Grout Line Uniform on All Side of Tile? [Archive] - Ceramic Tile Advice Forums - John Bridge Ceramic Tile


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01-15-2017, 02:39 PM
Hey Everyone!

Long time lurker since I started remodeling a small bathroom. I have two newbie questions:

1) I'm planning my layout for 6x24 ceramic floor tile but I'm unsure of the grout line width that's best. The actual dimensions are 5 3/4 x 23 15/16. I'm thinking of using a 1/8 grout line which would bring the width to an even 5 7/8 ....but if I use a 1/8 grout line for the length, it'll be 24 1/16". I was under the impression that TILE + GROUT LINE = NOMINAL TILE LENGTH. Is that not the case? Do I do a uniform 1/8 all around or am I supposed to use a 1/16 grout line for the length and 1/4 for the width to bring everything to 6x24?

2) I know I'm supposed to put thinset on the wood, then CBU, then screw CBU into the wood with Rock On screws (but not the joists). Do I screw down the CBU when the thinset is still wet or do I wait 24 hours for it to cure?

The info I've read on here has been invaluable so far. Thanks! :D

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01-15-2017, 02:47 PM
You typically picture-frame (not to be confused with staining) the tile...i.e., same grout width all around. Now, in practical situations, it might vary a little bit because the tiles may not all be perfectly sized the same or rectangular. Industry standards call for the minimum grout width to be 3X the variation between tile sizes from the max/min of the batch. You may or may not be able to do 1/8". You must take a bunch of them and check for variations before you can choose a grout width. Plus, they often aren't perfectly flat, and a tight grout joint will make it impossible to bridge that variation to make things all work out.

You absolutely need to screw down the cbu while the thinset is still pliable! You need to read the installation instructions. Otherwise, you'll not get the full spread, and you'll have trowel mark ridges and not get the full support you need.

01-15-2017, 03:40 PM
Thanks for the speedy response. The reasoning I heard about letting thinset dry before screwing was that if you did it while wet the CBU would conform to the shape/curve of the subfloor however your logic makes more sense.

The current subfloor has a few low spots I planned on filling with thinset before screwing down another layer of wood. Do I take the same approach and lay the new wood over the filled in spots while the thinset is wet?

I'll get to measuring my tile variations and plot my uniform grout accordingly :)

Also, my floor is out of level. The existing subfloor is level North-South but sinks by 1 inch over 4.5 feet East-West. It's a small 4.5 x 4.5 sq ft bathroom so I was NOT going to level the floor (the doorframe runs East-West so leveling the floor would result in the tiles being a good deal higher than the marble threshold that bridges the bathroom and the hardwood floor of the bedroom). I planned on laying the tiles with the long end running North-South. Am I asking for trouble by doing this?

01-15-2017, 04:56 PM
Welcome, KB. :)

You do not want to use any kind of filler over your existing subfloor before installing a second subfloor layer. You must install your second layer of subflooring directly over the first layer, with both layers having the strength axis perpendicular to the joist structure. Here is a good article ( from our (Liberry) showing what I think is the best method of installing the second layer.

You would then install your CBU into a bed of wet thinset mortar per the manufacturer's instructions, fastening it tight to the subfloor. If you still need some flattening, you do that on top of the CBU before starting to install your tile.

My opinion; worth price charged.

01-15-2017, 05:31 PM

Great advice and great article. I'll happily attach wood layer 2 to wood layer one as instructed and address any flattening on the CBU. It seems easier
to flatten with thinset (post redgard), let it cure, and then apply new thinset/tile vs trying to flatten while tiling. Is that correct?

And any thoughts on my out of levelness?

Thanks so much. I'm honored to get advice from the legendary CX. :yo:

01-15-2017, 07:24 PM

If you plan to use RedGard under your tiles (for what purpose, please?) you would flatten your floor over the CBU using an appropriate patching material rather than thinset mortar and apply your RedGard over that. Then tile.

Your tiles don't give a rat's patootie whether your substrate is level, they care only about flat. If you want the floor level, that would best be done at the joist tops, but your next opportunity in your application would also be on top of your CBU.

Please also note disclaimer in CX's posts before assigning any titles, eh? :)

My opinion; worth price charged.

01-15-2017, 07:33 PM
It was my understanding that Redgard serves as a crack-prevention membrane (as well as water barrier) so it seemed like a good idea to put on the CBU. Is it unnecessary?

You mention and "appropriate patching material" for flattening. What would you recommend?

01-15-2017, 07:37 PM
If I wanted things level (it sounds like you don't so the transition through the doorway isn't weird), I'd forget the cbu and install SLC.

Thinset is NOT designed to be applied thick (thus the name THINset) - it will shrink and crack if you do. There are other products that you can do that with, but you should not use thinset.

Your comment about cbu following the shape of the floor is true - it does, which is one reason why it is not considered in the structural strength of the floor. It does add weight, which is why 1/4" is called for on a floor unless you want the extra thickness. The thinset is there to fill any irregularities and provide 100% support for the tile, not to hold the cbu down. Not forcing intimate contact with the fasteners will leave voids underneath...the clamping force of the myriad number of screws will provide enough force to flatten them out when done properly (and conform to the shape of the floor as well - it MIGHT slightly flatten things, but don't count on it!).

01-15-2017, 09:06 PM
Hi Jadnashua,

Correct, I'm fine without level as long as it doesn't affect the tile.

Your point about voids is well taken but leads me to another question. The max dip in Wood Layer 1 (1/2") is about 1/8 (a few low spots in the field as well as a 3 foot span where the old subfloor meets the new subfloor). If I'm not filling this with anything before screwing down Wood Layer 2 (3/4") won't there be some voids between the two wood layers?

What kind of product should I use to flatten the CBU? Like, Custom Float pre-blended lightweight mortar bed?

Thanks again for the speedy responses. I'm trying to get my ducks in a row so I can make some progress on MLK day tomorrow.

01-15-2017, 09:17 PM
Woah! You're saying your first layer of subflooring on top of your joists is only nominal 1/2" plywood?

01-15-2017, 09:44 PM
Yes. The whole house (built in late 60s) is like that as far as I can tell. The bathroom originally had 1/2 inch on the joists, a giant layer of mortar with a lathe and the tile set directly in it.

I removed the original subfloor and replaced it with new 1/2 inch, which I plan to cover with 3/4 then cbu.

Your surprise makes me very nervous :confused:

01-15-2017, 09:54 PM
If you've added new 1st layer, did you glue and screw?
If no glue can you take it back up easily? Any low spots could be easily taken care of if you can access the joists.

01-15-2017, 10:00 PM
Yes. New subfloor is glued and screwed. The layout of the room dictated I needed to match the existing subfloor in terms of width. Why is 1/2 inch a problem?

01-15-2017, 10:23 PM
Two reasons 1/2" as a bottom layer is problematic:
- industry standards call for a minimum of 5/8"
- if you try to screw a second layer to it, it is tough to get screws to hold with 5/8" let alone 1/2" nominal plywood...IOW, it will be REALLY tough to get a second layer, especially a stiff one like 3/4", to get attached to that 1/2" stuff with no voids (which can spell disaster to long-term tile survivability).

1/2" with an unbonded mudbed on top of it and then tile means the subflooring is mostly providing support for the mudbed prior to it curing...not so much to actually hold it all up in the long term...the joists do that. CBU is not structural, so whatever is underneath it must be strong enough to hold up the tile without deflecting in between the joists (the joists hold everything up, but the subflooring deals with the gaps between them).

You might be better off treating the existing 1/2" ply as an expensive spacer, and installing the 3/4" as if it wasn't there.

01-15-2017, 10:27 PM
What Jim said.

Not to mention that there is no T&G available in nominal half-inch plywood, meaning that your first layer (aka the spacer) has no joint support between joists.

01-15-2017, 10:51 PM
Hi guys, I'm SUPER grateful for the advice so far. I really want to make sure I make the best of the situation I'm in. Just to make sure we're all talking about the same thing, here's how I saw my project happening:

Joists + New 1/2" plywood subfloor (i.e. Wood Layer 1) screwed to joists + 3/4" Plywood subfloor (i.e. Wood Layer 2) screwed to Wood Layer 1 + Thinset + CBU screwed to Wood Layer 2 + Thinset + 6x24 tile


So even though my total thickness is the same (5/8 + 5/8 = 1/2 + 3/4) the issue is the 1/2 layer have enough material for the screws to keep Layer 2 in place longterm. Can this be remedied with the kind of glue you paint on with a roller between wood layers and/or an obscene number of screws?

You also mentioned an "unbonded mudbed" but there may be some confusion. The floor I removed had 1/2 ply and a very thick mudbed. I'm not planning on replicating that at all, rather using additional wood (i.e. the 3/4 inch) and CBU to make up the height difference.

What exactly do you mean by "treat the existing ply as a spacer" since I was already planning on adding Wood Layer 2 (3/4")? What would I do differently?


I'm aware that joists hold things up and subfloor prevents deflection between them. I should mention I have put quite a bit of bracing in place between every joist so the subfloor is essentially sitting on a grid-like arrangement. There's no part of my subfloor arrangement that has a joint unsupported by either a joist or a brace I added. Am I following your comment correctly?

Thanks guys!

01-16-2017, 01:17 AM
If you ignored the fact that the 1/2" ply is there, place the 3/4" stuff down and screw it down tight through that into the joists rather than trying to attach it to the existing ply.

The reason the old stuff survived on a 1/2" layer is that it was a thick, unbonded mudbed. CBU is not a bonded mudbed and isn't strong on its own like a thick mudbed, so it needs an adequate subfloor beneath it, and 1/2" ply is not enough for tile. Industry calls for a minimum layer of 5/8".

The threads on a screw into 1/2" ply just won't have very much will be easy to just ream out the hole, especially if you were trying to bend a 3/4" piece of ply down tight to the layer below.

If you're worried about overall thickness, you could use a membrane verses installing cbu, then tile to the membrane. But, if you're going to flatten things, you need to flatten the floor prior to installing a membrane. If using cbu, because you'll be driving screws through it to anchor it in place, the stuff you'd use to flatten the floor first could debond and/or crack when you put a screw through it, so they tell you to flatten things afterwards.

01-16-2017, 08:31 AM
Jim and CX,

Okay, I can screw the 3/4 directly into the studs (through the new 1/2) but that leads to two more questions:

1) Wood Layer 1 has seams over the joists. When I cut Wood Layer 2 (3/4") I made sure the seams don't fall on the joists. To properly follow your advice and "ignore" Wood Layer 1 do I secure Wood Layer 2 to the first layer with a bunch of field screws AND screws into the joists (and since the seams aren't above joists, just use a ton of screws on them) OR do I recut Wood Layer 2 so there are stacked seams between layers (which seems like a bad idea)?

2) What's an example of a material to use for flattening on top of CBU?

I feel once I know the answers to these two questions I can get back to work :)

01-16-2017, 04:24 PM
1. Except around the perimeter of the room, you should be able to offset the 3/4" layer so it is on a different joist so that the seams don't line up. Around the perimeter, it isn't an issue since the tile won't be going tight up to the wall anyways.
2. One product that you should be able to use on top of the CBU is Ardex's Liquid Backerboard. There are others. Read the specs and limitations before buying.

01-16-2017, 04:28 PM

Thanks so much for all the advice. I'm clear on what to do now so I'm going to get back at it.

Hope you're enjoying your MLK day so far! :fish2:


01-16-2017, 04:57 PM
Argh! One more question lol.

I see that Ardex Liquid Backer Board is a self-leveling compound. My floor is fairly out of level and I'm not looking to fix that. What I am trying to fix are any low spots once the CBU is installed. Am I able to do this with a SLC or will it leave the low spots and run to the lowest end of the room?

01-16-2017, 06:02 PM
Some of the self-leveling cements have a thixotropic property which means that they'll 'flow', but then stay where you end up putting them. This means that you could shape it into a ramp or use it on a slope and it will stay where you end up putting it. I know at least a few of the manufacturers make at least might need to call their tech support department to find out which one(s) will work for your situation. I'm pretty sure that Mapei makes at least one, but they don't make it easy to find on their website! Laticrete makes at least one.

01-16-2017, 06:12 PM
Gotcha. I'll do some research to see what would work best of the SLCs. I should mention I'm using TEC Totalflex so if my dips are less than 1/4 couldn't I flatten them with that? And once my low spots are cured, apply the TEC and tile?

01-16-2017, 07:57 PM
It's not recommended to use thinset to flatten or fix your floor.

01-18-2017, 09:56 PM
Hey everyone,

Currently, there's a gap of 3/16 - 5 /16 between the wood/CBU I have cut for the floor and the outer ledge of an acrylic shower pan. I also have a marble threshold in the doorway on the opposite side of the room with a similar gap.

I'm thinking I install wood/CBU/tile, drop a foam backer rod in the gaps and fill 100% silicone....unless the gaps are too big :noid:

What size gap is acceptable?

P.S. I'm assuming I DON'T want to slide just the tile over to lessen the gap since it would leave about 1/8 unsupported :D

01-18-2017, 10:19 PM
KB, it'll help if you'll keep all your project questions on one thread so folks can see what you're working on and what's been previously asked and answered. We can give it a more generic title any time you'd like to suggest one.

If I understand your question, you really want a 1/4-inch gap there and around the entire perimeter of the room, anyway.

My opinion; worth price charged.

01-18-2017, 10:28 PM
Thanks CX! I'll keep things in one place.

I have 1/4 around the room but know most of that'll be covered by drywall and baseboards. It looked really big at the threshold and shower edge so I got nervous :suspect:

If you say it's good, I'll run with it and fill it with foam and silicone :shades:

01-18-2017, 10:31 PM
You can make those gaps smaller if you like, but what you've got sounds reasonable. Maybe a photo of how bad you think it looks would help.

You absolutely don't want the gap between the tile and those surfaces to be less than 1/8th-inch or one grout joint, whichever is bigger.

My opinion; worth price charged.

01-23-2017, 10:10 PM
I have a new question :D

There is original 1/2" subfloor on one side of the room that extends about 9 inches from the wall before meeting a panel of new 1/2" subfloor on a joist. I've already glued/screwed the new 1/2" in place (and left a 1/8 gap between old and new subfloor).

I now realize that the old subfloor on the joist is about 1/8 higher than where it hits the wall. To put it another way, most of the room is flat enough for me to screw down Wood Layer 2 (3/4" ply) but I have a wall length span of old subfloor that slopes down 1/8" over 9 inches until it reaches the wall.

Earlier, CX said not to have ANYTHING between the plywood layers but if I put ply on top of this part of the new underlayment will be unsupported between this joist and the wall.

What do I do?

01-23-2017, 10:35 PM
Any access from underneath? Assuming the sag is because they used 1/2" subfloor when they built the house and the weight of the wall is all in the center of that joist bay?

01-23-2017, 10:44 PM

No, I don't have access underneath at this part. I'm thinking the joist where I had the new meet the old is a bit high or the joist at the wall is a bit low. But that's neither her nor there since I can't do anything about that now.

I'm inclined to follow CX's original advice of nothing between the two wood layers but to have a gap between the two them seems incorrect :noid:

01-23-2017, 11:27 PM
Can you snap a pic? Sorry I'm visual.

01-23-2017, 11:37 PM
The ugly wood is the original. The part of it at the door frame (and wall the door is on) is 1/8 lower than the edge that meets the new wood on the joist.

01-24-2017, 01:00 AM
K, throw down your next layer of ply. Then if using cbu go ahead and put that in. You will probably need to feather out the low spot using a patching material of your choice. If using an uncoupling membrane you will need to fix your low spot with patch before you install the membrane.
My guess is it is sagging due to the fact its 1/2" which is a little on the thin side for a subfloor

01-24-2017, 03:21 PM
Thanks Ryan. I'll screwdown the second layer in a couple hours (unless someone tells me otherwise between now and then LOL). Then I'll do my thinset and cbu and see how flat the CBU is.

Regarding flattening on top of cbu I'm looking at using Mapei Planipatch. Any thoughts on that?