Screw Heads and Hardibacker [Archive] - Ceramic Tile Advice Forums - John Bridge Ceramic Tile


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Paul M.
08-15-2001, 09:05 AM
Help...I'm a first time installer and I'm working on a 550sf kitchen/laundry/bath porcelain tile project. I have a particle board over plywood subfloor. On top of this I have thinset and screwed 1/4" hardibacker. I used daltile thinset with liquid latex additive and 1 1/4" backerboard screws in a 6-8 inch grid pattern. The screw heads didn't all go down completely flush with the backerboard. Will the 1/4" troweled thinset going on next take care of this? Also, there are a few spots where the backerboard didn't stick to the thinset (I used 3/32" trowel size)...I am ready to lay the tile, but I want to remedy these issues first....

Any advice? Thanks

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Rob Z
08-15-2001, 09:50 AM
Hi Paul

Thanks for stopping by.

I have never installed Hardibacker, but several of the guys around here use it a lot. The question about the screw heads is one that really can be answered by seeing it. What I mean is just how high are they? A 32d of an inch is probably not a problem. High by an 1/8" is definately a problem. I do know that the users of hardi have said it can be a little more difficult to drive screws flush in Hardi because it is so dense.

The 3/32" trowel is undersized for the installation of the hardi. The recommended trowel is a 1/4" notch, and I have even used a larger trowel for installing cement board because I didn't get good coverage under the board with the recommended 1/4" trowel. Your Hardi may not be "stuck" in places because of the small notch size, or the thinset skinned over before the Hardi backer was set in it, or because the glues in the particle board prevented a bond with the thinset.

The thing that concerns me most is that you have particle board under the hardibacker. Of even more concern is that this is a kitchen/bath/laundry room installation. Particle board is not recommended as an underlayment of any sort for any ceramic product that I know about. Any moisture that gets to the particle board will cause it to swell like crazy.

Now, before we advise you to tear up the whole thing and start over, let's try to think of a solution. 550 sq feet is a large demo project in itself, plus all the time and materials to get you to this point.

We can help solve the water problem with the particle board by installing a waterproofing membrane over the Hardi. Waterproofing that much area will be an expense, but not as bad as tearing the whole thing out and starting over.

Next, we have to figure out what to do about the inadequate amount of thinset under the Hardi. There is a big debate in the tile industry about whther or not the thinset under the backer board should , in effect, "glue" the backer board to the floor. Without getting into a discussion about that, one thing that is agreed upon by almost everyone is the use of thinset under the board to fill in all the voids and cavities that result when you lay a rough piece of backer board over a rough piece of under layment. The thinset fills these little voids and adds strength to the whole tile underlayment.

Extra fasteners MAY help overcome this lack of material, along with the use of a premium, very flexible thinset used to set the tile on the floor. I emphasize MAY because I don't know. Let's see what ideas John and crew have to help you on this one.

I'll check back in later tonight.

Rob Z

08-15-2001, 01:21 PM
What thickness is the particle board?How old is it? Has it ever been wet? If so I recommend removing it and replacing with a good exterior grade plywood.I know that's not what you "want" to hear.Once particle board has been wet the deterioration process is greatly accelerated.If it is very old it will have begun to soften and seperate.If you can break a piece off the edge easily and crumble it in your hand it's garbage and will have to be replaced with plywood.I worked with this stuff extensively in mobile homes.Moisture,even from ground evaporation,will cause it to swell and deteriorate and even heave up or buckle.I've seen extreme cases where it was heaved upward as much as 3 inches....looked like a big bubble.

If it isn't feasable to tear it out and put in plywood I recommend using 1/2" backer,layed on modified thinset spread with at least a 1/4"x1/4" notch trowel screwed every 6"

Bad news...if there are "voids" under the backer you will have to fill them.Left as is they will allow deflection(up/down movement) to exceed the acceptable amount for a Tile installation.This can mean cracks or even loose tiles down the road.Let's see what some of the other guys have to say.We'll put our heads together and help you get the quality installation you want without breaking the bank.

Keith Alford
"Alford Tile Setting"

John Bridge
08-15-2001, 03:47 PM
Of course you're right, Keith, but I don't think Paul is going to tear out everything he's already done.


Have you covered the entire area with Hardi-backer? How much have you done?

If you've already done the whole thing, then we'll have to try to fix the areas that didn't get enough thin set. Keith is very right about that. The tiles and grout will crack for sure if there's any movement under them.

If you've got the whole area covered, I would agree with Rob that is should be waterproofed. I don't know how familiar you are with particle board, but it can swell to twice its original thickness if enough water gets to it.

So give us a very precise status report, and we'll give you our best advice.


Paul M.
08-15-2001, 04:21 PM
Thanks for all of the great advice...

Status: Unfortuneately I have layed, thinset, and screwed all of the hardibacker in place. There are a few places with minor flexing that I have added additional screws and it seems to have firmed it up. I'm using fiberglass joint tape and latex admix in hopes of tying everything together as tight as possible, but we'll see.



08-15-2001, 05:49 PM
You're right John.I wouldn't want to either.I think we need to make extra sure that the joists spacing is positively not wider than 16" on center to start with.You guys are right about the waterproofing too.We've got to figure out how to fill those voids under the Hardi.I'm afraid if we don't,when you put "extra" screws in it you'll have dips AND stress points in the backer.It could allow the screw in the area with insufficient thinset to pull through over time thus causing a failure.....Did I explain that right? I would actually be less concerned if it were Wonderboard instead of Hardi cause it's more forgiving.Wut chawl thank?

08-15-2001, 06:38 PM
Paul, particle board is the worst as a flooring material, and especially a bad combination with tile/stone. You've put in alot of work if you already have it in place. I guess its decision time. How much work do you want to do setting/grouting tile only to have the installation become a constant source of aggravation? (Continual repair at best, total installation failure at worst)

Hardibacker's minimum subfloor recomendations are 5/8 EG plwood or 3/4 EG OSB. This over 16" oc joist. The one possible benefit is your use of screws vs nails. Particle board will not hold a nail. If it gets wet, the screws won't be much better, but maybe you'll get lucky.

I've got a couple of ideas, non of them real good, but some may be the lesser of several "evils". Since you did use screws and they are not properly countersunk anyway, how about backing out all the screws and pulling the hardibacker. You might be able to salvage most of the hardi after you scrape the thinset/particle board off the bottom side. Then remove the particle board and replace with plywood. Alot of work and a demoralizing process, but probably the best bet waaayyy down the road. Someday this will be a vague memory, especially if the subsequent tile installation is alive and well.

Option #2, further secure the hardibacker where there are problems and use a roll-on/trowelable waterproof membrane over the entire installation and cross your fingers. To secure the hardibacker, you'll need screws/nails long enough to penetrate the particle board and bite into the plwood.

Option #3, ask Rob or someone else on this forum that has used Ditra about using that over the entire installation to not only waterproof, but maybe "uncouple" the tile installation from potential problems. I don't have any expertise here, and don't know if this is a viable option.

Option#4, gasoline, match, and insurance!!! Just kidding. Good Luck!

Rob Z
08-15-2001, 07:15 PM
Hi Latney

I thought about Ditra, as well, but figured the cost alone would cause such sticker shock that we'd lose the patient.

On second thought, the square footage (550 ft) is just under two FULL rolls of Ditra, so Paul could get a good price on the material and have little waste. With the Kerdi at the seams and at the coves, the floor would be waterproofed. And the uncoupling feature of the Ditra would absorb some of the flex under the Hardi.

Paul, if you're still with us and want to know more about this, let us know.


08-15-2001, 08:38 PM
How does the uncoupling handle the flex up and down?

As a wise#%@ once said, Curious minds want to know!!


Rob Z
08-15-2001, 08:51 PM

The flex up and down due to innsufficient thinset under the Hardi is probably on the order of a 16th of an inch. I'm guessing that the Ditra with the use of a very elastic thinset can handle that.

This is triage here. I don't have a clue as to whether or not this will work. Let's remember we have try to get paul some ideas short of "tear the whole thing out".


Bud Cline
08-15-2001, 09:09 PM

I personally think this job will not last as long as it could/should, but hey the money is spent now. I wish you had come here before the particle board was installed. But you didn't.

At this point I would add a bunch more "longer" screws so that they bite into and "thru" the plywood sub. I think the screws you have now are running out of "full" thread by the time they get to the ply and the holding power of the particle board is virtually non existant. In fact I would rescrew the entire job with longer screws. I would also think you also don't have a sufficient bond between the particle board and the Hardibacker. My guess is if the screws weren't there the Hardi would be peeling by now. How much did you pay for the thinset you used?

The joist spacing is a serious issue at this point because a couple of threatening violations already exist but it may not be the end of the world, YET.

I'm not sure what latex admix is that you refer to but I wouldn't use it either at this point.

This thing may be salvageable at this point considering the cost to start over.

John K
08-16-2001, 05:42 AM
Here is the deal Paul. Bud Cline hit it on the head. #1
the screws you are using are not for Hardibacker, they are for cement board. They will not countersink. #2 The best bet for you is to tear everything out, all the way to the subfloor. Put in exterior grade plywood, 1/2" wonderboard, or 1/4", thinset and screw or nail and now you have a excellent substrate for your tile. So at this point you will
be able to take your time and set the tile and sleep alot better at night.#3 Or you can back out the screws, go to
Home Depot and get 2/1/2" " Deck Mate" screws with the special bit and these will countersink in Hardibacker.

John K

08-16-2001, 07:04 AM
Hey Paul,
I hope we're not scaring you off with all this.The thing is to go ahead with the particle board there is a gamble and none of us can even guess how long it will last.

The bottom line is; If you want a worry-free installation and piece of mind,the particle board needs to go.I have,at the customers request,installed over particle board with Hardi but...The joists were on 16"center or less,(2"x12") the particle board was solid and there was a moisture barrier under the house with adequate ventilation.I you keep the particle board,put the plastic on the ground under the house,make sure there are plenty of vents in the foundation and install extra joists if the spacing is wider than 16" OR if the joist size is less tham 10" (2"x10")

Paul M.
08-16-2001, 08:47 AM
Thanks again for the great advice. Here's a little more detail on the installation so far:

The house is 9 years old and the joists are 16"oc and there is a moisture barrier under the house. The Hardibacker is solid in all but a few places. I think the longer deck screws are a definite option. I used Daltile basic gray thinset mix with Mapei acrylic latex additive (instead of water) for better bonding for the backerboard. I also plan to use the same mixture for setting the tile and I have bought modified Mapei grout. All of this is an effort to give this job the best chance for success. It sounds like the most cost effective solution for increasing the chance of success for this project is to install longer screws and more of them. All of your advice is right on and true.
Thank you. I now understand this wasn't the preferred method of installation, but is it doomed for certain failure? Call me stupid, but I'm not as worried about the possibliliity of water damage as I am about tiles popping up/cracking,etc. due to a lack of subfloor rigidity.




08-16-2001, 12:26 PM
With the steps you're taking,the joist spacing right and the ground moisture barrier I'd say the installation will give you decades of service.You still may want to consider Rob's suggestion about water proofing on the backer though.

Paul M.
08-16-2001, 12:58 PM
OK Keith, That's great news...I may can call the lawyers now and stop the divorce proceedings. haha I feel like I'm taking steps to over-engineer the part of the process I have control over...assuming I'm not removing the MDF (particle board) or pulling up the Hardibacker.

Does everyone agree that the use of the acrylic latex adhesive additive instead of water, both under and over the hardibacker is better than using water only. The grout is modified, so I don't use the additive there, right?

Also, I think the tile is pretty good, it's a 12" American Olean porcelain tile from Italy that retails for $4-5/sf and is also currently installed in the elevator bays of my office building.

I will surely know how to do it right next time.

This forum is terrific. Thanks everyone for the tremendous advice.

John Bridge
08-16-2001, 04:15 PM
Hold it. Not so fast. You now mention MDF and particle board in the same breath. They are not the same. MDF is a much better product that does not swell when wet. I'm hoping (for you) it is MDF -- medium density fiber.

Bud Cline
08-16-2001, 04:28 PM
I can't in good conscience tell you these procedures are OK because they have every potential to fail based on many years of research by others. At the same time I fully understand the investment thus far and the need to continue, without the additional cost of demolition and starting over.

I don't know that you will get decades of service from this installation, I don't think I agree with that, but I am reasonably sure it won't fail next week either.

I would say if you choose to move on, as is, based on what you have heard here then that is your decision and no one elses. You know as well as anyone the extend of your investment up to now and that it could all be for naugt after a couple seasonal changes.

It is now imparative that quality products be used from here on in.


Paul M.
08-20-2001, 08:12 AM
Thanks for the advice. I have a question. After reading another post regarding grout that was popping out made me wonder...How long should the thinset dry/cure before grouting for best results? I used acrylic latex additive instead of water in the thinset, if that helps. Especially considering the odds I'm up against with this installation, should I use acrylic latex additive in the grout mixture? It's a Mapei modified grout, so I'm thinking not. But would using the admix increase the flexiblitiy of the grout...i.e... be able to withstand slightly more flexing of the floor surface? These may be pretty silly questions, but this is my first attempt at laying tile.


John Bridge
08-20-2001, 04:23 PM
No Paul, don't do that. The two things don't really mix. Modified grouts work best mixed with water only. Read the back of the bag. If you want to go with the additive, you would use the older "AR" grout.

12-21-2004, 08:31 PM
Hi, I'm relatively new tile-setter in CO. Perhaps my following question would be better anwered by a seasoned (hoping a good one too) tile installer or builder.
The information I've discovered in these posting From point to the fact that OSB is a shotty underlayment and potentially will ruin most tile installations once water gets thru. In my short but "exciting" carrer I've observed that a lot of builders use OSB (instead of plywood) for flooring throught the entire house. Now, if you were very a savvy tradesman (tile-setter, per se) what would you do if you were to install tile in these houses? Or more exactly, would you voice your concern to the builder? Should one even bother thinking abt these matters since the builder himself knowingly takes a shortcut in building precess and potentially invites future problems into his/her creation a few years down the road? I understand that it is always the mattter of $ vs Time (waterproofing adds $$$) and I can only warranty so much and for so long (1year). Furthermore,a refusal to go with the installation might cost me potential jobs from this builder in the future? I do take pride in what I do but where do you draw the line? Are there any step i should take to cover my bihind? Thank you. Teach me Sasha. PS: forgive me for my grammar...English is not my first language.

01-10-2005, 08:11 AM
You were talking earlier about not having enough thinset under the Hardi. It seems to me that once you screw down the Hardi most of the thinset will be forced out? I have never worked with a CBU, but thats what I would guess would happen. Any comments on Thinset being forced out from under the CBU?

01-10-2005, 11:04 AM
Osb is an acceptable substrate for the instalation of backer boards. Not to be confused with particle board or mdf. THinset does not like to stick to osb, so direct to osb installations is a bad idea. You can however, use backers or Ditra over osb successfully. Advantech is probably the most widely used subflooring (at least in my area) and feels just as rigid as ply and there specs claim it is even stiffer than egp.

01-10-2005, 11:13 AM
With all the confusion I see here about OSB and particle board, is it possible that your subfloor has osb and not particle board? Around here 1/2 particle board cost more than OSB, so I can't imagine anyone using it as a subfloor.

Particle board looks like glued up sawdust, OSB looks like glued up wood chips (1-3").

John Bridge
01-10-2005, 06:51 PM
Particle board is sometimes used as an underlayment for soft flooring like linoleum.

Have you people noticed how old this thread is? You're taking us back a few years. ;)

01-10-2005, 07:55 PM
I guess so it does go back. And speaking of way back. What happened to Bud?

When screwing Hardi board spend the exra bucks and get Hardi screws. They countersink themself every time.