To Code? Hardie backerboard instead of plywood or ditra?

6 years ago

Hi everyone!

We have a front entrance reno, there was water damage from the old door which is being replaced. A tiler who just gave us a quote says he recommends using Hardie backerboard. Existing sub floor is 5/8" plywood, with a 1/2" plywood on top. He wants to take off the top plywood and replace with 1/4" backerboard so that the transition to the living room hardwood floor is flush with the tile. So:

1) is the backerboard on top of the 5/8" plywood to Ontario / Canadian code?

2) is the backerboard going to give it sufficient strength regarding flex? (ie like why the 1/2" plywood was used);

3) I hear that the silica dust ingredient in the Hardie backerboard is carcinogenic, so I envision asking it to be cut outside, but does anyone know about after it's installed? ie off-gassing or other issues?

4) if it's a question of the threshold not being perfectly flush with the hardwood, but better to use ditra or just 1/2" plywood again, I'd rather do that, if there are issues issues from the above questions.

Thanks so much in advance everyone!

Featured Answer

SJ McCarthy

6 years ago

Cement backer board does NOT add rigidity. By removing the top 1/2" plywood, the installer will be weakening the flooring system. The deflection rating of the subfloor construction (5/8" plywood over wood joists) may not be rigid enough to handle the removal of the extra 1/2" plywood.

Many tile setters love working with Ditra Membrane. It may be pricey (that's fine...the homeowner will end up paying for it) but it is fast, easy and worth its weight in gold when it comes to water intrusion.

In Canada, the best practices for subfloors = 2 layers with a total thickness of 3/4" or more. The argument will start with "Is cement board a subfloor/structural product or is it 'underlayment' for the tile?"

As for cutting hardy board/cement board, don't worry about carcinogens. The amount of cutting (maybe 5-10 cuts) isn't going to do anything. There are tile professionals who have been cutting backer board for 30 years...and they are still here.

You have to work in a Silica MINE in a 3rd world country with your T-shirt as your only method of "dust protection" to get to the point of lung cancer. Again, I wouldn't worry about a few cuts in a doorway.

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Comments (21)

Edmund Lee
Original Author
6 years ago
last modified: 6 years ago
Hi SJ!

Thanks for your reply! Just to clarify, you would consider Ditra to be just as good to lay on top of the 5/8" plywood, in lieu of the 1/2"? This is for a front entrance, not a bathroom, but I'm just wondering that besides the moisture benefits if it's a suitable replacement. Otherwise, I'd just stick to the 1/2" plywood. Or is the Ditra meant to go on top of the 1/2" ply as well, since it's only 1/8"?

Thanks!

Ed
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millworkman
5 years ago
Over as Ditra offers no structural qualities.
Edmund Lee
Original Author
5 years ago
great thanks both
PRO
Cinar Interiors, Inc.
5 years ago
If the installer is laying tile in the area you'll want a cement board used, not plywood or osb. Plywood and osb draw the moisture from the setting materials too quickly not allowing the product to cure properly which will cause the tiles to pop loose over time. If you're worried about the thickness of the product, 1/2" hardie is available.
SJ McCarthy
5 years ago
You need Ditra OVER TOP OF new plywood. Not instead of.
Edmund Lee
Original Author
5 years ago
At the moment, if we are laying tile directly on top of the 1/2" ply (which is over the sub of 5/8") the area is already slightly higher than the hardwood next to it. Maybe 1/8" and I'm ok with having to transition that not being flush. But if we add another 1/8" whether it's backerboard or ditra, that's quite a rise of 1/4". Of course having a structurally sound floor is more important but now I'm getting confused. I'm hearing that we should not be taking out the 1/2" plywood and only lay on top of that backerboard or ditra. The tiler says to take it out and replace with backerboard, as well as Cinar...
ksc36
5 years ago
"The tiler says to take it out and replace with backerboard, as well as Cinar..".

Here that wouldn't pass code. 5/8" ply always gets an appropriate subfloor, backerboard doesn't count.

You're much more likely to get cracks and pops with cement board over 5/8 than 1/2" ply. I've ripped up enough 1/2" ply with tile to know they don't "pop loose".
PRO
Cinar Interiors, Inc.
5 years ago
I don't care what your subfloor is constructed of or how thick it is. Any wood subfloor should not be tiled directly atop as the wood will draw the moisture from the setting material too quickly and not allow it to cure properly. Fix your subfloor to code for your area and then install a cement board atop and then tile.
PRO
Cinar Interiors, Inc.
5 years ago
If the subfloor + cement board + tile is too thick/high for the area, maybe a glue down vinyl plank/tile would be better.
ksc36
5 years ago
It's always best to read the manufacturers instructions than listen to unqualified internet posters. You get what you pay for...

Suitable Substrates

Exterior Grade Plywood

https://www.custombuildingproducts.com/products/setting-materials/polymer-modified-thin-set-mortars/flexbond.aspx
SJ McCarthy
5 years ago
OK...stop the madness. Let's get down to brass tacks. You need 3/4" of THICKNESS for the subfloor material (we already have established that hardy board/cement board does NOT count...nor does Ditra). That thickness MUST BE 2 layers.

Right now you have 5/8" (slightly thicker than 1/2") and you have 1/2" plywood on top of that. You currently have a total of 1 1/8" of subfloor grade material. This is an excellent substrate thickness to lay TILE.

Remember: tile is VERY HEAVY. So you need a RIGID subfloor to PREVENT the tile from sagging. Right. That's why we need thickness that is STRONG...not just thickness (cement board isn't strong...so it doesn't count).

The SIZE of your installation (this is an entranceway, not a 200sf kitchen) together with your joist spacing will point you in the direction you need to go. In other words you need to know the total SQUARE FOOTAGE of your project (square footage will give us SOME IDEA of what is going on here).

And you have thrown in another obstacle - floor heights. The entranceway is HIGHER than the surrounding hardwood. Oh jeeze. Here we go. I would love to know how THAT happened...but I digress.

So...you are faced with a world of confusion. How about we make it easy on you: vinyl. You can find some STUNNING vinyl tiles that can go straight over the plywood that you have.

But before you change the channel, how about you send some PHOTOS of what is going on. And just for fun, I would look to another tile setter. Someone who understands deflection ratings and subfloors, etc. Someone who *should know the answers.
User
5 years ago
Seems a few folks here need to order this. OP included.

https://www.tcnatile.com/products-and-services/publications/218-english-publications/188-handbook/957-2018-tcna-handbook-for-ceramic-glass-and-stone-tile-installation.html
Edmund Lee
Original Author
5 years ago
last modified: 5 years ago
It's not a big area nor wide. 72 sqft. Small powder room (toilet and sink) and closet on the left. The old tile was laid directly on the ply, old school style and not one was cracked. Perhaps because the area is quite narrow and there wasn't much play. That tile is at least 20 years old.

Joists run left to right of door, are 16" apart, 2x10s.

If I can get away with laying 5/16" Ditra XL on the 5/8" ply, I'd do that. If not, adding Ditra 1/8" on the 1/2" is going to bring a big height difference to the hardwood. Right now it looks to be 1/8" and that's without thinset. If I have to lay on top, then it'll have to be a longer transition of 3-4" maybe. The new tiles are small-ish hex tiles at 10x11", not long 12x24s.

http://www.geotiles.com/en/?portfolio=starkhex
PRO
Joseph Corlett, LLC
5 years ago
"Plywood and osb draw the moisture from the setting materials too quickly not allowing the product to cure properly which will cause the tiles to pop loose over time."

That's why you wet it first.
Edmund Lee
Original Author
5 years ago
last modified: 5 years ago
I just realized why don't I rip our the 1/2" ply, replace with 1/4" ply, add 1/8" Ditra. I get more stability. Have the Ditra for movement, and then we will be close to the level of the hardwood... thoughts? Seems like a decent compromise.
SJ McCarthy
5 years ago
@Edmund Lee...you are onto something there! Should work nicely.
PRO
Avanti Tile & Stone / Stonetech
5 years ago
Here we go......1/4" plywood has NO place in a tile installation. ANY moisture will cause delamination, so don't go there!

I would install regular Ditra over the existing and tile away. At your transition, that's what they make wood clamshell reducers for....or even possibly a "T" molding strip.
Edmund Lee
Original Author
5 years ago
ok,.. sounds like a good reason for not putting down 1/4" - but wouldn't the chances of moisture getting on there fairly low? I thought one of the 2 biggest benefits of the Ditra is that it's the barrier?
PRO
Avanti Tile & Stone / Stonetech
5 years ago
Yes, it is...but if you're not using 1/4 ply, it;s not an issue over regular plywood.
PRO
James Hardie Building Products
5 years ago
Hi, Edmund! There is no reason to be concerned about continued use of
HardieBacker Cement Board. Dust generation is only an issue if our cement board
products are not cut according to our guidelines. We have various solutions to
make it as easy as possible to use our products properly and safety. You can
learn more at: https://www.jameshardiepros.com/safety

To Code? Hardie backerboard instead of plywood or ditra?

SJ McCarthy

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