# Hardibacker Cementboard Strength



## kinglerch (Aug 28, 2008)

I am having trouble finding definitive information about 1/2" Hardibacker 500 strength when used vertically. The application is stone on a wall frame. The stone+thinset will weigh 15 pounds per square foot. Therefore the total weight of a 3'x5' board will weigh 225 pounds.

Can the 1/2" Hardibacker 500 be used for this weight? The manufacturer only lists flexural strength and compression strength. I guess I need shear strength.

Has anyone used Hardibacker 500 for vertical stone applications?


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## yesitsconcrete (May 11, 2008)

we use it for interior applications however we also staple-attach expanded wire mesh,,, no problems in 5 yrs.


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## kinglerch (Aug 28, 2008)

Thanks for the information. So you use Hardiebacker + matal lath + scratchcoat? In my application the lath would add too much thickness. Would you see a problem of the Hardiebacker holding by itself?


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## yesitsconcrete (May 11, 2008)

have no idea


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## Marvin Gardens (Sep 30, 2008)

kinglerch said:


> Thanks for the information. So you use Hardiebacker + matal lath + scratchcoat? In my application the lath would add too much thickness. Would you see a problem of the Hardiebacker holding by itself?


Yes I would.

Hardibacker is meant for tile, small rock walls, and prevent the wall from rotting in high water areas.

It is not meant for heavy rock walls and you will end up with a pile of rocks on the floors one day. This is just to much weight for the board itself to handle.


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## kinglerch (Aug 28, 2008)

Ok thanks. It's unfortunate because it is very difficult to find definitive information on the weight capacity of "cement board". The Hardibacker technical service guy said it is used "all the time" for 2" thick stones...but couldn't give me a maximum weight. 

I don't know that Durock or Wonderboard gives this maximum weight either. They just seem to list flex and compression strength. Is there a way to know what these "cement boards" can handle?


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## Marvin Gardens (Sep 30, 2008)

kinglerch said:


> Ok thanks. It's unfortunate because it is very difficult to find definitive information on the weight capacity of "cement board". The Hardibacker technical service guy said it is used "all the time" for 2" thick stones...but couldn't give me a maximum weight.
> 
> I don't know that Durock or Wonderboard gives this maximum weight either. They just seem to list flex and compression strength. Is there a way to know what these "cement boards" can handle?


Was the guy a salesman at the local store or did he really do this for a living? And did he say that it needed some mesh to make it stronger?

Granted it could last for decades on just the board IF it was done perfect in every step of the way.

But having a ton of rock come tumbling down is too much of a risk to do it without some lath to help secure it.


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## kinglerch (Aug 28, 2008)

It was at the 888 number tech service for J Hardie. To be extra secure I will add the lathe. It's gonna be a pain, but it won't hurt to add it.

I'm just a little confused on the scratchcoat part. The recommendation for the stone is type S mortar. I should put this mortar into the lath and also on the stone? Or is a different type of mortar used for scratchcoating the lath?


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## yesitsconcrete (May 11, 2008)

lemme expand on what i posted previously,,, we mechanically attach h-backer to supporting wall/2x4 framework/whatever,,, then we staple the expanded wire mesh,,, after that, we trowel mortar into the mesh & place the liteweight stone after thats cured - NEVER regular stone as we hate picking up piles of rock that've fallen on the floor


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## Marvin Gardens (Sep 30, 2008)

kinglerch said:


> It was at the 888 number tech service for J Hardie. To be extra secure I will add the lathe. It's gonna be a pain, but it won't hurt to add it.
> 
> I'm just a little confused on the scratchcoat part. The recommendation for the stone is type S mortar. I should put this mortar into the lath and also on the stone? Or is a different type of mortar used for scratchcoating the lath?


The mortar hooks into the lath and makes a secure connection. It can handle a lot more weight than just the cement board.

A scratch coat if for installing tile and similar materials since it is a lot easier for mortar to stick to a rough surface rather than a smooth surface. In fact mortar will not stick to a smooth surface at all unless there is some new technology that I am unaware of.

I am an old timer and do this type of stuff so little I have to refer to the real pros who do it way more often than I do. I do lots of sheetrock, plumbing, HVAC, electrical, heavy construction, finish work but do very little rock work.

For those details consult a person who is more in tune with that.

I just know that cement board by itself will not hold a lot of weight. I have had to repair walls that have had fall off. My last repair was a wall outside a building that had 20 vertical feet of rock fall off. It didn't have any lath and failing was so predictable.


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## kinglerch (Aug 28, 2008)

One issue you brought up that is a certainty is the outdoor exposure. With hot and cold temperatures, the bond is subject to much more stress than indoor applications. Although my guess is that the mortar or the mortar's bond with the cementboard breaks down, not the cementboard itself. The Hardibacker manufacturer said it was not recommended for heavy stonework outside, but could be used inside.

But let me describe my problem. The three major cementboard manufacturers (Wonderboard, Hardibacker, and Durock) only publish their flex strength and compression strength. And Hardibacker has by far the highest flex and compression strength. But only if you call them will someone on the phone say "you can use it for 15 lbs / sq ft"...and none of them will confirm what weight it's rated or tested for.

I have pushed in all directions a 15 lb / sq ft rock that was mortared to Hardibacker. Lemme tell you, this rock was going nowhere. I bet I could stand on this rock and it could hold my (now) 165 pounds. So my guess is that for interior applications, the cementboards could take much more than 15 lbs / sq ft.

But I don't want to go with a guess. I want them to say "we tested this to 100 pounds" or "we stand by 15 pounds in writing" or whatever. Because if they say "yeah, it could hold 15 lbs / sq ft" then what happens if a rock is heavier than normal, the mortar is weaker than it should be, someone leans on the stone, etc. I don't want to be on the edge of what the manufacturers are willing to admit to.

So because of their unwillingness to rate their product in this way, I will put up the metal lath even though I don't think it is necessary. And when I am done it could probably hold a minivan.


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## Marvin Gardens (Sep 30, 2008)

Let me tell you where I am coming from.

I am a retired Paramedic (still a Paramedic and do some volunteer work at my local ambulance at my vacation home).

In 25 years I have seen a lot of injuries and many from DIY'er doing sub standard work. Some even from pros who were just lazy.

Several come to mind. A wedding with loud music and the whole ceiling of sheetrock came down on the reception guests. No injuries but a huge mess and an even to remember every anniversary. There were screws every 18 inches or so. Way below standard. Work was done by church members. Note to self; screws are cheap, put in lots of them.

Guy climbing a roof with a nail gun. He falls forward and nails his hand to the roof. Note to self; nail guns are like real guns, keep your finger off the trigger till you want to drive a nail.

Family in great room with a nice big fire during a storm. Lights and heat out. Everyone huddled around the fire to stay warm. The rock wall lets go and comes tumbling down and hits everyone. Most had bruises and aches and pains except the 4 year old boy. He has a broken forearm. Was so shattered he had several operations to repair the damage. There was not lattice on the rock work and the rocks were heavy and large. Note to self; gravity sucks and is made worse by minimalist construction procedures.

Two guys under a house who are going to raise it to put in a foundation. They take out the roll blocks to drag out the dirt. A small gust come up the the house drops down as the joists tip to the side. Luckily they had dug out enough dirt to leave a cavity for them to keep out of being crushed. It took us 4 hours to dig them out as we had to put prop sticks against the house to keep it from tipping over. Note to self; don't mess with the structure if you don't know what you are doing.

A guy removes 5 feet of his bearing wall to put in a deck. He calls me to put in a tankless water heater. I go into the basement to see where I am going to route the gas line. I notice the base plate doing the slow mo bending and the 6 inch sag in the house. I ask him if his doors are closing okay. He says no and he is having sheet rock pulling off the walls. I go upstairs and to make matters worse he has a 150 gallon fish tank over the joists. Just another 1500 pounds he doesn't need there. Note to self; I will be making a lot of money here today and for the next few weeks.

So this is where I am coming from.

With DIY'ers over building is far better. If it is a non bearing wall that is underbuilt, no big deal really. Or a door that is installed without shims. Whats the worse that could happen? The door won't open down the road?

With putting rocks on the wall, especially 225 pounds per sheet of cement board it is better to be safe. Especially when you are asking questions about survivability of the work. The potential is high for injury if someone happens to be in the room and they come tumbling down. Also the liability is high if you sell the house and someone is injured.

This is always the danger or DIY. You have to live with your work and can't pass on the blame to someone else if there are unfortunate consequences.

If you notice there are issues where the experts tell the person to hire a professional because some things are just too complicated and dangerous to try and tell them how to do it over a web site like this.

So this is my perspective and why I suggest the lath.

Notice that they manufacturers you talked to didn't say anthing about shear or tear strength.


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## kinglerch (Aug 28, 2008)

I absolutely agree with you, and is why I am adding the lath. But not because cementboard is too weak. I am going with the lath because I want it to be strong enough and the cementboard manufacturers are not helping us understand exactly how strong their product is for this purpose. 

Thanks for your help.


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## Termite (Apr 13, 2008)

Why not attach the backer to a plywood substrate with lots of backerboard screws. That would take its strength out of consideration.


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## kinglerch (Aug 28, 2008)

I think ridgidty is only part of the equation for these stones. Flex strength for Hardibacker is excellent, even better than Durock or Wonderboard. But there is also the question of shear strength (perpendicular to the board) and tear strength (preventing the rocks from just ripping away from the wall).

If these manufacturers are selling this cementboard for this purpose of mounting rock, then they should publish their weight capability.


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## Termite (Apr 13, 2008)

kinglerch said:


> If these manufacturers are selling this cementboard for this purpose of mounting rock, then they should publish their weight capability.


They're not. Hardiebacker, wonderboard, and durarock are sold and marketed as tilebacker. That doesn't mean that they can't be used for applications involving thin rock or cultured stone though. An application with rocks of this size should be conventionally laid like a brick wall in my opinion (ties to wall structure, brick ledge or angle iron, rocks bear the weight of other rocks). Relying on the tensile/shear strength of a cement-based product for a wall veneer of that weight is just asking for trouble.


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## kinglerch (Aug 28, 2008)

Even if you are laying tile, thin rock, stone veneer, etc they should let you know how much weight they are capable of holding. But I think you are right that the bottom line is that this is partially a "misuse" of the cementboard product.


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## rtoni (Jul 18, 2007)

Marvin Gardens said:


> With DIY'ers over building is far better.


certainly helps me sleep at night


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## Tscarborough (Mar 31, 2006)

First, the primary concern is not compressive, flexural, or shear strength. Cement board exceeds the requirements in all three categories by a considerable amount. The reason the company does not test for shear is because it is a useless number for any application it is recommended for.

The only property that should concern you for this application is bond strength. Using a modified thinset, you will achieve bond strength numbers far in excess of what is required.

Any failed adhered veneer wall that you have seen failed for other reasons, most likely a freeze-thaw (or thermal expansion, like in the EMS's story above) issue on a wall that was stuck using type S mortar.

If you plan on using lath and stucco, then do not waste the money on the concrete backer board. It is an either/or, not both.

There is no real standard at this time for adhered veneers as to pounds per square foot, but most faux stone manufacturers specifiy a weight of around 11# PSF, with thin cut natural stone running 12-15#.


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## kinglerch (Aug 28, 2008)

I see you have commented on several threads regarding cementboard, stone veneers, and mortars. What are the advantages/disadvantages between modified thinset and type s mortar? 

One of the stone veneer manufacturers recommend cementboard stuck direct with mortar, the other says plyboard with lath and scratchcoat. Some say thinset, others type s, and another type s + sand. For interior applications, aren't all of these techniques/products more than sufficient for 15# psf stone?

Thanks for your help.


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## Tscarborough (Mar 31, 2006)

I prefer lath and stucco for exterior wall applications and board for interior walls and exteriour "shapes", like BBQ islands, mainly for speed. I do not use mortar on board, only only on lath and stucco. I do not feel the surface of the board is rough enough to allow for sufficent bond using mortar. Thinset only on board, mortar on stucco. 

The only issue wth thinset is short board life and a long "grab" time. You have to mix it drier than normal, and in small amounts. There are, however, some thinset manufacturers producing thinsets tailored for the adhered veneer market. Ardex X9 is one I have used that gave a 2+ hour potlife and very good grab (and will peel the cement board before it breaks loose).


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## kinglerch (Aug 28, 2008)

Outdoor worries me because of unknown heat, cold, moisture, shifting, wind, etc. But for interior my guess is that many different methods are sufficient.

I also noted as you did that the cement board seemed too smooth for direct application, although one of the stone veneer manus said exactly this. So I put lath on top of my board, not because it was necessary but because the cement board was already bought and installed.

But you bring up another good point about "cement board strength". Bond strength (or at least bondability) is important, but so is (for lack of a better term) breakaway strength. What good is a perfect bond if the backer board rips off...rock, mortar and all. Of course I think this won't happen until 200# psf or more, but it would be a useful spec nonetheless.


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## Tscarborough (Mar 31, 2006)

Do this simple test:

Thinset a faux rock to cement board. Get a hammer, a big one, and beat on the rock. Half of the time (80% of the time for real rock), the rock will shatter, leaving the thinset on the board. The other 50% of the time, the thinset will delaminate the concrete board. Unless you plan on using the wall as a climbing wall for cows, you won't need to worry about the cement board shear strength. As stated originally, that is why it is not tested, because it is not even remotely close to critical for any recommended use.


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## kinglerch (Aug 28, 2008)

I agree 100%, except that "you won't need to worry about cement board shear strength...it is not even remotely close to critical for any recommended use" would be a great and useful tagline for these products.


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## Tscarborough (Mar 31, 2006)

Because then some moron would use the product in an application where it DID matter, and sue them. The product has a narrow range of use and is perfectly suited for those uses only.


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## charlie453 (Jan 24, 2009)

*stone veneer over cement board*

Has anyone tried applying faux stone veneer directly to cement board in an interior application (i.e. fire place surround, indoor chimney, etc.) I am wondering why you couldn't skim coat the cement board and then apply the faux stone directly to the skim coat. This would eliminate the need for the wire mesh.



kinglerch said:


> I agree 100%, except that "you won't need to worry about cement board shear strength...it is not even remotely close to critical for any recommended use" would be a great and useful tagline for these products.


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## Tscarborough (Mar 31, 2006)

You don't have to skim coat it, it can be applied directly to the cement board, preferably with a latex modified thinset.


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## charlie453 (Jan 24, 2009)

My specific application will have the stone veneer going around the fireplace and the sides of the fireplace. Its height from the floor will only be 5 feet. 

Have you used the application process that you posted on any jobs you have done and how long have they been done?


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## Tscarborough (Mar 31, 2006)

The one one the left last year the one on the right 2 years ago:

http://www.5125851496.com/gallery-pages_0008_contemporary.html

Trust me, you can not beat the stone off with a hammer without destroying the board. Cement does not normally degrade over time, it gets stronger.


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## charlie453 (Jan 24, 2009)

Thanks for the reply; the two jobs look great. How did you apply the cement board to the studs? I have heard of some people using galvanized washers with galvanized screws plus gluing with construction adhesive. I have also heard of just nailing it on with roofing nails. I'd rather be safe than sorry.

Do you have a preference for the cement board you use? Durarock, Hardibacker etc. and what thickness?

I'm planning on using a veneer stone with mortar joints in between the stones. I'll use mortar to grout between the stones. Will the mortar adhere to the thinset without trouble?

Thanks,

Charlie


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## Tscarborough (Mar 31, 2006)

If it is bare studs, 1/2" cement board screwed and glued. Over sheetrock, 1/4" hardi-backer screwed and glued. Mortar sticks to the thinset and backer just fine. Here is one with big wide joints over 1/4" hardi-backer screwed to paneling:


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