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Need for metal lath under wall tile

ulepiiri's picture

I'm a little undecided on how to proceed.

We are currently looking at tiling a fireplace surround.  The structure is roughly 15' high, 8' wide, constructed of cement board screwed to structural steel stud on 16" centers.  We are lookin at using 1 foot by 2 foot slate tiles 1/2" thick.

Question is, can we get away with just using thinset mortar-directly applying the tile to the cement board, or do we need to apply a coat of metal lath under a mortar bed first.

Any advice would be greatly appreciated.

15' high? Wow! Tons of weight (post #214784, reply #1 of 10)

15' high? Wow! Tons of weight will come to bear upon the foundation of the fireplace. I will admit that I have not done anything quite like this, but here are a few essential considerations. The bottom row of tiles must bear upon something (masonry block) that telescopes loads to the foundation. Each successive row of tiles is temporarily supported by tile spacers while the thinset dries. Thinsets primary function is to  bond tile to substrate and not to act as a load-bearing device. Once you have grouted, you have achieved what amounts to a monolithic slab whose weight is borne by a concrete-formed foundation.

I'm less sure what to say about metal lath. Metal lath thickens the substrate to which the tile is bonded, but it is not, as I see it, a substitute for a firm foundation on which the entire load rests. It seems to me that 1/2" tile backer in one or two layers provides sufficient sturdiness for the tile.

Mel Fros froscarpentry.com

Hi there, As Mel noted, my (post #214784, reply #2 of 10)

Hi there, As Mel noted, my biggest concern with the scenario would be making sure there is a proper footing in place for all of this mass/load to land on.  As for the preparation of the substrate.  Cement board over structural steel studs is fine.  The wire and lath wouldn't add anything to your assembly as far as I can see.  I have seen very large fireplaces built this way with 1.5" stone veneer "glued" directly to cement board per the manufacturer's specifications.  

Used to be, when the standard (post #214784, reply #3 of 10)

Used to be, when the standard substrate for plaster was wood lath or "rock lath", expanded steel lath was used behind the base coat for tile.  But this was 50 years ago, and the various fiber-cement style products are much improved over what was available back then.

That said, more than a little care needs to go into the preparation for a 15-foot-high wall of slate.  That's an enormous amount of weight.


Of all the preposterous assumptions of humanity over humanity, nothing exceeds most of the criticisms made on the habits of the poor by the well-housed, well-warmed, and well-fed.  --Herman Melville

I beleive (post #214784, reply #4 of 10)

I believe one of your posters is in error.  Weight should not be much of a concern.  Think about how many cultrued stone projects you have seen and that "stone" is at least one inch thick vs your 1/2 inch.  Also thick rreal stone veneer which is also about 1 inh thick and supplies will tell you weighs about 12  pounds per sq ft with no foundation required.

I also don't beleive it is correct to say your first tile needs to set thight to the foundation.  Each tile or cultured stone or veneer stone is "support" independently by the bond to the substrant.

You can ask around if concerned to confirm which of the free advice you recieve here is correct.

I mihgt add that I have a project which is a 20 foot fireplace planned to be constructed with real stone veneer, so I have been asking around.  Maybe I get bad advice.

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excuse me (post #214784, reply #5 of 10)

typo ... Also THINK real stoen veneer

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It is interesting to note.. (post #214784, reply #6 of 10)

Veneer stone suppliers say you need to use steel lathe for their product.  Why for veneer and not for slate tile?????

I might add that for my project I do have a pony wall in my crawl directly below my perimeter for my fireplace, so I did plan that as much as product manufacturers state "no additional foundational requirements needed"..  I suppose that applies to more typical projects like an 8 foot stone wall and not 15 or 20 feet.  You would think they might want or need to make that disclosure.

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Hey DoRight, Those are all (post #214784, reply #8 of 10)

Hey DoRight, Those are all good points.  I agree that weight might not be an issue afterall.  The heavy stones/tile will be stuck to the wall therefore not that much weight is transferred directly to the floor, as it would be with a traditional masonry fireplace that is stacked on a footing.  But, if this is an interior partition wall with nothing under it, then the extra load on that wall may cause problems.  I would guess the stone manufacturer/supplier could get the OP the wall/substate requirements.  Good luck with your similar project too.  

sorry , OP? (post #214784, reply #9 of 10)

OP?

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Not an issue... (post #214784, reply #7 of 10)

As long as the structural framing holding the cement board is sufficient, and the cement board was fastened to the stud properly, the slate tile can be thinsetted directly to the cement board. Treat the seams in the cement boards panels with alkalii-resistant mesh tape and thinset. Use a high quality modified thinset (in the $30 per bag range) to set the tiles, or buy unmodified and add a latex modifier to it.

The weight of the slate. Think of it this way:

The slate tiles are 1/2" thick. 1/2" thick slate weighs about 7lbs per sqft. 8' x 15' x 7lbs/sqft = 840lbs of slate on the wall. About 105 lbs of slate per running foot of wall. Add in the weight of the cement board. The thinset. The grout. Still not a worry.

Before you put the slate on the wall, stack it flat on the floor in front of the surround, along the 8' length of the surround. The stack of tiles will be 8' long and 7-1/2" high. The floor won't notice. Neither will the wall after the tile is installed.


There are 10 kinds of people in this world; those who understand binary and those who do not.


Original Poster...   like (post #214784, reply #10 of 10)

Original Poster...   like OG, but for the computer folks...