Hardibacker seams [Archive] - Ceramic Tile Advice Forums - John Bridge Ceramic Tile


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Jeff Axline
04-11-2005, 01:57 PM
OK. I am ready to do the hardibacker seams prior to tileing my tub/shower.
With all the tools and materials around I'm not sure if the fiberglass tape I have is for drywall or hardibacker. Does it Matter? Why do I have to use tape on the HB seams at all? Why can't I just fill in the seams when I tile?


I read earlier about moistening the HB prior to thinset. What if I'm using acrylic mortar ad mix instead of water? Do I still moisten the HB?


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04-11-2005, 02:19 PM
OK. I am ready to do the hardibacker seams prior to tileing my tub/shower.
With all the tools and materials around I'm not sure if the fiberglass tape I have is for drywall or hardibacker. Does it Matter? Why do I have to use tape on the HB seams at all? Why can't I just fill in the seams when I tile?


I read earlier about moistening the HB prior to thinset. What if I'm using acrylic mortar ad mix instead of water? Do I still moisten the HB?


Funny that you would ask this question as I was just looking for the answer. I checked the Hardi site, and came back here to find your question. Their answer:

Q: Why do I have to tape the joints? What is the purpose of the 2" wide tape?
A: The tape will help disperse any movement of the substrate, therefore decreasing the probability of popping or cracking tiles along the seams.

The reason why I was asking is because I just put tile down in my half bath, and FORGOT to tape the seam. It's only one seam, so I hope I don't have problems...... :bang:

I don't know if it's THAT big a deal, but why take the chance?

04-11-2005, 02:37 PM
Jeff and Joe --

The fiberglass tape reinforces and strengthens the joint. It can make the difference between cracked or intact tiles over the joints. That should be a big enough deal to make taping worth the effort.

You should use alkali-resistant CBU tape because the alkali in the thiset can degrade regular fiberglass drywall tape. They appear the same, but aren't.

If you want, you can tape the joints with self-adhesive tape, and fill them with thinset as you tile. This method has the advantage of avoiding the possibility of hardened "speedbumps" of thinset over the joints, which makes leveling the tiles harder to do.

Dave Taylor
04-11-2005, 02:43 PM
The attached URL contains floor Install instructions for Hardibacker's CBU product.... follow them.... including using everything they recommend (and or sell) for:


a. Thin set mix specified between underlayment and CBU
b. Fasteners used to attach CBU
c. Tape to use for ALL CBU seems
d. Thin set mix to use to imbed surface seam tape

This way you will achieve the best of luck I wish for your project

Jeff Axline
04-11-2005, 02:49 PM
Thanks for the replies. I assume that the corners need to be taped as well? Obviously no tiles cross over the corners, but I've seen photos online here showing the corners taped. I find it strange that a little fiberglass mesh can keep walls from moving. I'm just having a hard time comprehending the concept. I will use the tape as I apply the tile. I need to make another trip to the hardware store for more tape. I should probably rent a room there so I can be more comfortable with all the trips I make.

Dave Taylor
04-11-2005, 02:50 PM
Your doing a shower not a floor... Sorry but helps to keep the posts together in one thread..... but still follow all Hardibacker install instructions for walls which are in the same URL I sent.

Jeeeze I feel dumb.

04-11-2005, 06:09 PM

I'm new here but am going to take a crack at this by applying some engineering principles. Like I said I'm not an expert on this subject but I believe I can use my engineering skills to piece together what I have been reading over the past few weeks. I believe the purpose of the CBU is to decouple the tiles from the subfloor. Wood (subfloor) and tile have different thermal coefficients of expansion.

If you were to lay tile directly on wood, when the wood moved relative to the tile due to it's different thermal properties, the weakest part of the tile surface would fracture, and that is normally the grout unless you have a weakened tile.

The cement board has material properties similar to the tile and therefore expands and contracts along with the tile. The CBU/tile combination can be thought to "float" from the subfloor since it is decoupled.

Now for your question. The tape will help to "lock" one piece of CBU to the next so they form a continguous mass, and not separate "islands." I'm likening this to plate tectonics. Otherwise, one CBU may move relative to another, thus cracking the grout. The movement here is small so the fact that the CBU is screwed down isn't relevent, the expansion/contraction occurs everywhere. Think micro not macro. But over even short distances the thermal movement is plenty to crack grout which has very little by the way of elasticity.

On top of these expansion/contraction concerns I've also learned that joist-to-joist deflection and header-to-header (along the joist) deflection must be managed with tile and natural stones.

All very interesting from an engineering point of view!


John Bridge
04-11-2005, 08:10 PM

Would you be in the market for a non-paying job? We have a very generous unpaid vacation policy. ;)

If so, report to Chief Injun Ear Bob Campbell ( bbcamp). If he's around here. He might have slipped off down to the Caribbean on another of his rum-drinking forays. :D

Seriously, we can use all the help we can get. You have to be able to put up with engineering jokes, though. :)

04-11-2005, 09:50 PM

I take it from your response that my explanation, which is theory upon theory upon theory, isn't too far off base!

Yes, I've heard (most) all the engineering jokes. The main point being engineers don't DO anything, they just criticize the "doers" and order the actual people that get things done around. I've always admired good site engineers (my brother is one) that will actually DO something. Not just walk around with the plans and act busy.

I'm busy preparing for NAB in Las Vegas right now but when I get back I might be interested in helping with the boards if that's what you mean. Truth be told, I'm just a DIY kind of guy that isn't afraid to tackle most any project so I don't know how helpful I can be but I'm always willing to learn. You know how it is, a guy learns a couple of equations and he thinks he can actually DO something!

Plus the unlimited unpaid vacation sounds fantastic! I could really use a break!

- Mark

04-13-2005, 12:01 PM
re: markbb's engineering analysis. here's a bench related question/issue: when we framed our tub deck/bench, we forgot to slightly slope the portion of the bench that extends into the shower. to fix this problem, we were going to attach hardi to the portion of the deck where the tub goes and then use that height to scree a line of thinset down to the edge of the plywood bench. Since the hardi is 1/4" high and the bench extends into the shower 1', that should provide a perfect slope. We will then kerdi over our "sloped" bench and extend the kerdi just a little ways onto the hardi tub deck (as it won't get much water there so we didn't feel it needed additional waterproofing). my original question was - What type of thinset/mud would be best for making this 1/4"x1'slope? The bench is about 4' long and 1' wide.

After reading mark's analysis - I'm wondering if our strategy of sloping with thinset will work at all - or cause problems down the road. thanks for your insights.

by the way, DH (dear hubby) will be at NAB too!

04-13-2005, 02:09 PM
As a civil engineer it hurts me to hear such an in depth explanations. Its really doesnt entail such a wild explanation. The idea is the hardibacker and the tile are one layer, together. By taping the hardi you help make this layer one. expansion and contraction occurs in the plywood, and the plywood takes the movement (thats why you dont tape or glue the seams of the plywood lol). This just gives meaning to why you wouldnt put a hardi seem over a plywood seem, that would obviously cause the greatest movement. Hence there are two layers, the plywood and the hardi+tile. Feel free to comment. MarkDD you do have the right idea with the decoupling...but i like the way i explain it better lol sorry. btw the screws doesnt matter cause its not like the hardi is doing a dance on the plywood (it isnt just moving by itself), the structure is deflecting, so there is movement in the plywood, thus movement in the hardi. by taping we want this movement to be uniform within the tile+hardi layer.
biggest thing to keep in mind is the deflection of the beams, as this site suggests.

When it comes to construction engineers have to realize their place, its to test the limits of materials and making construction more economical and effective and safe at the same time. Carpenters used (and still are to an extent) to be the "engineers" and you can be sure they werent worrying about thermal expansion coefficients of hardi and plywood lol. I am open to criticism for this statement i know, but its true. The majority of buildings of the past were over engineered, they were made on assumptions and experience rather than calculations. The engineering involved in the actual construction of a house is not that in depth.

04-13-2005, 03:14 PM
Stevo --

As a civil engineer, I would think that you would recognize the increasing importance of the engineer in the construction process as building standards are being decreased to bare minimums. There is much less leeway for error than there used to be.

In a stick-built house, if you want to remove part of an attic floor joist it's no problem -- just cut it out, nail in doubled headers on either side of the new opening, and you're good to go. But if you have trusses up there, you can't even think about cutting one without involving the manufacturer's engineering department.

And you may or may not be old enough to remember the 1981 Kansas City Hyatt skybridge collapse. There were two skybrides throught the atrium lobby, one directly over the other. As designed, the two bridges were suspended on long threaded steel rods, with a nut on the rod supporting the steelwork of each skybridge. But the construction company thought that it would be much easier to two separate rods at each location, one suspending the upper bridge from above, and the second suspending the lower skybridge from the upper. The same size steel rods and nuts as originally specified were used.

Sounds reasonable, right?

The change was made without consulting the engineers. The problem was that with the lower skybridge hanging from the upper, the nuts supporting the upper skybridge were supporting double the load they were designed to hold. The result was that when both skybridges were loaded with guests observing an event occurring in the lobby, those upper nuts (they were located in the wreckage by investigators) failed sending many of those guests to their deaths.

I still have a copy of the "as designed" and "as built" diagrams from the New York Times on the bulletin board over my desk.

So much for carpenters or steelworkers making engineering decisions on the fly.

04-13-2005, 04:29 PM
hahah of course i realize the importance of an engineer in the construction process. I work for a heavy highway contractor, not a designer. So of course i believe there is a place for an engineer on the job. The regency problem was a fabricator shop drawing problem that was failed to be picked up by the designer during the review process. This case just happend to be fatal, it happens all the time. The fabricator was just looking for an easier way to fabricate it. In any case its a classic lesson that is taught to all engineers studying nowadays, understandably. Unfortunately there are number of cases of engineering failures that are also stressed. As a side note, i have seen problems such as these happen, where things are not fabricated as they are intended, and when the ironworkers went to assemble the pieces the bolt holes did not match up. Having an engineer on staff allowed us to match up the the length of weld required to equal the shear stress in a bolt. Working on public projects, the ironworker could not assure the owners engineering reps that welding would equal without a PE stamp.

about the engineering minimums, that was my last point in the previous post. All materials in construction are now engineered for the most part, which means something like a nail you put in probably just makes the requirement of the quality of steel that it is made with. So yes it is important that engineers are sure the limits are not exceeded.