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Old 10-06-2008, 10:57 AM   #1
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Hardibacker Cementboard Strength


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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Old 10-06-2008, 11:07 AM   #2
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Hardibacker Cementboard Strength


we use it for interior applications however we also staple-attach expanded wire mesh,,, no problems in 5 yrs.
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Old 10-06-2008, 11:12 AM   #3
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Hardibacker Cementboard Strength


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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Old 10-06-2008, 11:59 AM   #4
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Hardibacker Cementboard Strength


have no idea
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Old 10-06-2008, 12:10 PM   #5
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Hardibacker Cementboard Strength


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Originally Posted by kinglerch View Post
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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Old 10-06-2008, 12:42 PM   #6
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Hardibacker Cementboard Strength


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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Old 10-06-2008, 01:11 PM   #7
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Hardibacker Cementboard Strength


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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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Old 10-06-2008, 01:26 PM   #8
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Hardibacker Cementboard Strength


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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Old 10-06-2008, 01:41 PM   #9
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Hardibacker Cementboard Strength


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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Old 10-06-2008, 01:50 PM   #10
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Hardibacker Cementboard Strength


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Originally Posted by kinglerch View Post
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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Old 10-07-2008, 07:09 AM   #11
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Hardibacker Cementboard Strength


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.

Last edited by kinglerch; 10-07-2008 at 08:56 AM.
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Old 10-07-2008, 10:24 AM   #12
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Hardibacker Cementboard Strength


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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Old 10-07-2008, 10:42 AM   #13
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Hardibacker Cementboard Strength


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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Old 10-07-2008, 01:13 PM   #14
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Hardibacker Cementboard Strength


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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Old 10-07-2008, 02:13 PM   #15
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Hardibacker Cementboard Strength


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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