Concrete countertop w/vessel sink

I am getting ready to try my first concrete countertop, which will actually be a small vanity for a half bath. I will be using a copper vessel sink and I have never installed or otherwise dealt with a vessel sink before. In making the countertop, what size should the knock out be for the drain? Does the sink mount directly to the countertop? Anything else I need to be aware of.
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Thanks, in advance, for any suggestions you can provide.
Replies
This can be a huge PITA. The drain for the vessel sink will generally NOT be deep enough to be used to grip the bottom of the concrete top and hold the sink to the counter. So your knock out will need to be large enough to tighten the drain through. And you may need to figure out an alternate (silicone) way to anchor the vessel.
Get the drain assembly and install it on the vessel and see if you can figure out a way to put a stepped knock out under the sink that will give enough of a lip to anchor the sink using the drain nut but also have enough of an undercut to allow a basin wrench to get in to tighten the sink. Work it out in Styrofoam before you cast the top and then make the match in foam and set it in your form.
Good luck with this.
Learned the hard way... More than one lesson.
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"You cannot work hard enough to make up for a sloppy estimate."
I've done this in my bathroom. It's not a PITA to set up the knockouts, but if you don't leave yourself enough room, it can be. I completely second what the other poster said.
The Stepped knockout is the way to go. Make sure though, have all the sink pieces before you make the forms so you know exactly what you're dealing with. I used lengths of PVC pipe and plywood circles to make the knockouts.
The tailpipe that came with my vessel sink was actually long enough to make it through and make a connection. make the expanded knockout large enough to put some kind of wrench around it to tighten.
I had to improvise mine. It wasnt the most elegant solution, but it worked. I can email you photos of what I did to make the knockouts, if you like. Just send me a message and i'll get them to you.
Vessel sinks mount the same way any self rimming sink ( except stainless) does----- gravity & caulk.
Size of knock out ( hole) depends on sink configuration----- I like a lot of room, so I always ask that the hole be about an inch & a half smaller than the sink will cover up, this allows for some play & still plenty of surface area for caulking to grab hold.
Here's a vessel sink that needs an 8" hole in the top - probably not the case with a copper one, but it's still a good idea to have the sink in hand before you do anything.
Jerry
Thanks for all of your advice. I already have the sink (on super clearance at Lowe’s for $15) but would not have thought to get the drain assembly prior to casting. I think I understand how to mold the stepped knock out – several plywood circles mounted to the outside of the PVC drain knock out? Does the concrete want to adhere to the plywood if you use it for the step form?
I'm not sure you meant to address this to me, but I'll answer anyway.I find foam knockout forms much easier to remove with less chance of damage than plywood. You can easily dig out foam forms of almost any shape.Jerry