NEWBIE Tile over radiant floor [Archive] - Ceramic Tile Advice Forums - John Bridge Ceramic Tile


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05-25-2019, 12:50 PM
Hi All,
My Family and I are in the process of building a house, and have decided to install tile in some of the rooms. I have volunteered to do the tile, and have been enjoying learning how.
Most of the rooms I am going to be tiling will have radiant heat in them. We are going to be laying down 5/8" OD PEX on top of our subfloor for the radiant.
The tile has to finish out at 1 1/2" above the subfloor, so my dad came up with an idea

Here is a cross section he drew:

Lay down a 7/16" layer of mortar,
Put 1/4" hardie backer on top. This will build the thickness up to 11/16" thick.
Put 7/16" thick layer of mortar on top.
Put our tiles on.

We are not sure what we will do about the gaps for the PEX, (highlighted in red)
We have LARGE FORMAT tiles (12x24 by 3/8" thick)

There are a few things I am worried about:

Is it OK (or even possible) to trowel medium bed mortar to 7/16" thick? I read that it ranges from 1/2" to 3/4" thick.
Will there be problems with the 1/16" inch difference between the backer and PEX height?

Is this a workable solution?
If not, does anybody have something better?
Thanks in advance!

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05-25-2019, 12:59 PM
Welcome, JohnPaul. :)

A geographic location in your User Profile is frequently helpful in answering some types of questions.

Tell us first what you have for a joist structure and subflooring if this is on a wood framed floor.The tile has to finish out at 1 1/2" above the subfloor... Tell us just why that is. I customarily recommend you make your subfloor suitable for tile, then make necessary transitions to adjacent flooring. You're not doing that.

There are a number of other problems with your dad's plan, but let's start there.

05-25-2019, 03:12 PM
Hi CX,
Thank you for replying.

We have 9" I-Joists spaced 16" o.c., and 7/8" thick engineered OSB for our subfloor.
As for the 1 1/2", the height can vary slightly, but changing to much would require rebuilding the stairs (this is a two story house). We can transition, but we do not want to create a trip hazard.

Thanks again,

05-25-2019, 04:07 PM
Ah, yes, planning is everything. And my customers usta think I was joking when I'd tell them I wanted to know what kind of window treatment they planned before we poured the foundation.

Sounds like a decent subfloor to start with, but without knowing the unsupported span of your joists and the manufacturer, that's never certain. Engineered floors are usually designed with adequate deflection, but only when the type of floor covering is known in advance. Your plans included these hydronically heated floors?

The thinnest possible tile industry accepted method of which I'm aware would be a Self Leveling Underlayment over the hydronic tubing. That requires a plastic lath and usually a minimum thickness of 1/2" of SLC over the top of the tubing. If your tile is 3/8ths" thick, that would theoretically give you you about 1 5/8ths" overall thickness, but in reality it would be a bit thicker than that and I'd also strongly recommend you use a full coverage crack isolation membrane under the tile installation. That would add another 1/8th" or so. The whole installation would add about 20 pounds per square foot of dead weight to the floor. Y'all planned for that in your joist design? That would also be a bit spendy. SLCs are not inexpensive to install.

Poured gypsum, which must be a minimum of 3/4" over the top of you hydronics would be the next thinnest and that one requires the use of a membrane of some sort under the tile and accounts for approximately the same dead load gain.

The very best method, in my opinion, would be an unbonded, reinforced mud bed over the tubing, which would amount to about 2 3/8ths" over your subfloor height at a minimum. The materials are less expensive than the other methods, but the added weight would be in the neighborhood of 30 psf.

If you can gain access to a TCNA (Tile Council of North America) Handbook you can research those methods as RH123, RH122, and RH141 respectively.

There is no acceptable way (and I can't imagine a realistic way) to install a CBU with a 7/16th" layer of thinset mortar under it. You'd need to follow the CBU product manufacturer's installation instructions, which do call for a supporting bed of mortar, but applied usually with a 1/4" square-notched trowel and mechanical fasteners. Just one of the problems you'll have with your dad's proposed method. The second problem that jumps out at me would be the 7/16ths" layer of bonding mortar under the tiles. While that thickness is not a problem for many of the LHT (Large and Heavy Tile) mortars, actually applying it in that thickness as a consistent layer would be.

How firm are you set in having this hydronic heating?

How far along is the construction of the stairs in question?

My opinion; worth price charged.

05-25-2019, 05:29 PM
Hi CX,
Thank you so much for your advice!
Our plans did include radiant flooring, but I am not sure we planned for tile.

About the TCNA handbook,
Do you think that a tile dealer would have one on hand?
I doubt Lowe's would. (There 'Tile installation class' banner has the thinset troweled wrong)

How firm are you set in having this hydronic heating?

How far along is the construction of the stairs in question?

We already have the boiler installed, and are pretty firm on it downstairs.
The stairs are finished except for the railings, however we can do wood floor around them.
I'll talk to my dad, and get you more details.

We did run it all through a structural calculation program, but I do not know what we were calculating :) .

05-25-2019, 06:35 PM
Hi CX,
This is Aiden, I'm Johnpaul's Dad. Thanks so much for your thoughtful replies and advice. If I understand you correctly, it appears that there just isn't going to be a way to install tile on a wood sheathed radiant floor without exceeding 1 1/2" overall thickness - is that the basic take-away?
Just FYI, the TJI's are only spanning 12' in this area and are well over L/480.

05-25-2019, 08:10 PM
Howdy, Aiden. :)

Yeah, pretty much so. Y'all are free to do that floor in any manner you see fit, of course. I don't like to rain on parades, but all I can do from here is tell you what the tile industry consensus is gonna tell you. That's pretty much what you find in that TCNA Handbook I cited.

Can you cheat those published methods and get by? Maybe, but consider also that they are all the minimum requirement in each case.

If you just can't live without the hydronics, and you can't possibly make accommodations in your stairs, maybe you could consider a smaller tubing and the SLC method. Gonna cost you some serious dinero if it's a large floor and you're gonna need a good bit of help to pour it unless you hire it poured (my recommendation), but you could maybe stay close to your 1 1/2" limit. Maybe.

My opinion; worth price charged.

05-25-2019, 08:20 PM
Thanks CX!

05-25-2019, 08:54 PM
I worked up the SLC idea and am getting a little over 2" thick from the subfloor. Am I missing something? I'm assuming that even with the SLC I still need 1/2" mortar bed for large format tile?

05-25-2019, 09:16 PM
OK - one last idea - would this work?

clifton clowers
05-25-2019, 09:18 PM
1/2" of mortar under tile seems awfully thick.

The Viega climate panels ( use 5/16" PEX and are thus thinner than you are accounting for.

The Viega installation instructions do not require any SLC or gypsum over it. They only need backerboard for tiles (and nothing for hardwood.) Instructions are linked from the above link.

Are your joists open from below? Could you install the radiant floor to the underside of the subfloor?

How about an electric mat?

05-25-2019, 09:20 PM
The last Idea I posted is similar to one being spec'd by a radiant floor system company.

05-25-2019, 09:30 PM
Aiden, you do not need a half-inch layer of bonding mortar under those tiles. It's commonly recommended to use a half-inch square notched trowel for those large format tiles, but that's not gonna provide even one-quarter-inch of bonding mortar under your tiles. And tiling over a very flat substrate such as you might get with your SLC installation, if your tiles are also very flat, you can get by with less than that. The tile industry standards require only a minimum of 3/32nds of an inch of bonding mortar under the tiles once set.

My opinion; worth price charged.

05-25-2019, 09:57 PM
OK - that helps to know. I appreciate your patience.
Clifton mentioned the Viega radiant system, which calls only for backer board above with a tile installation. If I were to use backer and a decoupler I'd be right about where I need to be thickness wise.

However, my son Johnpaul told me that the backer board needs to float on a mortar bed b/c otherwise when it is screwed down it somehow gets small voids under it. That creates a little spring that cracks the tiles.

Do you think backer board needs to float on a mortar bed if screwed down to a plywood substrate?

05-25-2019, 10:06 PM
Look at Schluter’s Becotek.

05-25-2019, 10:07 PM
Interesting thread and nice diagrams. A mistake was made on the stairs. That's in the past. What would be thinner than tile on a radiant floor? Not by much.
I know it's hard to take but CX is right. Think about proper tile install first then worry about transitions. I was guilty I the past of thinking about transition height first. I guess one stair solution is a step down off tile to landing at the stair bottom. Look into how hard it would b to redo the stair tread height spacing.
Look for the thinnest radiant floor heating system. Radiant heating is wonderful. You could find 5/16" tile and like CX said 3/16 of thinset on a flat floor may be fine. How far off are we on the bottom tread height? Surely it was made larger by some amount. Was this where the 1.5" number came from?

05-25-2019, 10:08 PM
I would not recommend a CBU that was not bedded in thinset mortar as required by all the manufacturers of which I'm aware.

clifton clowers
05-25-2019, 10:34 PM
To be clear, Viega, like everyone else, would require the backer board (which is 1/4") to be set in mortar. What they don't require is a thick layer of SLC.

05-27-2019, 06:41 PM
Bekotec is 1-11/16” plus your tile and mortar.