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Unread 04-15-2015, 10:12 AM   #1
ncgoober
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Install 25 sqft granite slab on top of radiant heated bench...bonding agent?

How's that topic for a first post?

So the short story is that I'm nearing the end of a big mudroom refurb where I installed radiant heated floors myself (Plywood subfloor / Roberts 2001 / RPM mat / stapled / radiant wiring / SLC). I had a tile guy lay the thinset and 12x24 tile on top of that (you must know your limitations). The counter tops in the room are granite and so I got a crazy idea to order a granite slab for a 30"x10ft long locker style bench divider for coats and such. To take the crazy to the next level, I thought it would be a splendid idea to make the bench radiant heated to act like a radiator on cold mornings.

So I got a well supported wood framework topped with a 2.25" plwood base (Three 3/4" pieces glued and screwed) and then 3/8" thick poured SLC top with wires inside. The SLC is FLAT...I mean dead flat. Anyway the granite slab (3cm) comes tomorrow and then I remembered I better get a thinset. Then I decided I better check the web and after some conflicting info...and reading about possible staining issues....here I am.

As an engineer (couldn't you tell already?) that's familiar with heat transfer, I know that I want a bond that has as little air as possible so that the heat makes it upward. Since the base is so flat I was thinking about something like just a 3/16" v notch or so (but I can be convinced otherwise). The granite is esentially white with dark and gray splotches (Delicatus White).

The install plan was to
1) Set the slab on edge against that back wall of bench with its bottom mesh visible
2) very thin skim coat the bottom mesh of slab (just fill in the mesh essentially)
3) Very thin skim coat the SLC base
4) V-notch the SLC base
5) Tilt the slab down to onto the base..... squish back and forth a bit..... stand back and admire

With that said....suggestions for specific type of thinset and trowel size?
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Unread 04-15-2015, 10:36 AM   #2
dhagin
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Welcome James.

Post some photos of the "granite" you've got there. Both sides. Some close ups of this "mesh" of which you speak too. Cement based mortars may not be recommended. Use the little paper clip icon to upload from your computer.

Oh, and how big is the 3m piece you're sticking to the SLC?

Have you planned on sloping this stone for drainage?
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Unread 04-15-2015, 10:58 AM   #3
ncgoober
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The granite is arriving tomorrow but here is a sample.

The mesh was pretty typical on most of the slabs that I went looking at in a variety of granite showrooms warehouses.

It's approx a woven mesh approximately 1/8" x 1/4" adhered to the stone in some kind of thin clear epoxy like substance.
I'd say the depressions in the mesh pockets is maybe 10 mils at most.
I can imagine that people might suggest to not bothering with trying to skim that mesh.

The slab is 30" x 108" x 3cm.

I thought about drainage but the kids are grown up and out so the sloppiness factor is not much an issue so the SLC is flat with no tilt.

Here is the pics
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Last edited by ncgoober; 04-15-2015 at 11:26 AM.
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Unread 04-15-2015, 11:06 AM   #4
ncgoober
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Here is a pic of the base.

Why of coarse it has a curve on it to make the trim harder.

Like I said earlier, I'll dry fit it at first to make sure there are no fitment issues but then I will tilt it back up against the back wall while we apply the bonding.

I am toying with the idea of resting that back edge of granite on a thin 1/16 - 1/8" thick strip of something (3cm wide)so that when we tilt it down that back edge will be supported by something that is approximately the thickness of squished thinset. Then again, I may just avoid that complication because there is no heating or expected dowforces that will ever be present along that back edge.

I will put some temporary securing mechanisms in place while it dries to insure it doesn't float away somehow. (although I'd think that's pretty unlikely).
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Last edited by ncgoober; 04-15-2015 at 11:18 AM.
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Unread 04-15-2015, 10:24 PM   #5
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Well the silence is disconcerting like people are wary of suggesting a solution due to the unknowns of the radiant heating.

I'm becoming more concerned about the water retention and possible staining especially when I see threads like this which would be totally unacceptable.
http://www.johnbridge.com/vbulletin/...ad.php?t=17208
Also the mesh gives me lots of pause wondering how that would effect proper drying.

We are having other of the same granite on the countertops and we cant have two colors of granite.

I just about convinced myself that I want to take a more conservative approach by putting a bunch of silicone down and essentially troweling it out with a fine v notch.
I don't think the thermal conduction would be as good but the odds of staining are zero.
I was just about ready to take this approach until I thought ...."I wonder what radiant heat does to silicon?'. Off to find that answer.
The max temp of the radiant floor is <80F and that would seem to be well within silicon that holds up to exterior and roof applications.
[Edit.... Service Temperature Range (after cure) = -55F to 300F ] Looks like I'm good there.
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Last edited by ncgoober; 04-15-2015 at 10:39 PM.
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Unread 04-15-2015, 10:58 PM   #6
Steve in Denver
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Granite tile has to get set with something, and it's not silicone...my guess is that a white thinset mortar might be a good choice. Do you have any scrap pieces you could test with? I wonder how much the thickness matters in terms of staining - 3cm is significantly thicker than the tiles in the post you linked.

I don't know how silicone cures, but it occurs to me that in the tube it stays liquid...how air tight is a 3cm slab on top and the RPM mat underneath? Assuming it is similar to a tile over kerdi / ditra situation, you probably don't want a modified thinset..

As flat as the SLC apparently is, maybe you could just put the slab directly on the SLC with some silicone around the perimeter to hold it in place.

I could keep making suggestions all night, but the truth is I'm just guessing. I would have figured someone around these parts would have some suggestions...
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Unread 04-15-2015, 11:31 PM   #7
ncgoober
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From the Technical data sheet for GE silicone

This material requires atmospheric moisture to cure from
paste to rubber and may not attain its listed final cured
rubber properties when used in designs or applications where
the silicone is encapsulated and without access to atmospheric
moisture.


Hmmmm....I guess its possible that a complete coverage with trowel of silicone might not be a good idea.

I wanna kick myself for not testing this out before hand with a test sample.
I may have to have them just put in place dry and do some tests with some samples.
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Unread 04-16-2015, 07:17 AM   #8
ncgoober
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Found a Fine Homebuilding mini-article talking about a heated granite countertop. They used a mat set in mortar but they didn't give details if they set the granite in wet mortar or put it on dry.

From Fine Homebuilding 191 (Kitchens & Baths) , pp. 18 October 5, 2007

The only option to view the article was an $80 all year subscription.

http://www.finehomebuilding.com/how-...en-island.aspx

[Edit]
Found somebody on Gardenweb who did a countertop from the FH article and they set the granite onto a finished dry mortar bed.
http://ths.gardenweb.com/discussions.../my-hot-island
It seems I don't have any photos with the thinset down. DH just filled it in and used a long straight edge to run it over the top to keep it level.

I know that will work but the heat connection just wont be as good as a thinset bond but I cant risk the look of some of the other pics I've seen of the stained granite.
I'm going to try a dry bond and see how it works.
I can always lift it up later if it's un-acceptable but I'm thinking the heat has nowhere to go but up.
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Last edited by ncgoober; 04-16-2015 at 07:42 AM.
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Unread 04-16-2015, 08:12 AM   #9
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If I had time I'd probably get a bunch of these thermal pads.
If I'm unhappy with the dry interface I'll investigate the price of these pads. I know they come in sizes upto 8.5x11 at least.

Stockwell Elastomerics is a leading manufacturer of thermally conductive pads. Thermal conductive pads are cut from materials such as thermal tapes, thermal coated fabrics, thermally conductive silicone (TC100), and thermally conductive silicone sponge (R-10404). These thermal pads are firm compared to gap fillers or gap filler pads. The thermal interface materials offered by Stockwell Elastomerics are designed to remove the heat generated by electronic devices to the ambient environment. Thermal management is a critical component in ensuring the reliability and proper operation of electronic and electromechanical equipment.

Thermal Gap Filler Pads
Gap fillers or gap filler pad materials are thermally conductive pads that have a dough-like consistency.

http://www.stockwell.com/thermal-gap-filler-pads.php
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Unread 04-16-2015, 11:37 AM   #10
dhagin
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Sorry, missed "mud room" in your first post, thought shower.

That thing ain't goin anywhere once you place it. Unless you've got big cantilevers/overhangs on the edges, I'd trowel some thinset down and smoosh it in. I'd try to use a rapid set mortar & maybe 1/4x1/4 trowel, it's just to fill voids and transfer heat. Use the flat side of trowel to "burn" a layer or mortar onto the mesh. The mortar may not hold onto the epoxy glued mesh long term, but no worries - it still ain't goin anywhere. If you want to stick the slab down, use epoxy mortar, something like Laticrete 300.
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Unread 04-16-2015, 12:41 PM   #11
ncgoober
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Granite got laid down today dry with only the thinnest bead of silicon on front edge.

At approx 400 #'s it smooshed it to nothing.

Going to go with the idea that heat rises and it will eventually migrate upward into the granite and eventually get up to temp and pretty much stay there.

With the whitish color I just couldn't risk having thinset stain or darken it.

I really cant test the heat until the SLC cures for 30 days and by that time all the locker style cabinetry will be built on top of it and there ain't no going back at that time and lifting it back up and putting some other kind of bonding agent there.

It is what it is.

The upper lockers will ultimately look something like the 2nd pic
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Unread 04-16-2015, 12:49 PM   #12
dhagin
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Quote:
Originally Posted by James
With the whitish color I just couldn't risk having thinset stain or darken it.
This is why white mortars are recommended with light colored or translucent stone, and glass. Once fully cured, no problems with either. With that 3cm stone, you'll likely be fine with the little bit of silicone. Like i said, it ain't goin anywhere, ever.

Room looks nice James. Thanks for sharing.
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Last edited by dhagin; 04-16-2015 at 12:55 PM.
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