# cement board provides no structural value!?!?



## greythorn3 (Jun 5, 2011)

Ok I am about to head to the store and get my 2nd layer of 1/2 durock to put on my metal studs and was searching the internet to see if there was a special screw to use to attach the cement boards to cement boards.. well i was reading and several google searches say cement board provides zero structural value 

Well my woodstove of 500lbs will be sitting on it atop the metal studs, now this ia making me very nervous. so i will have tile, 1/2 cement board and another layer of 1/2" cement board with no joints overlapping set in motar and this will not provides any structural value ? 

Last time i did this i did it over wood studs and plywood and it supported my blaze king very well. but it required just ember protection and not a high r value like the englander im building for this time. thats why i didnt use wood, instead used metal studs and some sheetmetal on top some of the studs.

getting worried now.

as they stated it didnt matter what was used 1/4 or 1/2" non of it would be of any structural value ! 

man i dont want a hot stove come crashing thru the cement board! maybe i should stack some bricks under the cement board to the floor also what do you guys thing? am i worried for nothing?


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## blades (Jun 5, 2011)

Make sure the cement board you use isn't of the light weight type, and yes with the Englander you will need more support.  Those steel studs are not all that strong. Remember that they were designed for vertical use and their load bearing is from end to end not across a span. They are stronger on edge than face up or down ( weakest position) just like a 2x4. A steel tube made out of the same material would be much stronger due to not having an open face on one side.


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## Dakotas Dad (Jun 6, 2011)

Two layers of 1/2 inch, thinset between. They make a special screw for attaching to steel studs, that's what you need. I used the same for field screws. 

You are correct that the cement board is not a structural material. It doesn't need to be, it isn't carrying any load, only distributing it down to the structural elements, the 2x's. 

My hearth has literally had close to a ton on it. Didn't move or crack/creak. 

Hundreds of people have gone before you, many are on this board. You will be fine. If you look at the pictures in my link (about #39), you will see where I cut and built the ceiling for my alcove. When I cut it, I swear a sneeze would have snapped it in half. After screwing it to the studs I stood in it with no measurable deflection across 5 feet. When I set the chimney in place it didn't make a sound or move.


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## greythorn3 (Jun 6, 2011)

ya i got it screwed to the studs, im just thinking if the feet dont hit on the stud exactly or the sheetmetal it might poke right thru the porcelin tile i got and the 2 layers of 1/2" cement board if it isnt structual, but like you say maybe the tile /  cement board / sheetmetal will transfer the weight to the 16  gauge steel studs.

i wonder if i put 1/4" steel 4x4 squares under each foot of the stove if that would help some too.


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## raybonz (Jun 6, 2011)

greythorn3 said:
			
		

> ya i got it screwed to the studs, im just thinking if the feet dont hit on the stud exactly or the sheetmetal it might poke right thru the porcelin tile i got and the 2 layers of 1/2" cement board if it isnt structual, but like you say maybe the tile /  cement board / sheetmetal will transfer the weight to the 16  gauge steel studs.
> 
> i wonder if i put 1/4" steel 4x4 squares under each foot of the stove if that would help some too.



My understanding is to use a layer of 3/4" plywood then layers of cement board.. This will handle the weight and support the cement board..

Look here: http://tinyurl.com/2bnx9wd

Ray


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## Dakotas Dad (Jun 6, 2011)

raybonz said:
			
		

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Can't use plywood if you need a high rvalue or Clearance to Combustibles.. My hearth needed an rValue or 6.something, all non combustible..


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## Dakotas Dad (Jun 6, 2011)

greythorn3 said:
			
		

> ya i got it screwed to the studs, im just thinking if the feet dont hit on the stud exactly or the sheetmetal it might poke right thru the porcelin tile i got and the 2 layers of 1/2" cement board if it isnt structual, but like you say maybe the tile /  cement board / sheetmetal will transfer the weight to the 16  gauge steel studs.
> 
> i wonder if i put 1/4" steel 4x4 squares under each foot of the stove if that would help some too.



Take a piece of your tile. Place it on the hearth. Now stand on it. Unless you weigh less than 150 pounds, you are placing the weight as it will be carried.. at the worst.

Did you fall through? 

Didn't think so. 

Move forward..


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## begreen (Jun 6, 2011)

raybonz said:
			
		

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That is one method, but not what greythorn is doing. He is making a raised hearth and wants to maintain a high R value for protection. His hearth is fine. It will be solid as a rock by the time the tile layer is on it.


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## begreen (Jun 6, 2011)

greythorn3 said:
			
		

> ya i got it screwed to the studs, im just thinking if the feet dont hit on the stud exactly or the sheetmetal it might poke right thru the porcelin tile i got and the 2 layers of 1/2" cement board if it isnt structual, but like you say maybe the tile /  cement board / sheetmetal will transfer the weight to the 16  gauge steel studs.
> 
> i wonder if i put 1/4" steel 4x4 squares under each foot of the stove if that would help some too.



You are seriously over-worrying this. Your hearth will be fine.


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## Highbeam (Jun 6, 2011)

I am one of those folks that has been there before you. Your stove weighs 500 lbs and the weight is applied at four points. Throw in 50 lbs of wood and ash for good measure, heck make it 100 if you choose to add a kettle of water. So 600 lbs means 150 lbs per point load and the point loads are distributed out over a larger area than the stove's shadow. Assume the stove's shadow will take all the weight and that it is 2'x2'  for 4 square feet and 150 lbs per square foot.  

Stand anywhere on the hearth with your feet together, this is one square feet and you probably weigh more than 150.  

Have you got a family? friends? Load 600 lbs of them up inside the stove's shadow, pick up the dog if you have to. May as well put 800 lbs on the area. 

This is just for confidence of course, the two layers will hold the stove fine. I was not confident at first either. The second layer of bonded durock makes a huge difference in apparent strength.

Oh and the source that was talking about durock having no structural value was mistaken or taken out of context. Everything solid has structural value. Try poking your finger through the durock, can't do it, that's because of its structural strength. I am guessing that your source was a tile setting site and referring to the durock used as an underlayment. In that application, the durock does not provide enough strength to make up for an inadequate floor structure. Like not enough beams or joists. You'll get people with bouncy floors trying to put tile down and a layer of durock won't take out the bounce and make the floor rigid enough for tile.


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## SnapCracklePop (Jun 6, 2011)

Ditto on what all the other guys said about your overthinking this.

I have the same stove, but I left the pedestal on instead of installing the feet. I incorporated an airspace under my stove to help achieve the R value, so there are parts of my hearth where tile lies on just two thicknesses of cement board. But I might add that I did cut strips of cement board about four inches wide that I placed under the pedestal. You could do something similar by cutting squares maybe 6" on a side to place under the feet.

And, the Englander people are being super cautious in their recommendations. I checked the tiles when I had my stove roaring and I was not at all concerned with how warm the tiles got.

Enjoy the Englander.

Nancy


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## webbie (Jun 6, 2011)

If one is really worried, put a large painted square or round washer under each leg to double or triple the spreading out of the weight....but as other have said, glueing and screwing together (to hold until the glue or thinset takes) should do the job finely - and as long as the span of the double or triple cement board is not too far, it should be very strong when finished.


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## greythorn3 (Jun 6, 2011)

well i put the steel studs every 16"s but you guys are right just having the 2nd layer laying on top the first layer its alreay allot more rigid! once i get the thinset in between the 2 layers and more screws holding the 2nd layer to the first  that things gonna me really solid! and thats before the tiles! once the tiles are on i think we gonna have a winner. but i think i will use the washer advice too to help the porclean tiles last under the feet themselves

I am a little worried about the 2nd layer with screws in it making the first layer crumble more in the screw area since there is so many screws already in that pieces of cement board. would it be a bad move to not install any screws in the 2nd layer?

Thanks guys!


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## Highbeam (Jun 6, 2011)

Yes, a bad move. You need to laminate the two sheets together so that they will combine strengths. Glued and screwed. Once the layers were glued you could probably get away with removing the screws but why bother at that point. The screws will suck the two sheets together to get an even and consistent bond between the layers with no air pockets and no lumps which would make setting tile a bugger.

For giggles, and more confidence, you can screw the hell out of a scrap piece of durock and see that the prodcut stays together quite well with many holes in it. The mesh makes it pretty tough and not prone to cracking. When I had to punch a large hole in the durock to route my outside air duct, I did it by drilling a zillion 3/8" holes in a circle and knocking out the center. That durock held up very well to the adjacent holes due to the mesh grid.


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## greythorn3 (Jun 6, 2011)

Highbeam said:
			
		

> Yes, a bad move. You need to laminate the two sheets together so that they will combine strengths. Glued and screwed. Once the layers were glued you could probably get away with removing the screws but why bother at that point. The screws will suck the two sheets together to get an even and consistent bond between the layers with no air pockets and no lumps which would make setting tile a bugger.
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> For giggles, and more confidence, you can screw the hell out of a scrap piece of durock and see that the prodcut stays together quite well with many holes in it. The mesh makes it pretty tough and not prone to cracking. When I had to punch a large hole in the durock to route my outside air duct, I did it by drilling a zillion 3/8" holes in a circle and knocking out the center. That durock held up very well to the adjacent holes due to the mesh grid.




ok then i will screw the he// outa it after i smear the thinset. 

thanks!


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## Dakotas Dad (Jun 6, 2011)

I didn't send any screws in the 2x's from the top layer. I only put them in the field. 8" x 8"or so grid, offset from the studs.


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## begreen (Jun 6, 2011)

Don't overkill this. A screw every 8" is all that is required. There is nothing to be gained by adding more screws.


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## greythorn3 (Jun 6, 2011)

ok guys i will do a 8x8" grid of screws then! u think i should start from the inside and work out wards? or start at a edge and work across?


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## Highbeam (Jun 6, 2011)

The notch trowel will introduce lots of air. plan on squeezing the air out as you go so you can start on one edge and squirt it to the side or start in the center and work outwards but just don't start on the outside and go the middle. That's a great way to end up with a big hump in the center. 

Pick an important edge like the front edge and start there.


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## summit (Jun 6, 2011)

maybe a redundant question at this point, but do they make a bottom heatshield for this unit? If so, the structure could have relied less on total noncombustibles for support.


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## egclassic (Jun 7, 2011)

Just my .02, but being in the construction trade, the only thing I have seen durock used for is tile (floor and wall). It has always been refered to as "backer board".
Now that being said, I do not not all of its uses only what I've seen in the field.


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## begreen (Jun 7, 2011)

greythorn3 said:
			
		

> ok guys i will do a 8x8" grid of screws then! u think i should start from the inside and work out wards? or start at a edge and work across?



I can't see why either would make a difference. Take your pick. 

PS: I forgot to ask. Did you end up skinning the top of the studs with sheetmetal before you put down the cement board?


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## greythorn3 (Jun 7, 2011)

BeGreen said:
			
		

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yes i skinned with some ducting sheetmetal i had the center area around the studd i put across the top because thats where i think the stove will go, gave my durock a wavy appearance tho. but im sure the thinset will fix that.


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## John_M (Jun 7, 2011)

egclassic +1. 

FWIW, somewhere within the bowels of Hearth.com are details of how I and others constructed our raised hearths. My recommendation would be to NOT use Durock as a structural element.

John_M


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## pen (Jun 7, 2011)

I agree that this should be pleanty stong.  

Out of curiosity (or I missed it) what was your spacing on your studs?

Will the legs of your stove be resting on the studs themselves?

pen


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## begreen (Jun 7, 2011)

greythorn3 said:
			
		

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With the skin of metal the top of the hearth is like a drum. Add the cement board and it is going to be very solid. The waviness though may be a problem unless it is only a 1/16" or so. On the top layer, you want a completely flat surface for the tile and stove.


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## greythorn3 (Jun 8, 2011)

pen said:
			
		

> I agree that this should be pleanty stong.
> 
> Out of curiosity (or I missed it) what was your spacing on your studs?
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studd spacing is 16" i have no idea if the legs will set exactly on studs tho as im waiting "again" for the stove to come in.


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## johnstra (Jun 8, 2011)

I made 1/8"x3"x3" steel pads and put my stove feet on those.  It looks clean and gives me peace of mind that the feet won't crack my slate tile.


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## Highbeam (Jun 8, 2011)

If I could rebuild my hearth I would make it a few inches deeper (so I could install a BK) and I would have set some sort of physical support under the leg area. I have some 4" iron pipe that would be prefect if cut into short lengths and placed on end between the subfloor and the bottom of the durock. 

It is not needed but I would feel warmer and fuzzier.


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## greythorn3 (Jun 9, 2011)

Highbeam said:
			
		

> If I could rebuild my hearth I would make it a few inches deeper (so I could install a BK) and I would have set some sort of physical support under the leg area. I have some 4" iron pipe that would be prefect if cut into short lengths and placed on end between the subfloor and the bottom of the durock.
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> It is not needed but I would feel warmer and fuzzier.



heck that pipe idea is good, i was thinking of stacking some pavers underneeth.


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## Bub381 (Jun 10, 2011)

I went looking for something to keep the legs off the tile if we get the Rangeley which is over 500 lb's.Homedepot carries 4"x4" granite blocks that are 1" thick and believe i'll go that route.No need for em i'm sure but would hate to crack the tile.Also another 1" off the floor wont bother me at all.A little more clearance.These are granite not junk they glued together.


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## Highbeam (Jun 10, 2011)

Stoves tend to walk around a bit on hearths with heat and cool cycles. Be sure to keep an eye on these pucks to make sure one doesn't squirt out. Also be sure your stove stays in place so that none of your stove joints open up. I see very little reason to try and distribute the stove weight over tile. Properly set tile already distributes the weight very well.


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## Bub381 (Jun 10, 2011)

As long as the stove has a somewhat flat surface for a foot,not just an angle iron piece.Adjustable so should be ok. Forgot to look.


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## greythorn3 (Jun 11, 2011)

well i got the 2nd layer on it, getting strong! used a whole box of the rockon screws in it, the smaller box they sell anyhow. been hard hand mixing that thinset! hehe forgot to get my mixer from the other house! oh well.. this things gonna be tuff and super heavy i bet by the time the tile is on it!

I have some large granite pieces left over from when i did our countertops, i might use a piece of that for under the stove, i really like the tile i picked out tho and hate to cover it up. i may just do the washer thing, weld them on the legs, depending on what the legs look like, i havent seen them in person yet. maybe they have a larger pad on the bottom of them.


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## begreen (Jun 11, 2011)

Once that is tiled it will not need anything else under the feet. That's a nice solid construction there.


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## Intheswamp (Jun 11, 2011)

Was reading one of the faqs on the USG website and here is a quote from it....

_"Installation  Q: Can DUROCKÂ® Cement Board Next Gen be attached directly to floor joists instead of using plywood first?  A: No. DUROCK is not a structural product. It cannot support live or dead loads without a structural sub-floor like plywood underneath."
_

I'm not sure that doubling up the cement floor satisfies this or not, the bottom sheet of durock would be a type underlayment but I don't think it would be considered subflooring.  ????

Ed


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## begreen (Jun 11, 2011)

It is doubled up on a tight skin of sheet metal. All layers have been screwed and mortared together. That should be quite rigid. Granted, I would have put the studs on 12" centers for less free span, but the sheet metal skin, plus brace in the center should beef up the area of the stove weight quite a bit.


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## greythorn3 (Jun 12, 2011)

i think the really thick metal of these studds helps allot also, infact they are super hard to drill the screws into that are suppsed to go into metal!


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