you have a double layer of flooring,And never use K-15 when Ardex featherfinish will do the job without lathe.In fact, Ardex is way too proud of K-15 and if you buy much you'll know what i mean.
Pay attention to Jim, he knows his business,get rid of the OSB or cover it with Plywood before pouring anything.

Any self leveling pour over wood subfloor is a 2 layer requirement period.If you do not put a layer of underlayment down before pouring,you WILL redo this floor in less than 1 year,i can almost gaurantee it !!

08-11-2004, 05:09 PM
May I suggest everybody but Bennet start their own thread to avoid confusion. :)

08-11-2004, 05:55 PM
Good call Jim, I just realized there were a few differant things going on here.

of course on the brite side i havent been able to do this for awhile

Steven Hauser
08-12-2004, 07:30 AM
Hello all,

I do second or third Jm's advice about seperating the threads.

If any one wants to take their questions here and start a new thread just PM me and I'll set it up for you.

I concur with the opinion of removing the OSB and starting out correctly. While I use SLC it doesn't represent a cure all for me.

My .02

Setter Wannabe
08-12-2004, 10:05 AM
Hi all, I got to thinking about it more and realize that even if I did do a self-leveling pour I would still have to flatten the floor after that since it isn't level. I got to looking on the Ardex website and found the sd-f (feather finish) that tileguytodd referred to. That seems like it would do the whole job (flattening the floor) in one step without the lath.

The Ardex site didn't say anything about suitable substrates for sd-f other than it works on "wood." Todd, since this won't be a self-leveling but rather a trowel pour, does your two-layer underlayment caution still apply? This OSB is 3/4" as Ardex recommends and the area won't get much if any foot traffic, since it's from the sides of the toilet to behind the toilet.

Any other opinions about the sd-f? Thanks all.