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Do stainless steel kitchen undermount sinks always attach to the granite / quartz or do I have to make a precise (routed?) hole in the 5/8" plywood to drop the sink and, have it be supported by the plywood and then just have the quartz counter sit on top?

This is my exact sink

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I have installed sinks that are supported by the plywood sub base many times. Sometimes it was framing added strategically, since there was no room for a plywood base.

FWIW, countertop installers, all of them in my experience use bolts epoxied in slots under the stone top. This is for 3 cm stone, I would not want that in 2 cm stone, then I would only use plywood or framing for supports. That is my preference.

I have had some installers complain for my base material being in the way for their bolts, my miscommunication. Be certain how the sink is to be held in place so there are no problems during install.

  • +1 to actually supporting an undermount sink with ply or framing. There are plenty of stories of sinks letting go after a while when they've just been glued in place. I know this is the land of DIY, so maybe you're doing your own stone as well, but if you aren't, I'd just let the stone installer cut (jigsaw accuracy, not router) the hole in the ply. Have them cut generously at the back so the airgap/faucet can attach directly to the stone, rather than have to go through both stone and ply. – Aloysius Defenestrate Sep 23 '15 at 13:59
  • Maybe it was just the installers we used in Maryland, but the stone countertop guys would not cut plywood or set framing for the support of a sink. They would leave the sink and it would be used to size up the support by the carpenters, usually me. – Jack Sep 24 '15 at 2:16
  • interesting comment... we're always doing plywood here (never framing), so it's 90% of the time that the installer will cut the ply. The other 10% they'll draw a line and ask us to do it. Maybe they don't trust us to cut the hole in the right place ;) – Aloysius Defenestrate Sep 24 '15 at 3:14
  • Plywood has been used for 2cm stone on my jobs and it has, typically a has a thickened edge to hide the plywood edge. When it comes to 3cm, plywood is not needed, but framing is needed to support the sink, at least in the cases I worked with. – Jack Sep 24 '15 at 4:57
  • Plywood has been used for 2cm stone on my jobs and it has, typically a has a thickened edge to hide the plywood edge. When it comes to 3cm, plywood is not needed, but framing is needed to support the sink, at least in the cases I worked with. – Jack Sep 24 '15 at 4:58
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Use a thick bead of caulk around the sink and some 1/4 brass anchors and screws. Drill hole at every corner of sink and a few in the middle. Use a quarter inch drill bit to drill 1/4 holes in the granite to insert the brass anchors into. Trace sink outline on granite. Apply glue and mount sink and fasten down with anchors and screws. Let it set overnight. It will never come off unless you got a hammer and chisel and then you will totally destroy the sink.

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