New joists? [Archive] - Ceramic Tile Advice Forums - John Bridge Ceramic Tile


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06-16-2019, 12:40 PM
Hello, I've used your site for years and am very grateful. I'm starting a new bath, 1957 house, 16" oc, don't know span yet. The old tile is on a thick layer of cement over planks over joists. Once I know my height requirements, I will figure out the right 3/4" ext tng plus 1/2-3/4" (?) cc ply with 1/8" gaps, then Hardi or ditra.
Here's my question- contractor spoke of just ripping out joists and starting over, new joists, use joist hangers, and I assume a possible blocking/new beam situation (I will know more tmrw after he takes a look and we talk).
Do you see any red flags here? I want porcelain.
Thank you:)

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Tool Guy - Kg
06-16-2019, 02:36 PM
After all these years....Welcome, Jane! :wave:
...I will know more tmrw after he takes a look....
....Do you see any red flags here?At first glance, it seems like intense over kill if they have neither seen the joist condition, nor have a reason to suspect some sort of deficiency.
Has the existing tile job failed in some way?
Is it in good condition?
Are you planning on a barrier-free shower or something?


06-16-2019, 03:13 PM
Thank you:) Planning a corner jetta tub, with probably a partial glass partition on the right side. His reasoning seems to be it's just easier to rip it and put it back instead of working to get it flat- the floor is not flat. Old tile looks ok, but mostly covered in lino so dunno.

Tool Guy - Kg
06-16-2019, 03:36 PM
Well, joists typically extend into hallways or other adjacent replacing them isn’t confined to the bathroom. And rarely are joists spaces exactly at the edges of a’d typically need to leave the last joist on the two sides of the room.

I can’t imagine replacing joists that have an enclosed ceiling below as “easier” than working with what you’ve already got. Perhaps they know more than I. But sight unseen, it’s off the deep end for someone to say that it’s easier to replace joists than to get a flat surface to build upon.


06-16-2019, 05:55 PM
It is only one story. This bathroom is over a crawl space on the ground. But I hear you... I think the joists go across the hall to the other bedroom (making it 13-16'. What plywood situation would work on 16'? (For porcelain) That's why I was considering his proposal...

Also, if anyone here is an experienced tile layer and/or Schulter fan in Tulsa, OK, please let me know!

06-17-2019, 08:48 AM
I'm thinking of just asking him to install new joists in between old joists giving me 8" oc joists which should help me lay porcelain over a 16' span, instead of ripping it out, these new joists will help easily attain the flat/level he's looking for. Does that make sense? I'm meeting with him today...

Also, I'm going to ask him to glue (any glue?) and screw 3/4" tng (exp 1? Cc?)
That has to be long ways across joists, right?
Then I will handle the second layer of ply once I figure out floor height and deflection.
Can I use lowes poly modified? Or get latricete somewhere?
I've used thhermosoft in the past, but folks around here like warmly yours, yes?

06-17-2019, 11:15 AM
I'm thinking of just asking him to install new joists in between old joists giving me 8" oc joists which should help me lay porcelain over a 16' span,

Agree with adding the additional joists but I'd just sister to the existing ones using Locktite PL adhesive and 3" Grip-rite deck screws.

Also, I'm going to ask him to glue (any glue?) and screw 3/4" tng (exp 1? Cc?)
That has to be long ways across joists, right?

Either Locktite or Liquid nail sub-floor adhesive will work. Youll want to use T&G plywood similar to 23/32 in. x 4 ft. x 8 ft. Southern Pine Tongue and Groove Plywood (

06-17-2019, 12:36 PM
Thank you so much!
Thinset Rec for large porcelain and floor heat?
I keep reading the schluter underlayment article trying to understand...
Both layers of ply are to be perpendicular to joists? Second layer screwed to first, but not joists, and overlapped accordingly. ?? Neither needs to be exterior, and top layer just needs to be cc?

06-17-2019, 12:58 PM
Also, please help me understand proper sistering- proper screws as mentioned spaced ?? And simply screwed laterally into old joist, nothing coming in from the beam into the end of the new joist?

06-17-2019, 06:32 PM
Jane, there are several good thinsets for larger tiles. Kerabond T is one. But let's start at the beginning.
Let's see the joists before any talk of replacing them. Did the guy go into crawl space yet. Maybe they are fine. Wait until he demos the floor.
Sounds like you understand the two layers plywood floor installation.
What floor heat? Different floor heating systems have different requirements.

06-17-2019, 07:19 PM
Sistering joists needs to be long enough to cover a minimum of the center 2/3 of the joists; I typically go to within 12” of each end.

I do a heavy bead of the adhesive in a serpentine pattern. I place the screws in a alternating sides pattern about 12-15” spacing. If I’m screwing into old joists, I’ll put 1/4” washers on the screws as shown in the photo below.


06-17-2019, 07:24 PM
Second layer is typically a BC grade like this one:

Re you the orientation of both layers, you are correct. Long way goes perpendicular to the joists.

You have a good understanding of the screw pattern for attaching the second layer. The number of screws (“schedule”) is one every 4” along the edges and 8” in the field with lines spaced at +/- 4” of center line of joists.

06-17-2019, 08:16 PM
Thinset: versabond white or grey. Your choice of color at Home Depot for around $15

06-17-2019, 08:17 PM
Thanks for the info! Ripout the joists guy didn't show up but another guy did- he was very open to all my silly requirements. He agrees about sistering, but he thinks the sisters need to sit on top of a perimeter beam, and that he may even need to add a beam on some concrete piers under the joists to raise up the sunken areas. Still don't know my joist span, but you guys are saying put a 14' sister on a 16' joist I think... If all that makes any sense...

Haven't decided on floor heat. Don't know how to decide that.

Also... I have 3 1/4" total. 1/4-1/2" plus tiles leaves 2 1/2" or so of ply. Do I add a third layer of 3/4" ply? Or just two, but thicker first or second layer?

Does Hardi get put under the tub? For height reasons? Or just ply, then Hardi up to edge of the tub?

Lastly, he asked me why not just pour two inches or so of concrete over the first layer of ply. How do I answer that?

06-17-2019, 08:28 PM
Welcome, Jane. :)

I'm not understanding the part where you're planning new joists, but still lack 3 1/4 inches in finished floor height. Why would you not simply install the new joists at the proper height for the subfloor, substrate, and tile installation you want?

If there is some reason to keep the subfloor at the lower level, you also have the option of using reinforced deck mud to raise the floor to the perfect level to accommodate your tile installation and make it very flat for the installation.

But If I were to install new joists, I'd want to make the joist tops as perfectly in plane as possible and as close to the correct height as I could.

My opinion; worth price charged.

06-17-2019, 08:34 PM
Sistered joists are the perfect solution for “out of plane” floors. Just make sure the tops of the new joists are all level or lined up to the same plane and you are good.

14’ is great for 16’ joists. You can even go a little longer (15’) since you’ll have to buy 16’ board anyway. You’ll find that it’s nice to have a little room on each end to allow you to work them into place.

I usually screw a 2x4 off to the bottom of the joists on each end to have something to rest the new joists on while I’m setting them (see below) Once the sisters are installed, you remove the 2x4.

Sometimes you have to jam a temporary board under the bottom of the sisters to force them up into alignment where you want em (also shown below)


06-17-2019, 09:09 PM
Thank you, PC. I guess I'll get back to you with a joist span and seek more clarification on why I don't need any new beams or blocking to support the new joists or raise up the existing joists.. and so I can explain it to him...

That's a damn good question, CX. Would these joists need new beams? Or sit on or hang on (with hangers) to existing beams or perimeter frame?? Maybe that's why the first guy just wanted to rip out old beams??

Do you Rec the mud idea? how would that work? One sheet 3/4" tng? Then 1 3/4" cement mud bed??

06-17-2019, 09:45 PM
Yes, I would. You must first lay a cleavage membrane over the subfloor (roofing felt, polyethylene sheeting, etc.) and you must install an appropriate welded wire mesh in the vertical center of your deck mud. But it can make the very flattest surface available for setting ceramic tile. It can be made level if desired, too.

Note that you do not want to use poured concrete for this purpose. It must be deck mud or dry-pack as it's sometimes called.

My opinion; worth price charged.

06-18-2019, 06:52 AM
Jane - I might have missed it but what is the size (height) of your current joists?

I missed the part about your floor being 3 1/4" low. So assuming Ditra Heat you floor stackup is:

Thinset (all layers): ~1/4
Ditra Heat: ~1/4
Tile: 5/16"
Primary Subfloor: 3/4"
Total: 1 9/16"

Remaining delta: 1 - 11/16" or about 1-3/4"

So with that large of a delta you have a couple options:

Option 1) Place the new sister joists at the about same level as current joists while ensuring a flat (and level if possible) plane. Then add 3/4" T&G ply and the reinforced deck mud CX described to get to the level you want.

That much mud will be a good way to figure which of your tile contractor knows what they are really doing too. The old school tile pros will go for this option. The one who are more comfortable with a hammer than making up deck mud (like me :D) will do option 2.

Option 2) Place the new sisters 1-3/4" higher than existing joists. If you go up this option, I'd make the sister as long as possible (edge to edge of the room is best).

I'd also consider using the up next size in the joists to maximize contact surface between the joists.

Decision Tree:

If the existing joists are 2x8, I would absolutely use 2x10 for the sisters.
If the existing joists are 2x10, I'd would stick with 2x10 sisters.

If the existing joists are 2x12, they are already plenty strong for tile and I'd go to option 1 (skip the joists sister part)

06-18-2019, 08:45 PM
Thank you so much everyone.
These guys are not tile guys, I'll be doing the tile.

Is ditra heat the new best thing?

I'm trying really hard to understand...
Should I rip out old joists and start over??
1957- but they look in good shape, other than the floor dipping.
Should I sister 2x10's and raise them?
PC, why washers? Do I screw in from the new joist side? Alternating up and down every 12"? And NO additional beams, piers? The new joists are ONLY attached to side of the old joists, NOwhere else?

Can't y'all recommend someon in Tulsa?!?

06-19-2019, 09:56 AM
Why is the floor 3 inches lower than what you want? Is it because the other side is an additional? Or do you have a settling problem.

The only thing washers will do is to keep the installer from over driving the screw.

06-19-2019, 10:21 AM
See post #1, Spencer. Previous floor covering was apparently tile over a mud bed.

06-19-2019, 01:38 PM
1) Should I rip out old joists and start over?? 1957- but they look in good shape, other than the floor dipping. Should I sister 2x10's and raise them?

- Sister 2x10's offset 1-3/4" from top of old joist. Make sure the tops of the new joists are in plane so you end up with a nice flat floor.

QUESTION: do you have 2x10 joists?

2) PC, why washers?

I always use washers when glueing to old rough surface face joist (pre-1950's typically). I'll some time use them when working with curved joists to ensure the sistered joist is pulled in snugly. The washer are cheap and easy to place on the screws once you get the hang of it.

3) Do I screw in from the new joist side?

Yes. just like the picture show.

4) Alternating up and down every 12"? And NO additional beams, piers?

Somewhere in 12-15" range. Just need to clamp the joists securely so the glue can do the work.

5) The new joists are ONLY attached to side of the old joists, NOwhere else?

Yes - The sisters are attached to original joists.

Additional beams are not required if the existing joists are properly reinforced with sister joists.

6) Why no beam?

Adding beams is often a significant challenge since its difficult to get the beam into the crawlspace and requires adding poured concrete support foundations. for the beam posts.

06-19-2019, 03:21 PM
Thanks PC!

I busted up the floor some... does 1957 cement have asbestos in it?!?

The first pic is the concrete block wall exterior of house. Second pic is other direction. Joists are almost 8" and 16" oc. There is a piered (with blocks) beam about 7' inbwteen exterior wall and what looks like the other beam (14' but with some type of beam at 7'.

First guy (contractor) seems knowledgeable but keeps arguing with me (rip out guy) tells me "relax, I've been doing this 35 years."

Should I insist he sister? Or let him rip out? Does that beam on blocks help or count? Can someone help me determine deflection please? I'm guessing it is 14' if the added beam doesn't count.

PC, can you help me with that math again? Adding in underlayment (3/8" or 1/2") but really, should I do 3/4"
I was thinking, how is a 2x10 going to raise up above the 2x8 if there's ply on the joists in the hallway and other rooms?

I was thinking thermosoft heat, maybe with SLC- any recs on heat?
Hardi or ditra? Have to get my math right before I argue with him anymore...


06-19-2019, 07:40 PM
why rip out those joists? what is his plan?
Yes that support qualifies assuming it is sitting on a good footer and touching each joist. what people are saying is it is not easy to rip out the joists because they are not just under this one room. How will he support other walls or will he replace one at a time? Tell him to make sure that mid support is on a good footer and in contact with each joist. He maybe able to shim the floor flatter,,, go with CX idea of a mudbed to get your height and flat floor. maybe insulate under the subfloor if you want floor heat. pick a floor heating system, and tile so you know your heat, tile stackup.

06-19-2019, 08:54 PM
After reviewing the beam and joist information you provided I’d leave the structure as is and go with CX original suggestion (as seconded by Teddy) for a deck mud bed built up to the required level.

The 2x8 are plenty strong with a 7’ span and there is no reason either replace or sister provided there is no rot or other physical damage (none noticed in photos you provided).

So just say NO to Mr. let’s just rip it all out!

Assuming 5/16” tiles and 1/4” Ditra Heat system you’ll have a max of 3/4” of tile, Ditra and thinset. Subtract that from the height diffencr of the bathroom subfloor and the finished level of the adjective room floors and you’ll have the required mid bed thickness.

06-19-2019, 09:08 PM
Rip out guy is gone, didn't listen well and had trouble showing up.

We plan to sister, yet the sister can't rise too much above the old joists due to subfloor elsewhere... can probably get a 10-11' sister on a 14' span, by eyeing it.

I can't do a mud bed, that's not in my wheelhouse. Going to do 3/4, 3/4, Hardi or ditra, heat, porcelain. Will ty to raise sisters 3/4-1" to make up difference.

I'm still needing above questions answered... but I have another:
When I lay the 3/4" ply across the joists, I've got overhang near the wall off10" or so. How much ply can be unsupported by a joist?
Does the second layer of ply also go perpendicular to joists?

06-19-2019, 09:55 PM
My helper asked me about cement but he's not a tile guy.
Does anyone have a detailed description of deck mud? Or a link to the product?

So, leave joists as is, no sisters at all?? How do I get the joists flat? There's a big dip in the middle of the floor- ply then deck mud correct?
How do I get joists flat if I don't do mud?
Joists, 3/4" tng ply, 1-3/4" deck mud, I assume it's screeded with a 2x4, I have no idea... then heat, thinset and tile. ??

And the ply overhang?

Thank you so much!

06-19-2019, 11:14 PM
Jane, in the dark blue bar near the top of the page you'll find a link to our Liberry. In that Liberry you'll find a thread titled Shower Construction. In that thread you'll find a link to an article John Bridge has written about Deck Mud. That should answer a lot of your questions in that arena. It's about as simple a product as you'll ever find yourself making and a very useful one.

06-19-2019, 11:19 PM
Awesome, thanks!

06-20-2019, 05:09 AM
I’d reconsider the sistering, you don’t need the extra strength with the center beam and it will be a lot more complicated and expensive than mixing deck mud.

If you really don’t want to do the deck mud, you should consider just glueing and screwing 2x4 flat to the tops of the joists and setting your new subfloor over that surface. It will get you close to the required height and is much easier than sisters. The remaining height difference can be made up using another layer of plywood if necessary.

The photo below shows an example of something similar I did to strengthen ceiling rafters and increase depth for additional insulation.


The big dip can be address by using composite shims (from Lowe’s or Home Depot) between the joist edge and the center beam you showed in your pictures.

The shims are tapered and can be hammer between the joist s and beam to raise up the sagged sections. If the joist are sagging more than a 3/8” use 1/2” plywood instead of the composite shims. A bottle jack is a very useful tool when lifting joists back into plane.

06-20-2019, 09:16 AM
Thank you, PC. I'm reading about deck mud and will discuss with him. I guess sagging joists are not a problem if I'm leveling with mud? Also, my plumber is now confused... ugh. He can set the tub right on deck mud surface, yes?

*edit- my drain location is changing from standard to corner, so do I need to leave a new drain pipe hole? Or is plumber supposed to drill through deck mud??

The dip is noticeable, perhaps more than 3/8. I don't understand how 1/2" ply would be used- just a small section placed under the joist, on top of beam?

What about where subfloor extends past joist? Probably 10" near vanity wall with nothing to adhere to. ??

Any advice on heat? If I don't use deck mud, someone mentioned insulation under heat, what would that be?

06-20-2019, 10:53 AM
He didn't think 2x4's flat sounded very strong and suggested sistering 2x8 sisters 8-10' maybe? Idk (length of bath is 10') just for getting a flat floor and a little added height. If we do that, I suppose plumber will easily drill through ply.

06-20-2019, 05:30 PM
Regard the 2x4 on top of joist, nothing stronger that one board resting directly on another! When done right, it will double the strength of a 2x8 joist.

Funny your plumber would be worried about framing strength. I spent a good bit of my time reworking framing they cut in a carefree manner to run their lines. :D


06-20-2019, 05:36 PM
Use Ditra heat over an approved subfloor and you’ll be fine. Regardless of the floor heating product you choose, I’d be sure to add a R30 batt between the joists. It gets cold in Tulsa and you’ll want to hold onto all the heat you can.

I saw a bunch of old iron pipe in your photos. Is that domestic water supply or for a hot water radiator?

06-20-2019, 05:43 PM
It gets cold in Tulsa

Not since the ice age. :D

They're at 92 degrees today.....

06-20-2019, 05:46 PM
The dip is noticeable, perhaps more than 3/8.

You can easily raise the center of floors up using either purchased or site made shims.

I don't understand how 1/2" ply would be used- just a small section placed under the joist, on top of beam?

You got it. Doesn’t have to be ply. Any solid material of correct thickness will work.

06-20-2019, 05:50 PM
They're at 92 degrees today.....

Ah, joys of the Midwest, hot and sticky in summer and subzero temperature with arctic winds in winter.


Edit: just checked temp ranges in OK and gotta admit much better than Nebraska where I spent my younger days.

06-20-2019, 07:19 PM
How do I put the batt between the joists- screw 1x4's under joists every 15"...??

Iron pipes all over...

We had a mild winter and have had a mild spring, I'm not complaining... a bit too much rain, but my garden looks great:)

My plumber isn't my demo/subfloor helper. We've decided for double ply, not mud, so I assume plumber will just cut a new drain hole. I might use 1/2" Hardi if I need the height, I assume it's cheaper than ditra XL.

Soo... I think this is coming along nicely... I still need to know about that ply much is ok? Do we use blocks between joists for overhang?

BTW- how do I purchase 2x4 or 2x8 - anything untreated or certain grade?

By next week I'll be Buffon you with shower wall questions!

06-20-2019, 08:18 PM
Very easy to cut plumbing holes through subfloor, just keep him away from tbe structural stuff!!!

Def add blocking between joists at all unsupported plywood edges.

The paper backed insulation is typically stapled off to the side of the joists.

Standard 2x4 etc are fine. No need to use pressure treated lumbers.

Are you replacing all the old iron water supply lines? Good time to replace with CPCV lines. That’s assuming the pipes aren’t feeding radiant heating system.

06-20-2019, 08:22 PM
By next week I'll be [buggin] you with shower wall questions!

Ha! I’ll defer shower construction questions to Kevin, CX or one of the other tile wizards. :yipee:

06-20-2019, 08:38 PM
Yes, replacing what pipes we can.

Ply edges are every 6 or 8"? That means a 2x4 every 6-8" as well?

Thanks so much!!!

06-21-2019, 03:45 AM
Ply edges are every 6 or 8"? That means a 2x4 every 6-8" as well?

Afraid I don’t understand. You’ll need to make sure that there is blocking under all unsupported edges.

For example, you’ll see there is blocking around the edges of this bathroom for the new subfloor.

If you are using T&G plywood, mated T&G edges are considered supported. [edit: so NO blocking is required]


06-21-2019, 08:06 AM
Shit. I just meant the short edge that overhangs the joists...forgot about all the tng seams! Ugh.
Are those doubled 2x4's? I see little strips, like 1x4 beside the 2x4 sand which. For level?

Then going the other way, 2x4's are flat? Are these just screwed in with deck screws? Griptitie? 2 per 2x4? Glue?

06-21-2019, 07:05 PM
Hey Jane, Happy Friday!

No blocking required under T&G joints between boards. Only unsupported edges.

Lay the 2x4 flat centers on the joists. Use a bead of PL3 and 3” Griprite (or similar) deck screws. I’d put a screw ever 16”. Try to get 2x4 which can reach end to end of the bathroom if possible. If not, use two board with the joint over the center beam.

Shim up the joist BEFORE you install the upper 2x4.

Re the picture of the floor framing; the 2x laid flat around the edges are to support the plywood. That floor had a lot of ugly going on so don’t look too hard at the stuff in the center. :D

06-21-2019, 09:12 PM
Ok. He looked at me kinda funny when I said the tng seam needed blocking...
That tng ply is not bc, but it's the one from the link you sent, so I guess that's ok.

I understand washers on an old joist, but not going into a new joist... ??

Also, flat 2x4's might get me too much height... have to see tmrw. Prob just sister 2x6's for flat and a little height.

Question... if I need height, should I use 1 1/8 tng + half?
3/4 tng + 3/4" ply should work, but just asking.

I've included a pic of the stamp on the 3/4" tng ply recommended from Depot. Something in the liberry made me question whether this is exactly what I need. Also, could you tell me screw spacing on subfloor please?

PC, you're 2x4 blocking on the left side shows them flat, between joists and flush- seems like they should be on their side- for unsupported ply edges.

06-22-2019, 07:13 AM
The ply in the picture is good.

I’d avoid the 1-1/8”. Expensive and stinkin’ heavy. Use combo of 3/4, 5/8 or 1/2” to get the thickness you want.

No need for washers unless the sisters joists are not being pulled in tightly to the original joist. Sometimes if the boards are warped, the screw head will pull into the joists before it gets tight against the other. That’s when you’ll want to put washer on the heads of the griprite screws.

06-22-2019, 07:15 AM
PC, you're 2x4 blocking on the left side shows them flat, between joists and flush- seems like they should be on their side- for unsupported ply edges.

Nope, on side is fine. The ply will be screwed to the blocks and will be plenty strong.

06-22-2019, 08:19 AM

How many screws does first layer get? The 3/4" tng- every 16" I hope lol

06-22-2019, 08:39 AM
Jane, I believe the APA would recommend a fastener schedule of 6" spacing on supported edges and 12" in the field of the panels.

My opinion; worth price charged.

06-22-2019, 08:45 AM
But my joists are 16", does mean a whole lot of blocking?

06-22-2019, 09:10 AM
Screw spacing CX provided is along the joists.

06-22-2019, 09:39 AM
Oh thank god lol

07-15-2019, 10:40 PM
Where to begin...
I plan to install a new beam, just for fun. Gonna dig 14-16" or so I guess...
How wide? 12"x12"? 18x18"? Or 24"x24"
Frame some 2x4's so the pad rises above the ground, attach 4x4 post under the beam.

When blocking my unsupported edge of ply is every 12" good?
Should T&G go pretty side up?
Do I glue the tongue?

I'm having a hell of a time with the joists:(
Been jacking and shimming...
Got a couple of joists that have been chopped off to out in AC unit/vents. I figure if I get the rest of the beams right, maybe I could use joist hangars on those two? Not sure how to explain...

07-16-2019, 11:00 AM
Do I spend $28 on a western red rough cut 4x4 or $11 on #2 prime Douglas fir??
Help quick! Lol. Thanks!!!

07-16-2019, 11:14 AM
Fir is fine for enclosed structure...not so much your outside deck.

07-16-2019, 04:08 PM
Also, I'm looking at a piece of 4x4 that's maybe 4-6" tall. Should I just pour pad higher to touch beam? Or use short little piece of 4x4?

07-16-2019, 04:33 PM
Hi Jane,

I’d build up the support pylon using 6-8” long pressure treated 2x6 sections laid flat on tbe footinng.

07-16-2019, 05:00 PM
Thank you, PC. I thought this was not a good use of PT? I'm using this video as a guide:

Can anyone answer the above Plywood questions? Thanks:)

07-16-2019, 05:15 PM
When blocking my unsupported edge of ply is every 12" good?
Should T&G go pretty side up?
Do I glue the tongue?1. No, you need all edges of your subfloor panels fully supported.

2. Yes, if possible, but it's not a deal breaker either way so long as neither face is of grade lower than C.

3. If recommended by the panel manufacturer. I tend to do it anyway, but I know there are some that do not recommend gluing those joints.

My opinion; worth price charged.

07-16-2019, 05:32 PM
I thought this was not a good use of PT?

You will have to use PT for the portion against the masonry footing and there is no issue using 3-4 stacked flat if all you need is 4.5-6” to support the beam as you said in your earlier post. The larger surface of tbe 2x6 will make it easier to shim between the supports and the beam.

07-16-2019, 06:30 PM
I'm sorry, CX, I just DO NOT understand... I'm about to reread johns book if it eases your frustration:)
In the pic, the unsupported left side near the left wall... joist is about 15" away... so if I add blocks perpendicular to the wall between joists, would I add them every 12"?

07-16-2019, 07:06 PM
Jane - to support the edge next to wall, simply glue and screw 2x4 to bottom of wall plate offset slightly so the 2x4 edge will support othe edge of the plywood. You can use one long section of 2x4 or several shorter ones.

07-16-2019, 07:46 PM
What PC said. Or some method that provides support parallel to that edge of your subflooring. Always more than one way to skin any cat involved in remodeling work.

And don't worry about my frustration level. If that were a problem for me, I'd certainly have stopped participating on this site years and years ago. We're here to help. Keep axin' questions 'till someone has helped you understand whatever it is you need to know. :)

07-16-2019, 08:05 PM
OMG, why didn't you just say so?!? A 2x4? That's easy!
So, 6" along edges and 12" in the field?

CX, since most ply calls for 1/8" gap, I am surprised you're glueing the tongue- but I trust you. Can you help me understand how to screw the second layer? A second layer is required yes? I don't think I have as much height as I thought now that I'm jacking up the floor, so I might go down to 5/8. I plan to use ditra.

BTW, CX, can you find my raised exterior deck project from 10 years ago? (JaneM maybe)
2 kansas inspectors said it couldn't be done, and it's still solid!!!

07-16-2019, 09:17 PM
Yes, nearly all square-edge plywood panels call for a 1/8th" gap, but requirements vary on the T&G edges. Some still want the gap on the top surface, some call for the the top surface to have no gap, but not to be forced tight, some called for the panels to be forced together and the gap was whatever it was, some called for construction adhesive only on the bottom lip of the groove side, and there may be others that I've forgotten. I know I've worked with at least one that specifically said not to glue that joint.

Some have the center match (the T&G) milled much more closely than others. Back before the Internet (you old enough for that?) it was sometimes difficult to find the manufacturer's requirements, but purchasing as I was from real lumber yards, they would have such data on hand if you knew to ask for it. Mostly I suspect people didn't ask for it and did whatever they had learned from somebody else. Sometimes you get information overload, eh? :)

I find no record of a JaneM.

[Edit] This is perhaps you? (

07-16-2019, 09:23 PM
Schluter has a lot of specs with just one layer of ply.
I'm definitely running out of height. The old mud bed sank the joists so low that I thought I had more space than I do. My first layer of 3/4" is already almost too much.

I want to do some type of heat (Recs?) ditra then porcelain.
Is one layer 3/4" T&G enough?

I'm sistering the joists and adding a beam.

07-16-2019, 09:30 PM

It was a big job. I'm 44, but I couldn't have tiled without the forum (HD folks give out the worst advice...) I did two layers ply, durock, ditra/Kerdi, Rondak, dilex... porcelain. It was intense but so beautiful. And still there:)

07-16-2019, 10:20 PM
That's it! Wow, that takes me back... tiling was so much easier before I became a single working mom 🤷🏻*♀️

Please someone say I can use one layer of ply!
I'm not sure id have room for ditra xl and heat...
Maybe thermosoft or wamrlyyours?? Idk...

07-17-2019, 04:58 AM
Yes to single layer of 3/4 ply when using porcelain. Apply the Ditra heat over the plywood using a Versabond thinset. You can buy it at Home Depot for about $15 a bag, it a lightly modified thinset and will work well for your entire project.

I believe Ditra heat is about the thinner option since it combines the decouple layer and the heat into a 1/4” thick layer. The standard Ditra is about 1/8”.

You can buy the Ditra heat system at Homers as well.

07-17-2019, 05:48 AM
As PC said, Ditra Heat is the way to go for your project, Jane. The physical installation of it is very DIY friendly. It is available in individual sheets and in rolls. My advice to you is to use the individual sheets.

Keep in mind, too, that virtually all the floor warming products suggest or require a dedicated electrical circuit, which would be more important if the area you're going to warm is large. If it's a small area you could possibly tie it into an existing circuit - depending of course on what's already on that circuit.

07-17-2019, 08:28 AM
Thanks! Got an electrician coming in next week.
Do they sell ditra fast set at HD? I'd hate to void the ditra warranty by using mod over it.
Also, it states T&G with 1/8" gap between sheets.

Does anyone know what schluter would say about tightly connecting ply or glueing?
And I will glue to joists, then screw 6" on perimiieter and 12" though field, yes?

Trowel size? Porcelain tile 12x24
1/4x1/4 for ditra?
3/8x3/8 for tile?

Any advice on a good toilet?

I'm reloading my pic to ask if joist hangers will be fine to fix these cut off joists.
Their beam runs under about 12" from edge and the hanger area will practically be under the Hvac closet, not my tile.

07-17-2019, 11:31 AM
Re the screw schedule, that sounds correct for 3/4 T&G

Re the Ditra Fastset, I’ve not ever seen it or used it. Unless you plan on inviting Herr Schluter over sometime you’ll be perfectly fine using the versabond under and above the Ditra. :)

Re toilet preferences, I typically use the closest one! :lol2:

Sorry about that. - I tend to use Toto Drake terlet; I think you can get in the ADA height which is good for older or taller folks.

Joist hanger would be good but sometime they’re hard to install in tight areas. I use two heavy gauge Simpson TA9Z Staircase Angle brackets in those situation.

07-17-2019, 11:00 PM
Wow! I found the toto drake on Amazon for $169, even got my boys a bidet topper:)
I think I finally have all my answers, will refer to schluter for trowel sizes.
BTW, they said stick the tongue in the groove a little bit but leave the seams gapped 1/8", and no glue. That sounded a little dirty...

If all goes well, I'll be back in two weeks to talk about the shower walls... ugh.

07-19-2019, 08:50 AM
Ok, I'm using either PT 4x4 or pieces of 2x6 flat on top of my concrete pad but I'm using UNtreated doubled up 2x10 (I think, maybe 2x8?- that probably matters huh?) I'll check later. Just to verify, only PT between pad and beam?

I shimmed the other beam with some T&G 3/4" ply scraps- is that bad?

07-19-2019, 09:32 AM
Jane, the only place you actually need treated wood is the part that actually touches the ground or the concrete pad you're using for support (See Post #61). If you use the flat pieces of 2x material as PC suggested, it's OK to use treated wood for all of them, but you need to be prepared to tighten up your shims next year as that treated wood will shrink by then.

You could also use CMUs of some sort instead of the stacked wood and shim on top of that.

Plywood would not be my first choice of support material in your application.

Actually, whatever you use you should plan to be able to adjust your shims next summer after everything settles a bit. Hopefully not enough to cause any cracking in your tile or grout.

My opinion; worth price charged.

07-19-2019, 10:00 AM
Ok, I'll replace plywood shims.
Where do I get a CMU? I was asking about shims at Home Depot and nobody could tell me anything.
The good thing about my PT pieces is it's been in my garage since I built my deck a few years back. Pretty dry I'd guess.

Does my new beam have to be coupled 2x10, or would doubled 2x8's suffice?

07-19-2019, 10:26 AM
How long will the new beam be?

07-19-2019, 10:54 AM
10'. There's already a beam that I've shimmed up about 8' from stem wall, but it sits on pavers and I thought it best to add a nice strong beam with poured piers at about 4-5' from stem wall right across the center of bathroom floor.

07-19-2019, 12:30 PM
I'd say that a double 2X8 will be fine. Set the 2 support posts 2 feet from each end of the dbl 2X8.

Selecting straight, non cupped 2X8's will make your life easier.

07-22-2019, 04:45 PM
I'm pouring the pads and attaching the post anchor and Just wondering how long does this cement need to dry before it can fully support the post and beam??

07-22-2019, 06:03 PM
Typically, 7 days is the required cure time for concrete in structural applications such as slabs and foundations. That’s the time it takes for the concrete to reach 2500 PSI (70% rated strength); it will continue to slowly cure over 28 days. For situations like yours with limited load, I’ll set posts under the beam after 3 days.

Back in the day, I worked at a prefab concrete pace where we made really big septic tanks for pork farmers. We would pour those things one day and pop em out of the mold the next. I’m sure the concrete was a fast set blend and the tanks only had to hold their own weight in the yard until they cured and were shipped out in a few weeks.

I’m currently beginning a project where we will be underpinning 60 linear foot of foundation in 4 foot sections. Those sections will have to cure for 7-8 days before we do the next sections; probably 3 different pours. Lots of time to wait between pours but not something you can rush.

07-22-2019, 06:14 PM
Thank you. Plumber starts Wednesday. I guess I'll tell
Him rip out and replace pipe, etc, but plywood and tub install will have to wait a week. Thanks!

08-04-2019, 09:25 AM
Floor is down, insulation, glue, screws, the whole bit. Even moved the duct myself- AC guy wanted $250, pffttt Lowes had these twisty metal elbow joints that did it for $30.

Soooo, I have a tiny high spot around the toilet and I have left over ditra. But I'll likely use ditra heat mats near the vanity. I figure I can use the ditra around the toilet and a little extra thinset if needed to match the ditra heat mats. Unless you yell at me...

Also, need to talk about shower surround soon. What is the absolutely easiest way? Help before I buy the damn acrylic walls and quit.

08-11-2019, 11:41 PM
Ditra in toilet area where it's slightly higher, Ditra XL with heat by vanity area, seems fine to me, is that crazy?
What size trowel do you like to put ditra down?

Also...Easiest way to tile tub surround??

Any tips on building tub skirt? Wood or tile, whatever is easiest

Thank you:)

08-12-2019, 07:23 AM
I have a tiny high-spot...

What is the unit of measure for “tiny”? :D

08-12-2019, 08:14 AM
Aw, c'mon, PC, I thought you was a injineer. Tiny is measured in units of Itty Bitty and is quite easy to picture.

For somebody. :D

08-12-2019, 12:25 PM

08-12-2019, 04:10 PM
Just one itty bitty 1/8" or so

08-12-2019, 05:10 PM
Good to know. What size tile are you using? Is this a smaller area (2-3” across) or a larger area?

11-26-2019, 08:38 PM
I AM making some progress...
Planning to install Kerdi with Versabond soon. Drywall doesn’t quite extend over tile flange in a few spots. I can Kerdi fix some Kerdi but didn’t know if I should fill in a little thin set where drywall meets 1.5” flange (or leave it hollow with wedgied Kerdi and Kerdi fix.
Putting large format tiles on floor. Still have a minor (1/8”) high spot that I thought I’d sand down. Still want to use some ditra to fill is edges around ditra xl.
Simply cannot understand Ditra heat wiring instructions, really wish y’all knew a tile guy in TULSA....

11-26-2019, 10:05 PM
Hi Jane,

I’d fill the gap with polyurethane sealant (Kerdifix, sikaflex) and overlap the kerdi as you described.

Making progress on my project too although the foundation had to go through a redesign to simplify the execution. Walls have been in for 30 days and slab/stairs were poured today! We start framing tomorrow so hopefully I’ll be onto my curbless shower before too long.


11-27-2019, 12:10 PM
I’m a little worried about the tile overhanging 1.5” beyond drywall. The drywall protrudes out beyond flange, but doesn’t drop down over. I can Kerdifix and wedgie the Kerdi but that won’t fully fill the void. Is a little extra thin set ok, or is thinset adhered to drywall and touching flange (or on top of Kerdi on flange) mess up the movement requirements?

Filling with kerdifix might take a lot of tubes...

11-27-2019, 02:07 PM
How large are the tiles that would cover the drywall/flange gap, Jane?

Still want to use some ditra to fill is edges around ditra xl.
Simply cannot understand Kerdi heat instructions
Kerdi is Schluter's water proof membrane, while Ditra is their tile floor underlayment. I think Ditra comes in two thicknesses, and I know Ditra Heat does, but I don't quite understand what you're attempting to do. I do know a bit about Ditra Heat.

11-27-2019, 11:03 PM
I was thinking of small subway tiles for the shower, 3”x5” maybe?

I have enough ditra XL for most of the floor. I wanted to use ditra regular with a little extra thinset (either under the ditra or under the large format tiles) around the perimeter of the floor, bc I have a bunch of it- instead of buying more XL.

It’s the electrical wiring part that is beyond my comprehension.

11-28-2019, 09:52 AM
I think you're going to have a tough go of trying to get regular Ditra to be at the same height as your Ditra XL, I think the difference between the two is 1/16th. Not impossible, but will a challenge.

Still confused though, because it sounds as though you are planning to use Schluter's floor warming system. If that is true then you need their Ditra Heat, or Ditra Heat Duo mat, and NOT their regular Ditra/Ditra XL product. The floor heat wire must be installed into the Ditra Heat mat, it cannot be installed into the regular Ditra.

The electrical part is fairly straight forward, but most manufacturers of floor warming systems want a dedicated circuit. If you attempt to use an existing circuit you really need to calculate the load that's already on it, and the load (floor) that you'll be adding to it.

I'd be a bit squeamish about using 3" high tile to cover that 1 1/2 inch gap.

11-28-2019, 11:57 AM
Me too! After Kerdi, can I fill the gap with thinset? Or choose larger tiles?

The regular ditra would be under the floating vanity behind the toilet, etc, where i don’t need heat. I figured I could make up the 1/16 with thinset.

11-29-2019, 11:09 AM
I’m going to put extra kerdifix on flange and bottom of drywall, then Kerdi, then I’m just going to have to add a little extra thinset below the drywall, but on top of Kerdi. And I will probably try and get more like a 5”x10” tile. Unless you say otherwise...

11-29-2019, 11:27 AM
I would recommend, again, that you not fill that gap with anything at all, Jane, but afix your Kerdiband (or Kerdi) to the tub flange with the appropriate sealant and to the drywall with thinset mortar.

My opinion; worth price charged.

11-29-2019, 11:36 AM
I’m sorry if I missed or misunderstood a previous comment, CX. Don’t don’t fill, I assume for movement reasons. Therefore my tile will hang down about an inch over a somewhat hollow space?

Tool Guy - Kg
11-29-2019, 01:06 PM
That’s okay. That resulting 1” gap, while unsupported, is in a corner area that is fairly well shielded from possible damage. It’s pretty hard to image something being in the tub area that would be sufficiently hard and pointy that was accelerated at a speed to cause impact damage to such a narrow band along the corner.


11-29-2019, 01:28 PM
Ok, thank you. I just want to get it right. The tile will only have thinset on its backside where it sits on drywall (after kerdi). A 3”x5” tile will work but larger tile might be better.
The bottom 1” of the tile will be dry (no thinset) and hollow, except for whatever silicone caulk that goes into the void when I caulk between tub and tile. I hope this is correct. Thanks!!

Tool Guy - Kg
11-29-2019, 11:08 PM
Yep. :tup2:

11-30-2019, 02:44 AM
Is this unfinished raw whatever travertine a bad shower tile?

11-30-2019, 08:45 AM
For so many reasons, Jane, IMO, though it would look great in a shower that is never actually used. ;) Aside from the installation challenge, cleaning that tile will be quite a task, I think. It's going to hold onto everything.

Tool Guy - Kg
11-30-2019, 12:14 PM
Tile that is butted together like that (not properly grouted) will cultivate mold in the cracks like crazy. I know tile like that looks really good, but to use your word, it is a very bad shower tile.


12-17-2019, 04:09 PM
What about this marble?

12-18-2019, 07:35 AM
What about this marble, Jane? You mean as an accent in your shower? If so, where in the shower?

On a wall it'll be fine I think, and that it has joints for grout is a good thing.

12-18-2019, 09:22 PM
Yes, for the entire shower surround (around a Roman tub). I will do 1/8” joints I think. Just wanted to make sure it wasn’t too porous or would be hard to keep clean. Going with dark gray grout.
Thanks everyone!!

Tool Guy - Kg
12-18-2019, 11:56 PM
Probably not so good: probably too soft, porous, and easily damaged.

Dunk one end in water and see if it turns darker. If so, it’s a poor choice. Next, expose it to a single dose of a harsh cleaner to see if it’s immeduately and permanently damaged/etched. If so, it’s a poor choice.

Natural stone is beautiful, but the upkeep is on another level from durable material like ceramic/porcelain.


01-02-2020, 08:33 PM
Ok, Tonto, it’s been sitting in water for days and I sprayed it with tub cleaner and bleach- it doesn’t appear to have sustained any changes or discoloration. Sooo, will it be fine for shower walls??

I’m still wondering if I can supplement ditra around some of the floor perimeter (bc I don’t have enough ditra XL) and just use a little more thinset to get it level. Is this crazy?

01-04-2020, 03:35 PM
Thought I might revitalize an old discussion...I want to use ditra xl in my bath where I want heat and regular ditra around the toilet and near some walls just bc I have a lot of it. I figure I’ll use a little more thinset below and above and it will all be fine. Thoughts??
Large format porcelain.

01-04-2020, 03:43 PM
Anyone on the ditra? I’m ready to lay this stuff down and freaked out to do what I want to do.

01-04-2020, 04:14 PM
Hey there. You may want to call schluters tech line to make sure, but doubling up the ditra shouldn't be a problem. If you're lining it up with ditra xl, the thickness should be pretty close. It was never made super clear, bit it does sound like you're using ditra heat membrane, not ditra xl. In which case, two layers of ditra will end up a bit thicker than the ditra heat. You may be able to make up the difference when setting the tile, but it will make your job harder.

All that being said, I would just bite the bullet and pick up some more of the proper material. That way you'll avoid headaches during installation. My two cents.

Tool Guy - Kg
01-04-2020, 07:57 PM
I figure I’ll use a little more thinset below and above and it will all be fine. Thoughts??Easier said than done...and it reduces the effectiveness of the uncoupling (though there’s no industry standard for ‘uncoupling’ and I’m taking Schluter at their word).

Unless Schluter has changed their position, I’ve heard the same thing as Dan: Schluter says doubling up on the regular Ditra is perfectly fine. That’s what I’d do.


01-05-2020, 12:50 AM
Just to be clear, I don’t intend to double up. I intend to use a little ditra on the edges around the ditra heat. However... Ives discovered I have plenty of ditra heat, so it seems to be a moot point. But I considered using regular ditra under the toilet.

01-05-2020, 10:48 AM
Ok, thank you. Double up as in lay ditra normally, then use same amount of thinset to lay ditra normally again- thinset/ditra/thinset/ditra. Yes?