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Unread 04-23-2020, 02:46 AM   #1
SWB04
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CBU Install for fireplace surround, to support ledger stone

This a long story that I'll try to wrap up into a shorter post (failed ).

Built a home (actually, finished a previously abandoned build) in 2016. Last minute direct vent fireplace install. Installed to code, but had the drywall guys throw together prep for a fireplace surround with CBU over studs. Was never comfortable with the overall design, or that they did the install correctly. After 3+ years of looking at the roughed in fireplace, I'm able to combine time + pennies + H.D. discount on ledger stone, so it's time to finish the job. I started by stripping off the CBU skin. There are problems with the stud construction which I'll fix, I'm expanding the ledger stone wall to 16" on the wings of the fireplace insert. I'm cutting off the existing hearth, and will rebuild and reattach a larger one. (FYI: I know that those two studs in the middle extend below the height of the safety stand-offs on top of the insert; they will be cut to finish above that height.) All of this is background to my questions.

1) For several reason (damage to the fireplace metal face, too much metal showing), I intend to overlay part of the black fireplace insert face with stone. It meets manufacturers safety requirements, as long as anything attaching to or near the fireplace door is a non-combustible. CBU, mortar, and stone meet those specs. I'll add a picture. I intend to install stone to about 1" over the top of the fireplace eyebrow, and about the same 1" on the sides. So, I have a couple of options:

a) Butt the CBU plus with the edge of the fireplace insert face, use mortar (e.g. Flexbond) across the CBU on to the edge of the metal fireplace, and lay the stone across both surfaces. Flexbond is a polymer-modified mortar, and supposed to bond to metal. Problem is, the metal and CBU expansion and contraction properties are different, and the bond at the edge of the fireplace, and across the metal top face, will be weak.

b) Do (a) above, but instead of Flexbond, use some sort of 300F tolerate bonding glue to attach the stone to the fireplace face. Same concerns as (a).

c) Probably the only correct way to do this, install the CBU across from the frame to overlap the metal face of the fireplace. To do this, I would skin the entire fireplace surround with 1/2" of plywood flush to the fireplace edge, mechanically fastened to the studs with a 2" flooring substrate screws (left over from another project) and glued with a poly based construction adhesive. That would give me a solid backing for the CBU. I'd snap chalk lines over the plywood marking the studs, and screw the CBU with CBU screws 8" apart like any other installation. Then I'd install the ledger stone on the CBU to the edge of the fireplace, slightly longer than the CBU, and finish the edge "somehow", i.e. probably leave the edge of the stone exposed, and use mortar, or a mortar colored heat resistant caulk to fill in the edge behind the stone, between stone and fireplace.

Q1) Assuming I do (c) above, should I use 1/4" or 1/2" CBU? 1/2" CBU is standard for walls, but over plywood, I really won't need the (nominal) extra strength or height.

Q2) Assuming I do (c) above, should I also glue the CBU to the plywood, i.e. again, with a poly based construction adhesive?

Q3) This isn't a wet area. Do I need a moisture barrier behind the CBU? (I doubt it; just being thorough.)

Q4) Assuming I do (c) above, I still have an issue of how to attach the CBU to the metal fireplace face. Current plan: Glue the CBU to the metal face. Since this is a sealed, direct vent fireplace, piercing this unit anywhere with screws is a no-no. I know that I could seal any holes, and it's probably OK to do, but I'd feel better from a safety standpoint not drilling holes and using CBU screws to attach the CBU. Does anyone have recommendations for a glue that I can use to glue the CBU to the metal face, that's heat resistant to 300 degrees, and strong enough to substitute for screws? Better ideas? I'm looking at a product on H.D.'s web site, and have a question in about it: Rutland 500°F RTV High Heat Silicone. Comes in clear or black.

I'm probably not done with the questions, and I appreciate any feedback.

Thanks,

Scott
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Unread 04-23-2020, 07:15 AM   #2
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Welcome Scott,

Is the fireplace metal surround face flush with the framing? If not, it is possible to make it so?

If yes, my first thought is to use a 4X8 sheet of cement board horizontally to span the whole thing from side to side above the opening.
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Unread 04-23-2020, 12:29 PM   #3
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Hi Dan,

CBU flush to the unit is no problem. The DVI (direct vent insert) is made with a 1/2" flange around it for a drywall application, which I intended to fill with 1/2" plywood skin covering the entire fireplace surround area. The status quo before I pulled this apart was in fact, CBU flush around the unit. However, the top has that (ugly) black, steel face about 10" high, which I want to reduce to 1" or 2", and I want to do the same to the sides, leaving only 1" of black steel margin showing, by overlapping with ledger stone. (I probably could add stone right to the edge of the opening, but there's a removable steel screen and glass door that might be in conflict, i.e. bumping the stone claddng/fascia during removal, so, I'll keep at least a 1" steel margin around the door clear.)

How do you recommend I do this install? Should I install the CBU flush, and then simply apply mortar over both CBU and steel fireplace flush to the 1" margins, and install the ledger stone over both? Is that standard with fireplace insert installations?

The only restrictions I've read on covering the fireplace face is not to cover louvers (don't have any on top), and combustibles restrictions. I supposedly can bring a wooden mantel support as close as 2.5" down to the top of the door, when using the eyebrow canopy. The mantel height from the top of the door depends in the width of the mantel, e.g. for an 8" wide, I need 10" space above the door. In fact, the set-back diagram for the mantel illustrates an example with 3/4" marble attached to the upper steel face down to the top of the fireplace opening.

I'm at a point where I can do almost anything to build up or reduce the fireplace surround structure, i.e. altering studs, adding shims or plywood to build it up, and etc.. I'm going to ensure I have a completely flat surround from left to right, parallel to the wall. (It wasn't that way when the drywall crew originally installed it.) The only thing I don't want to do is move the actual insert, since there's a rigid vent out the back of it, and I'd have to reinstall that, plus perhaps mess with the gas and electrical below. I'd probably eliminate the raised hearth altogether if I were starting from scratch, but at this point, it's too much extra work.
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Unread 04-23-2020, 03:58 PM   #4
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Would the 1/2" CBU over the studs be adequate support for this relatively heavy (heavier than just wall tile) ledger stone, over a wall and above the fireplace? Can I skip the plywood skin, as long as the vertical wall studs underneath are at least 16" on center spaced? Just glue & screw the CBU to the studs, and then apply mortar and stone?

I'm leaning towards using the plywood to strengthen and tie everything together on the wall, since CBU itself provides very little strength. I also think 1/2" should be sufficient strength for a wall, but at this point, I'm open to all ideas (and/or references). This forum is my first place I go for tile & stone advice; never have failed me before.

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Unread 04-24-2020, 06:42 AM   #5
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What I meant was this; if you can adjust the framing and/or the insert so that the face of the surround - the part that the manufacturer meant to be visible after installation, is flush with the surrounding framing you could simply hang a 4X8 sheet of CBU above it, letting the bottom of the sheet drape over the top of the insert face.

An un-notched sheet of CBU, with no vertical butt joints will be more stable, won't crack, and if you keep your framing spacing tight the CBU will be sufficiently well supported. Remove those two vertical 2X4's, install a horizontal 2X6 at the proper height, then install the rest of the framing as needed. I doubt that any mortar will adhere to the metal face in any helpful way but it will provide support for the CBU curtain so it doesn't move in or out.

You'll still need to figure out how to hide the edges of the CBU.
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Unread 04-24-2020, 11:54 AM   #6
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I'm pretty much in agreement, Dan.

If CBU is securely fastened and well supported to all surfaces, however it's done, the job will hold. I have to build out the fireplace surround from the wall a couple of inches in any case, and the studs will get me to within 1/2 inch. I can either use spacers on the studs and around the fireplace, e.g. like 1x4's, or as I first posted, just skin it all over with 1/2 inch plywood, which will guarantee an even (versus wavy) wall base. Screws will mechanically anchor the plywood to the wall, and I'll use 1 5/8" CBU screws for the CBU. Where the CBU drapes over the metal fireplace, I'll use a high temp glue to anchor the CBU to the metal face. (That bond will only be as good as the powdercoating, but it should hold.)

The side gap between fireplace and the stone will be the width of the CBU and mortar, probably 3/4" to an inch. I can fit the stone at the edge, interleaving it like a hand built stone corner, but have done that before, VERY labor intensive and messy. Other options (other than 1" troweled mortar) is to use ledgers/stacked stone lengthwise sections, kind of like pencil molding, and just frame the opening. I'll figure it out when I'm dry fitting stone. If you review pictures of the old block & brick built fireplaces, it's not unusual to see the inside of those openings finished with a troweled mortar edge, so that may end up be the best look.

I've been stalled thinking about this for a couple of days, so I'm gonna pull the trigger and get started (before the wifey gets fed-up with the mess and decides to cash in my life insurance policy early. )
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Unread 04-24-2020, 10:47 PM   #7
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ProLite Gray Tile and Stone Mortar - Alternative??

Can't believe I'm asking this question. I'm looking for an appropriate mortar for installing MSI Ledger Stone (24" x 6" sections) to a wall, i.e. a fireplace surround. MSI recommends this product, and WEEEee, it's available at the local H.D. big box store.

In white. Not gray. I can order gray, with a 1 month + delivery schedule.

I'm looked at everything sold at H.D. and Lowes. All mortars that I've seen are:
  1. Not for stone
  2. For floors only
  3. Not for large format tiles
  4. White
  5. Dryset/unmodified

I almost grabbed unmodified mortar and a 2 GAL bottle of latex additive, but I've never used that stuff before, and would rather just mix a polymer modified thinset with water.

The H.D. tile clerk recommended VersaBond LFT, and a tile pro there buying supplies assured me that they use this on walls, so I picked up a bag. However, I've read everything published by Custom Building Products about this product, and while it doesn't come out and outright specify "floors only", as MAPEI UNTRAFLEX LFT** does, every reference is about floors, and it doesn't indicate that it's a "non-sag" product, i.e. for walls.

** Interestingly enough, the MAPEI ultraflex gray states "floors only", the white is walls and floors. MAPEI doesn't seem to market a gray version for walls. WTH?! What does Custom Building Products and MAPEI have against making a freaking GRAY non-sag mortar for LFT on walls?!!

This ledger stone is heavy, and I really need a high-grab mortar. I suppose I could use a speed set type mortar, if I mix a new batch every 5 minutes, but past experience tells me fitting this ledger stone will be time consuming (particularly since there's already a 25% breakage rate in the first three boxes).

Short story long: Any recommendations for an alternative mortar that's for LFT, high grab/polymer modified, non-sag, that doesn't cost $70 a bag, and that I can pick up at a big box store on a Saturday?

Thanks.
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Unread 04-25-2020, 04:08 AM   #8
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Ultraflex 1
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Unread 04-25-2020, 08:37 AM   #9
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Hi Scott. I've yet to see a true non sag thinset. All that I've used will sag a little and I don't have time to keep going back and sliding them up, even 1/16-1/8. And, you're talking about your stones being heavy so I think you'll find yourself supporting them with something.
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Unread 04-25-2020, 08:43 AM   #10
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Thanks for the tip, Shawn. Ultraflex 1 would work about as well as the versabond LFT I already have. It's not a non-sag mortar, and a non-sag mortar should work better for my project.

FYI, to all. Laticrete 4-xlt i supposed to be one good alternative. However, only one tile store is open today, but at this point, I won't be done with the prep until Monday anyways. I'll see if I can get a bag of this in gray. (Long drive to any of these stores, though.)
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Unread 04-25-2020, 09:09 AM   #11
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You want a true non-sag mortar ardex x77 is the one to get. I have seen this demonstration in person

https://youtu.be/qRfb5racNH0
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Unread 04-25-2020, 10:51 AM   #12
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ardex x77 looks perfect. Prolite looks good, too. Problem with both is that I can't buy them.

When I start hunting tiles stores in my area, I'll look for Ardex, too. Their web sites (both Ardex and MICROTEC) don't have the standard "dealer locator" option, so other than blindly stumbling across the product, I have no way to find it.

Thanks again, for the tips.
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Unread 04-25-2020, 11:02 AM   #13
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Home Depot has Prolite.

I've heard good things about Ardex but it's not available for me. It doesn't make sense to drive 50 miles for thinset when I have Home Depot and Lowes around the block and Daltile a few miles away.
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Unread 04-25-2020, 11:13 AM   #14
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HD has Prolite white in stock. ETA for delivery of gray by special order is end of May. No HD for 100 miles around, has Prolite gray. Lowes and other local building stores in my area don't have what I need, or only have it in white mortar. I don't know if it's CoViD-19 related, seasonal, or just their inventory choice.

There are some tile stores locally (20 miles) that should something available on Monday.
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Unread 04-25-2020, 11:19 AM   #15
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Why do you have to use gray? Is the ledger stone dark? If so, use the Versabond or Flexbond. You're making it sound like it'll fall off the wall if a certain thinset isn't used.

I would think Lowes has a gray modified thinset.
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