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Old 10-10-2008, 02:46 PM   #1
briandegraveGC
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Here's a new one for ya!

I'm doing a shower with a 3' X 6' onyx slab set into the back wall as a center piece. No big deal right? Wrong!

I had the brilliant idea to back-lite it with led panels so it will glow through the semi-transparent stone.

Problem is securing the slab to the wall. I need the substrate to be water tight but still clear.

Here's my idea. I'm going to get a 1/2" sheet of plexiglas 1 1/2" larger than the onyx slab.

I'll attach the plexi to the back of the slab with a dozen or so screws and threaded sleeves.

I'll then attach the Joined plexi and slab to the framing- screw through the plexi into the studs, install my backer, silicone, redgard and then tile away.

What do you think? see any problems?
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Old 10-10-2008, 03:30 PM   #2
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Example of
Don't write a check with your mouth that the body can't cash.

No.
I think your idea is sound.
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Old 10-10-2008, 03:38 PM   #3
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The only part that scares me about your plan is the screws. I have worked with onyx before and found it very brittle.

Also make sure that the paper or plastic is on first.


Good luck but go slow drilling.
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Old 10-10-2008, 03:43 PM   #4
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I'm afraid of the screw and molly idea, i think when you tighten the screws in the mollies will expand and crack out the slab,especially that close to the edge,and as veiny as it is.unless it's 3 cm.which i doubt.You may want to use the type you epoxy in instead(unless of course that's what you're talkin about). Chad beat me to it, Chad, don't ever do that again.
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Old 10-10-2008, 04:00 PM   #5
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Sorry Man ... Next time I'll type slower..

Plus I'd listen to your advice before mine you have many more years in this than I .
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Old 10-10-2008, 04:01 PM   #6
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I would lay the slab down and attach Kerdi to the side with Kerdi fix and let dry.
then put all the wiring the wall, set the slab in a cut out area of the wallboard, finish the Kerdi and seams then tile.

you could also install it all then Redgard around it.
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Old 10-10-2008, 04:50 PM   #7
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thanks for the thoughts guys.

the slab is 3/4 thick and the edges polished, so the slab will actually project out a 1/4" or so. I'm going to have the stone company drill and install the threaded sleeves with epoxy. I would'nt attempt that myself.

I talked to my glass guy and he suggested using an windshield adhesive/epoxy in spots also- just for added support.

As for the lighting, it will all be accessible through an adjacent storage closet. I'll be building sliding tracks that fit in behind the shower wall.
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Old 10-10-2008, 05:14 PM   #8
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aston ehh? not far from me
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Old 10-10-2008, 05:18 PM   #9
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Make sure that the slab guy is aware that this is going to be lighted up from the back . You don't want those sleeves anywhere that is transparent or it'll look terrible ( black dots ) and you'll be doing this one again.
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Old 10-10-2008, 08:33 PM   #10
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i've done some work with plexi and this can be an expensive project. you should be aware that any machining of plexi (drilling holes, gluing) introduces stresses in the material that may cause "crazing" and eventual cracks and failure long after it has left the fabricator's shop. especially if under physical stress-supporting that heavy slab. for long term success, the part must be annealed after all fabrication (drilling, edge polishing, etc). that means bringing it up to around 200 degress for a period of time and then slowly decreasing the temp according to an annealing curve, typically 1 hr per mm of thickness. here is a fab manual for plexi that might help with some of that (note mod of drill bits) - http://www.plexiglas.com/literature/pdf/134.pdf
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annealing after machining - http://www.plasticsmag.com/ta.asp?aid=2514
http://www.polyfab.biz/annealing-acrylic.htm

i'd test that slab with the LEDs before such a big project. but if you do it, please let us know and show pictures!!

there's an interesting art-form that involves carving a design into the back of such slabs. then it is invisible from the front until it is backlighted, whereupon the design appears. very dramatic if it is depth-carved.

hmmm. just thinking out loud here, but with such a thick monolithic slab without cracks & mounted vertically, i doubt you'd have any vapor penetration, so one could drill small holes in the back of the slab just deep enough to mount the LEDs directly in the slab letting the light diffuse more through the rock. carefully locating corresponding holes in the CBU would allow conventional mounting and then inserting wiring from the back. it's low voltage DC so you shouldn't have shock/fire hazards. to avoid that altogether, use fiber optic cables inserted into the slab.
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Old 10-12-2008, 07:17 AM   #11
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here's the lights I will be installing.


I got a panel overnighted to me yesterday and will be testing to a sample of the slab on monday. $93 for a 12 x 12. I'll need 18 of them!

heres a pic of a backlit onyx wall


i'm looking into plexi-glue for the adhesion along with anchors. I think I'll have them drill the anchors into the dark non-transparent area of the stone so you don't see them.

thanks for the links on the plexi- I'll take it all into consideration.
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Old 10-15-2008, 08:24 PM   #12
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Quote:
heres a pic of a backlit onyx wall
VERY nice !!! so, how was this one mounted? is that photo of natural daylight lighting? it appears to a large wall?

FWIW, i see 36 LEDs on that panel -- if you want to get some tech to solder it up for you on a perfboard, you can buy high-brightess white LEDs from hong knog direct for about 50 cents each. i bought several hundred last year and had them by airmail in less than a week.

also, don't forget that for decades outdoor sign companies lighted huge signs with cold-cathode florescent lights. it seems that you have the room to accommodate this as well.
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Old 10-17-2008, 05:58 PM   #13
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william,

the pic above are actually onyx veneers with lighting already installed. They sell them as skins that fit together.

I'm ordering the led's monday. The sample I got is so bright, you can barely look at it. I sent the homeowner out to test the led's to the actuall slab and they were happy with what they saw, so it's a go.

I reframed the shower wall today to allow for the led's to slide in from a closet to the right. I used some paneling tracks and some 1/8 plastic boards to mount the led panels to by means of velcro. all is working out like I've planned ----so far.

I made some calls and came up with a couple of different silicone adhesives for use on glass and plastics. I made a sample board of plexi with 4 different samples of adhesive and stuck some natural stone to each. I'll give it 24 hrs and see which wins.

as if this wasn't challenging enough, I realized that the house has a quarry nearby that does blasting twice a week-------it shook the whole house last week. Just one more reason to make sure I do this right. I have one salesman checking on a seismic silicone adhesive that they make.

I'll post some pics after I get the rest of the led panels installed.
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Old 10-18-2008, 07:14 PM   #14
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i like these kinds of project and have been thinking about it and wondering if any adhesive at all is a good idea, even it if doesn't lead to crazing. the problem i see is in the very different coefficient of expansion of acrylics (and all thermoplastics) and the stone. if you glue the panels together, the expansion of the stone will be considerably different from the plexi. see - http://www.maizey.co.za/index2.php?o...230&Itemid=235 in the section entitled 'screw unions' -- all thermoplastics have a relatively high coefficient of thermal expansion and may also expand due to moisture , , , ". with such a big sheet, the expansion could be significant and lead to cracks if the union can't move. maybe the uion should be designed to move (also because of the brittleness of the onyx, if expansion of the sandwich sections is different); see their examples of acceptable screw unions -- "Large drill holes, scope for movement at the ends and systematic installation with fixed and sliding points prevent harmful stress generation within the plastic materia",l

you are definitely in uncharted waters and have much to consider. and man . . . especially with that blasting going on nearby. whew
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Old 10-19-2008, 06:04 AM   #15
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William,

I also like these kind of projects. It breaks up the monotony.

I've considered the expansion issues, the coldness of a winter day followed by a steaming hot shower. The sample glue ups I've done have given me some reassurance. The standard ge silicones I tried failed miserably. The silicone and stone peeled right off as expected. However the GE silicone for plastics will not come off without extreme scraping and I am not able to remove the stone from the larger glue up I did without extensive pulling. It took me about 5 minutes of tugging at it and the 1/4 plexi almost broke.

I feel if I was to spot glue the slab to the plexi it should allow for expansion. I would also peremiter seal the entire sandwich with the silicone. The silicone remains flexible. I am feeling more comfortable with no mechanical fasteners at all----only between plexi and framing.

this is a $3700 project just for the illumination process (yeah, I know). I think I'll contact Plexi for support. Thanks for the link. Now I'm more confused!

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