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Unread 02-24-2020, 09:00 PM   #1
bottlefed
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Redgaurd on concrete slab?

Hello all, long time browser first time poster, I work doing various forms of construction and have done some tile but never any outside work.

I am getting ready to tile a screened in room for a client. I will be laying porcelain tile.

Its a pretty small room just under 200 sq ft and I will by covering the previously painted slab (paint is mostly gone and I think I should be able to complete the process with a power washer) with customer purchased (Home Depot) 9 3/4" square porcelain tile.

My question is should I apply Redgaurd to the slab before laying the tile or just apply directly to the slab (Customer says no mention of isolation membrane in PDF from tile manufacturer whatever that's worth).

Any other tips are appreciated.

Thanks in advance,
Richard
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Unread 02-24-2020, 09:24 PM   #2
cx
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Welcome, Richard.

I doubt seriously that you can get a painted concrete SOG sufficiently prepared for a ceramic tile installation by using a pressure washer. I would suspect you'll need some mechanical scarification.

The tile manufacturer has no idea where or how you would intend to install the tile and would be very unlikely to recommend any sort of crack isolation product. I'm guessing that is your intent with the RedGard?

But that would have no bearing at all on whether you need or want to use such a membrane. Does the concrete currently show any signs of cracking? Do you know how the slab was designed and placed?

My opinion; worth price charged.
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Unread 02-24-2020, 10:07 PM   #3
bottlefed
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Hello CX thanks for your reply,

The slab appears to be in good shape for its age, no cracks and pretty flat. I assume its been there for at least 10 yrs. It meets the brick/block wall of the house but I am not sure of the attachment to the house or other slab details.

Yes I was planning to use the Redguard as an isolation membrane. The other reason for wanting to use Redguard was me thinking the Redgaurd would better bond to non scarified concrete (assuming all the paint is removed) than thinset, but maybe that's a poor plan...
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Unread 02-25-2020, 08:54 AM   #4
cx
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I ask about the foundation because it's common to see those patio-covert-to-closed-porch slabs poured as flat-work (driveway, walkway, etc.) rather than as foundation slabs.

I would certainly not expect the RedGard to adhere better than thinset mortar to a poorly prepared concrete surface. Even on a well prepared surface the adhesion of the RedGard is required to be only 50psi and the shear bond of the thinset mortar for the tiles to the RedGard is also required to be only a minimum of 50psi. More than adequate in most cases, but it's all measured under optimum surface conditions.

A crack isolation membrane may be good insurance, but only if all the right preparation is done.

My opinion; worth price charged.
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Unread 02-25-2020, 06:22 PM   #5
Davy
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If the slab is painted then you may have hairline cracks and not know it yet. Get rid of the paint and bond directly to raw, clean concrete.

A cup wheel on a grinder will get it down to raw concrete. If dust is a problem then get a shroud with a vacuum attachment for the grinder. Sweep, and mop the slab to see if there's any cracks. If not, stick directly to the slab if you want.
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Unread 02-25-2020, 07:16 PM   #6
bottlefed
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Thanks all,

At least 40% of the paint is already flaked off the slab and I did not see any cracks.....but as you say when its prepped some may appear.

I have a grinder so I went ahead and ordered a cyclone separator earlier today for my shop vac to keep from packing the filter and I will get a vacuum attachment once I am sure of the model number and a cup wheel.

I had considered renting a commercial tool but with the job only being 180 sq ft I thought I would rather just use elbow grease.

Really appreciate the help.
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Last edited by bottlefed; 02-25-2020 at 08:17 PM. Reason: Missing word
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