Tiling over micore 300 (indirectly) for fireplace surround [Archive] - Ceramic Tile Advice Forums - John Bridge Ceramic Tile


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06-20-2019, 01:43 PM
Hi all. Long time lurker who learned much about bathroom tiling from reading your forums. Thanks so much to members about that!

We have removed an old crappy fireplace and installed a new direct vent. Per install instructions we used micore 300 mineral fiber board as a noncombustible backer.

Now onto the finishing. We will be using 3 x 6 marble tile with unmodified thinset.

What is the best option to use on top of the micore board - Durock or metal lath with a scratch coat of type S mortar? Why?

If anyone has done either method, please share your experiences!

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06-20-2019, 06:27 PM
Hi Deb, welcome. It depends what the marble tiles look like. Can you post a pic of what you have? Usually, the cement board would be best for 3x6 tiles.

06-20-2019, 08:25 PM
Welcome, Deb. :)

I think you'd do best to contact USG for a specific recommendation on that. They do not recommend tiling over the Micore 300 (https://www.usg.com/content/usgcom/en/products/industrial/mineral-fiber-boards/micore-300-mineral-fiber-board.html) and I expect that would apply to using it as a backing material for your proposed mortar bed, but I don't know that for sure. And I know they don't publish any installation method for applying their Durock cement board over such a backing material, nor would any other CBU manufacturer.

I'm curious about your stove/fireplace installation (could we have a make and model there?) requirements. If the specification was for a "noncombustible backer," I'd think the Durock would suit that purpose as well as being a suitable backing material for your stone tile as Davy has indicated.

My opinion; worth price charged.

06-20-2019, 09:26 PM
Davy, here are pictures of our fireplace install and the tile. It is statuary marble.

The tile is on the stove because I just baked it. While I ordered a honed finish what I got is a semi polished and I wanted to make sure there was nothing on it that would change color on high heat.

CX - our fireplace is a Mendota FV34 (https://mendotahearth.com/pdfs/manuals/FV34-PF2-IM-1015%20INSTALLATION%20MANUAL.pdf). I think it puts out almost 32k btu. We used the micor because of the R value. Durock was on the list but a much lower R value.

We had taken the temperature of the fireplace metal which extended above the viewable glass before micore install and it was in the 300 - 350 range. With the micore the surface temperature is now about 115 so the Micore
Is doing its job.

The online version of the install manual and the one that shipped with our fireplacce are a little different. The online version states you can sandwich a metal lath between micore and durock and fasten with screws. My print version just shows a picture of metal lath but no detailed instructions so I had assumed (perhaps incorrrctly) it got mudded in.