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hardi backer thickness under tile

bleubassplyr's picture

I'm going to install new tile in my kitchen and was told the best thing to do, to keep the floor near the current height, is to take out the sub floor down to the 3/4 plywood that's on the joists. The long and short of it is, if I use 1/2" hardi backer the finished floor will end up being about 1/2" higher than it currently is. FYI--I'm planning on adding an under floor warmer embedded in thinset. Do you think the thinset and 1/4" hardi board are sufficient under the tile or should I definitely use the 1/2" hardi?


The current configuration is 3/4 sub floor, 3/8 ply, 1/4 luan, vinyl. What was recommended for the tile installation is 3/4 sub floor, heating unit in thinset-1/8-3/16, 1/2 hardi, thinset & tile-3/8. What do you think?


 


Bleu


There are more old drunkards than old doctors.  Ben Franklin

There are more old drunkards than old doctors.  Ben Franklin

(post #82906, reply #1 of 21)

My tile floor cracked with 3/4" plywood, thinset and 1/4" hardi backer.  It was kind of the same deal where I wanted the finished floor to be of the same/similar height to the adjacent 3/4" hardwood.  Granted the tile cracked along a bad joist that bowed (this was new construction) but still, I wasn't especially impressed.  That's just my expierence.   My feeling is that 1/4" hardi backer is better for wall applications.

Matt

(post #82906, reply #19 of 21)

>>>> My feeling is that 1/4" hardi backer is better for wall applications.<<<<<


Exactly backwards from the manufacturer's application recommendations.


Jim


Never underestimate the value of a sharp pencil or good light.
Never underestimate the value of a sharp pencil or good light.

(post #82906, reply #20 of 21)

Thanks everyone.


I'm 99% sure I'm going to rip out the 1/4" luan and vinyl, leaving 3/4" subfloor plywood and 3/8" plywood, add 1/4" hardi using thinset and screws as recommended, then tile. I'm still uncertain about installing the heating pad. My cold winter feet may push me over the edge toward warmth (and cost).


There are more old drunkards than old doctors.  Ben Franklin

There are more old drunkards than old doctors.  Ben Franklin

(post #82906, reply #2 of 21)

3/4 ply usually isn't sufficient.


1 1/4" of "subfloor" is the usual recommend starting point.


"subfloor" ... means wood.


 


backer board don't count as "subfloor".


makes no difference between 1/4" and 1/2"


the only difference is 1/4 more or less in height.


 


"if" ... it's not underframed ... a layer of Ditra "might" be suitable over the 3/4 ply.


and the heat mats go over the backer , not under.


 


Jeff


    Buck Construction


 Artistry In Carpentry


     Pittsburgh Pa

    Buck Construction

 Artistry In Carpentry

     Pittsburgh Pa

(post #82906, reply #4 of 21)

The floor heater I'm checking out, stepwarmfloor (http://www.warmfloor.com/component/option,com_frontpage/Itemid,1/) , does go under the hardi in a bed of thinset. The other brands go above the hardi, under the tile. So based on what I'm reading, i think i should leave the 3/4 and the 3/8 ply in place. I'll remove the 1/4 luan, use 1/4" hardi with the heating mat. How does that sound? FYI-the joists are 16" on center and it's a 16 year old house.


Thanks for the advice.


Bleu


There are more old drunkards than old doctors.  Ben Franklin

There are more old drunkards than old doctors.  Ben Franklin

(post #82906, reply #5 of 21)

Ff the mat is below the hardi how can you reliably screw the hardi down and insure you don't hit a wire rendering the mat useless?

Plus the heat has to "penetrate" the hardi and the tile.

Seems like it is a bad translation of chinees instructions.

TFB (Bill)
TFB (Bill)

(post #82906, reply #11 of 21)

I actually ruled out the stepwarmfloor because of the high cost. It was $1,800 for a kitchen with about 130 sq feet. you can screw into the middle part of the product. you have to be careful not to hit the two leads in each roll.

There are more old drunkards than old doctors.  Ben Franklin

There are more old drunkards than old doctors.  Ben Franklin

(post #82906, reply #6 of 21)

I'm an old mud guy so I'm voting for a 3/4 dry pack over the stripped down subfloor. I don't know about your heat mat though so ignore my post if it's not a compatible solution.

If you are really interested in getting the floor flush, take up the 3/4 subfloor, add ledgers and drop a new floor between each joist. YOur 3/4 mudpack will be flush with your current subfloor then.

In the olden days, we always dropped some of the subfloors in this way to accomodate a 3/4 mud pack. There is a lot of ceramic tile laid over 1/2" cdx plywood in the metro detroit area. I suppose the strength of the installation was in the metal lathe and 3/4 mud.

Bob's next test date: 12/10/07

(post #82906, reply #9 of 21)

Thread hijack alert!!!
Sometimes I think I'm the only old guy around here who's messed around with dropped mudpacks...
I happened to catch TOH this weekend, showing a new product to re-secure plaster to lath that involved drilling lots of holes, squirting a primer in them, and then gunning in a proprietary adhesive. Made me feel all warm & fuzzy, since I spent the week digging out every tenth tile in a mosaic floor & squirting superglue under the adjacent tiles (actually more like letting the capillary action pull it under....
Huge PITA, but the room has full tile wainscoat in good condition, and I don't have the time or $$$ to demo the whole thing out right now, so I'm hoping to get a few more years out of the floor-which bring me to my question.
Do you think it's possible to demo out the floor, but save the walls, without jumping through so many hoops that it isn't worth the trouble?

(post #82906, reply #10 of 21)

Why can't you lay a new floor right over the old floor?

Bob's next test date: 12/10/07

(post #82906, reply #13 of 21)

because the old mosaic tile is poorly adhered to the slab, and the additional height would mess up the sanitary transition to the wall tile......

(post #82906, reply #3 of 21)

The best you can do and match the floor height...


3/4 sub under #30 felt under thinset (1/2" notched trowel) under 1/2" Hardie screwed on 4-6" centers under thinset/heating grid under thinset under tile.


and it may still have issues. 


Hardie is NOT structural, but the 1/2" seems to set better on the thinset bed.


Keep some spare tiles.


A La Carte Government funding... the real democracy.

(post #82906, reply #7 of 21)

You should be sure that 3/8 ply is glued to 3/4 subfloor and nailed with at least shanked nails every 6" to make it "structural".  Check the joists under and they should be at least 2x10 on 16".  Add 3/4 ply or blocks to stiffen the joists further.  Check discussion on bouncy floors.


Your choice of heating mat says they have long history, but you can't be sure what kind of application this was.  The fact that they recommend this under the Hardi raises a red flag for me.  Maybe this was fine for commercial application and was embedded in thick dry pack and tiled as normal, but it may not survive well in residential use if tiles were laid directly over it, therefore, the need for Hardi over it.  I am just guessing here, but I assume the Hardi would not be screwed down but glued to the mat with thinset.  I think the company may have commercial history but not residential and they are recommending the use of Hardi as substitute for 1-1/2" dry pack, and I don't think the dry pack and Hardi are the same.  Hardi, in fact, may insulate the heat from getting to the tiles.  I apologize if I am wrong and I have not read anything from this site, yet, but just some logical thinking heads me this way.  I will read up on this kind of mat.


I don't think either Hardi or regular cement boards conduct heat well (wood fibers and loose pack cement) and if I want radiant heat I would want the tiles in direct contact with whichever heating element that was used.


Kerdi separates moving subfloor from tiles but again it is not structural so you should be double sure that the subfloor system is stiff enough.  Hope this helps.


 

(post #82906, reply #12 of 21)

Do you think it's wiser for me to leave the 3/4 sub floor, the 3/8 plywood, then add either 1/4" or 1/2" hardi then tile? Would that be sufficient? The floor joists are 2 x 10's, 16" on center. I don't imagine the mat type heating pads add any structural strength, do they? i know the mat would probably add a solid 1/8". I'm already planning on raising the countertop height so I'll be sure to have enough room to fit the dishwasher.


There are more old drunkards than old doctors.  Ben Franklin

There are more old drunkards than old doctors.  Ben Franklin

(post #82906, reply #14 of 21)

As far as I know, several ways were tried in the past, although current trend is for hardibacker or cement boards.  Another way was to nail on expanded metal sheets (I'm not sure about this name) then thinset then tile as normal.  As posted by others, tile backers do not add any strength to the structure.  You can use 1/4 hardi.  I think 1/2 hardi probably will hide imperfect floor better, bumps and so forth.  On walls, 1/2 hardi is a must unless there was another backer.


2x10 joists is minimum accepted structure but then you have to see the span of these joists.  I think another poster said 2x10 can span up to about 13' and you can tile over such floors.  You can try jumping on the center of the floor and see if it flexes, that's usually what I did.  Try tile association website for possible span chart for tiling.  There is one website often mentioned in this forum, Johnbrowntile.com?  I forget the exact name.  You can search this forum under tile questions for the link.  I only searched that site for sealing recommendations but he may also have suggestions for having the right wood floor for tiling.


There is no special trick to construction.  Either your method works or it does not.  If you want level floors, something has to be built up or down.  If you build down, that means you are taking something away from the structure.  If you have to take away, you can add from below.  If you have to add, and if the work is acceptable to you, try removing 3/8, if not glued, and add thicker ply to level the floors.  I think this would be better situation for tiling.  Use construction glue that does not creep or move.  Use screws instead of nail, and find the joists as much as possible.


If you must have radiant floor, you must have it, but I think just insulating the joist bays from basement may give you some comfort without the feeling of walking over cold floors.  In fact, however, when I renovated the current house, I initially regretted not considering radiant floor.  I just went with forced hot air because that is what the original house came with.  But air system reacts quickly to my temperature controls and I can leave the house fairly cold when noone is in the house and warm it up quickly later.  With energy price these days, I don't know if radiant heat is good idea when it needs to be warmed up over long period and kept warm all the time.  I am sure someone who is affected has or will ask this question.


Your dishwasher consideration is a good catch.  Don't go overboard, though, try a mock up with a cardboard and see what height you need for dishwasher to fit through an opening.  The adjustable legs may let you fit the washer without lifting the countertop.  You have to go in far enough with tiles so the bottom plate will cover the tiles.  Hope this helps.

(post #82906, reply #15 of 21)

Thanks. I think I'm going to leave the 3/4 and 3/8 plywood in place. I'm going to remove the vinyl and 1/4 luan. I'll add 1/4" hardi and tile over that. That'll raise the current floor by about 1/2". If I raise the counter 3/4" there will be plenty of room for the dishwasher installation. I have to figure this out soon since my new granite counter will be cut and installed by next week. I'm going to raise the counter by making some decorative molding and adding it to the top of the cabinets.


Why use hardi at all if it adds nothing structurally?


There are more old drunkards than old doctors.  Ben Franklin

There are more old drunkards than old doctors.  Ben Franklin

(post #82906, reply #16 of 21)

Tile will stick better to cement related backers then wood.  For one thing, hardi will move less.  Some people prefer Kerdi for this reason.  I saw more than a few tiles glued to ply subfloors using mastic, and these tiles can separate and crack.  Your ply subfloor also may not have even joints and even surface.  Many perfectionists will level the hardi backer with thinset, but for surface that is level by eye, most people will just use screws.  Wet the hardi with sponge so thinset does not lose too much moisture.  And also stagger the ply and hardi joints, for better surface.


By the way, if your subfloor is wavy, you may want to even it out by using thinset under the hardi.  Hardi will follow the curvature of the subfloor and it will show up on the finished floor.  I am not talking about gentle curve from wall to wall or straight and even floor that may be out of level from wall to wall.  I think you should ignore such problems.  Don't try to level the floor with the tiles.  But if you have dips and rises across the floor, you can even them out with hardi and thinset under it.  This way you don't have to try to level the tiles one by one.  Use 1/2" square notch trowel, anything less will not give you enough thinset for floor tiles.  Lay out straight and square lines and as many lines as your arm will reach.  I found pencil works best.  Pencil lines can become covered but as long as you are a bit careful not to scrape the thinset across all the lines, you can see it.  Lay out the thinset as much as your arm will reach and it won't get out of control.  If you are using really small tiles, your work area should be smaller.  Don't let the thinset dry out but if it looks a bit dry, you can back-butter the tile thinly for good adhesion. 


Actually I like tiling, as you can see, but I'll stop here.  Good luck.

(post #82906, reply #17 of 21)

Jeff Buck gave you the right dope early on; your subfloor needs to be 1¼" thick--and it needs to be plywood. As he stated, Hardibacker (or any of the other CBU brands) is not structural. Neither is luaun or OSB.


The thicker subfloor is needed to avoid flex between joists set on standard 16" centers. If you don't have that much structural subfloor, you risk having tiles crack even if your joist depth and span meet the L=360 maximum deflection for ceramic tile.


CBU is used primarily for wet areas, because cementitious underlayments don't rot or get mouldy if they are damp or wet. The use of CBUs has nothing to do with tile adhesion. Tile will stick just as well or even better to plywood if the thinset is properly mixed and slaked. I have torn out tile jobs over both plywood and CBU, and it is often easier to pop a tile off a CBU than it is to get one off plywood.


Using thinset to level a subfloor isn't a good idea. Thinset is not structural, either; it is an adhesive. If you have a substantial deviation from level, you should level the floor with a latex-modified patching concrete designed for that purpose, or with an SLC. You can, however, use thinset to bed the second sheet of plywood to an old subfloor, although PL Premium is more commonly used for this. You should use thinset to bond any CBU laid over the plywood.


All that said, if you want your floor to be flush with the existing floors, you've got a couple of choices. 1) You could do a dropped mud job as Blue (Jim Allen) suggested, or 2) you could cut down the joists and install a total of 1¼" of plywood subfloor low enough so that the additional height of the Hardibacker, heating mat, and tile will still come flush with everything existing around it. Depending on the unsupported span and how much you cut the joists down, you may have to sister the joists to keep your deflection below L=360.


I have no opinion or advice to offer on the heating mat; I don't do those.


 



Dinosaur


How now, Mighty Sauron, that thou art not brought
low by this? For thine evil pales before that which
foolish men call Justice....


Dinosaur

How now, Mighty Sauron, that thou art not brought
low by this? For thine evil pales before that which
foolish men call Justice....

(post #82906, reply #18 of 21)

"your subfloor needs to be 1¼" thick-"

Where does that come from?

TCA method F144 calls for 19/32 min ext plywood subfloor over 16" OC joist that meet 1/360.

Must be flat within 1/4"

Then use thinset ot form supporting bed and then CBU or fiber cement underlayment fasten per manufactures specs.

Now for a direct to wood install F150

19/32" ext ply sub-floor.

Then ext ply underlayment with fastener into sub-floor, but not joist. 15/32" residential, 19/32" light commercial.

1/4" flatness and no adject plywood edges more than 1/32" difference.

then latex modified thinset to set the tile.

.
.
A-holes. Hey every group has to have one. And I have been elected to be the one. I should make that my tagline.
. William the Geezer, the sequel to Billy the Kid - Shoe

(post #82906, reply #21 of 21)

"your subfloor needs to be 1¼" thick-"


Where does that come from?


From me. From Jeff Buck. From a bunch of people posting over at JohnBridge. And also pretty much from the TCA specification that you quoted:



19/32" ext ply sub-floor....Then ext ply underlayment with fastener into sub-floor, but not joist. 15/32" residential, 19/32" light commercial.


Okay, I used the word 'subfloor' to comprise both the actual subfloor and the underlayment. Mea culpa. So sue me. The OP's gonna be running Hardi on top of all that anyway, so it don't make no never mind to him if the second layer of ply is isolated from the joists or not.


And okay, maybe he can get away with a total of 1-1/8" or even 1-1/16". If he's running 2x2 porcelain and not 16x16 terracotta; if the joisting is in perfect shape; if the floor really is flat to <¼"; if there are no 300-pounders in his house; if he's a pro and does everything else perfectly....


That's a lotta 'ifs', Bill. I'm more comfortable giving him the standard 1¼" rule-of-thumb. An extra eighth of an inch won't do him any harm, and it might save him some grief.



Dinosaur


How now, Mighty Sauron, that thou art not brought
low by this? For thine evil pales before that which
foolish men call Justice....


Dinosaur

How now, Mighty Sauron, that thou art not brought
low by this? For thine evil pales before that which
foolish men call Justice....

(post #82906, reply #8 of 21)

I just read over the website for this heating mat.  I am still not assured.  I saw a picture of it as installed under a subfloor from the basement.  This mat is series of strands connected together and that may be one reason they want you to use Hardi over the mat.  The strands would make the thinset troweling almost impossible.  The strands would catch all the time on the trowel.  Under the subfloor installation is also troubling because these mat strands are not in contact with the subfloor.  For example, hot water radiant heat installed this way always use aluminum panels that get direct contact with the subfloor.  This mat would be heating the air then the subfloor, and even if the joist bays are well insulated, it would have to heat the joists as well .   


I wish this company would explain more about what type of plastic this is and give more reference to where this was used and how long the plastic would last under real life conditions.  It is also a mistake for them to showcase the diy tv show since these shows don't care about life after the installation.  Also if you have a power-out in winter, this mat would be useless and would take aweful long time to heat back because of its low energy requirement.


This mat sounds interesting as day time bath or foyer heater if it can run itself with minimum number of solar panels.  If it can be connected to one or two panels just to heat the least used spaces in a house, it may be worth the money.  Cometo think of it, I wonder if this can be used to heat the pool.  Hope this helps.