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yhfwd05 09-30-2009 05:01 PM

Tiling in a Zero-Radius Undermount Sink (How to Seal Properly)
Doing a remodel on my kitchen. Planning to use tile countertops with a zero-radius undermount stainless-steel sink.

I want to be SURE that the edges of the sink not only look good, but are sealed properly. The sink came with clips, and has a 3/4" lip all the way around.

I plan on putting new plywood down, followed by new cement board, followed by tile. The sink mounting hardware works by placing a threaded insert down through the plywood from the top, then using brackets and screws from underneath to pull the sink up to contact the bottom side of the plywood. Visually, that makes the metal edge of the sink begin just below the plywood level.

That will leave me with a visible edge of plywood, cement board, and tile all around the edge of the sink.

QUESTION: what is the best way to seal and tile over this exposed edge? Can I just put mastic directly onto the exposed plywood and cement board edge, place my tile on, then grout it? Do I need to prepare the plywood edge surface with a sealer in any way? Then, what about the seam at the bottom of the side tiles (the edge facing toward the bottom of the basin)? Should this be grouted all the way around, or sealed with silicone?

Bob Mariani 10-01-2009 05:48 AM

undermount sinks are not designed for this material and method. One solution is to get an undermount kit used for making the undermount sink work with a laminate top. This provides a plastic edging that seals the inside edge. Nothing else you do will work well. With tile you really should use a top mount sink. The plywood edge is one concern and the tile edge is the other.
another solution would be to tile the vertical edge slightly past the sink bowl top edge using bullnose tiles. The rounded edge of the tiles will be vertical with the top up. This will give you a sealed and smooth edge you need.

ccarlisle 10-01-2009 07:46 AM

You can protect the tile edges with a Schluter profile product called RONDEC. It covers the edges of the tile only and allows an undermount look.

yhfwd05 10-03-2009 07:09 PM

Thank you both for your input.

It looks like the tiles that we ordered include bullnose pieces for the purpose of tiling around the edge of the sink.

Clarify this for me if possible: I should tile up to the edge of the sink and use the bullnose tile all the way around the sink. That would mean the bullnose pieces would extend down to the top lip of the sink, and would therefore cover up the plywood layer all the way around? Thin-set mortar is OK to apply directly on to that plywood in order to stick on some bullnose tiles? Will that create a waterproof seal for that plywood edge?

Then, you've got the seam around the edge of the sink lip where it meets the lower 90-degree end of the bullnose pieces. Should that be sealed up with silicone or what?

I did check out the RONDEC product and it looks pretty good. Except with the bullnose tiles it wouldn't be able to help much in terms of sealing the seam between the sink and the 90-degree end of the bullnose. That seam wouldn't be that visible anyway since it's down inside the sink cutout a ways.

Thanks again.

ccarlisle 10-04-2009 06:47 AM

IMO you'll need to waterproof the edge (underneath the bullnose) with a trowel-on product, or Kerdi, as that edge gets a lot of water. Just thinset alone is not enough...

And yes 100% silicone along the bottom of the bullnose/sink joint.

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