Bathroom vanity countertops made of a single slab or material like stone can have a finished edge around the sink cutout. This edge means that the sink itself can be mounted to the underside of the counter, rather than hanging from the top and concealing the cutout’s edge. The sink is held in place on the underside by special sealants that both bond the sink into place and help prevent water from leaking out through the join at the sink’s rim.
Mount the Sink Before the Counter
While vessel sinks and drop-in sinks are mounted after the counter has been placed on the vanity and sealed into place, undermount sinks are installed first. It takes time for the sealant to cure, holding the sink securely to the underside of the stone. If the top is in place when the sink is mounted, gravity may work against the sealant, possibly pulling the sink away from the counter before it has time to dry. Instead, flip the counter upside down on a flat surface and attach the sink there. After 24 hours, the counter can be flipped back over and installed on the vanity.
Start With a Clean Surface
When stone counters are produced, a lot of stone dust may get left behind on the underside of the counter, which is rough and unfinished. This textured underside can trap and hold a lot of debris, which can interfere with the bond between the sink and the stone. Before attempting to mount a sink to the underside of the counter, clean it well with a stone cleaner and a soft cloth. Let the stone dry completely; a damp stone will appear darker in color than dry stone will. Wait for the color to return to normal before applying the adhesive.
Seal the Sink in Place
For most bathroom undermount sinks, all that is needed to mount the sink to the stone is a silicone adhesive. Squeeze a very thin line around the sink cutout on the underside of the stone. The weight of the sink will compress the adhesive slightly, making it spread out, so a thin line is all that is needed. Lower the sink into place, upside down on the countertop’s underside, then lift it straight up again. This will cause the adhesive to pull away in small “strings” from the counter to the sink. The air hitting these strings will make the adhesive bond stronger. Lower the sink back into place and leave it undisturbed for 24 hours to dry.
Using Mounting Clips
The majority of undermount sinks only need adhesive to bond them to a vanity counter, but some large or oversized bathroom sinks will also use mounting clips. This small metal clip fastens on posts to both the sink and the stone top. Epoxy is used to attach the posts to both materials, then the clip is slipped over them and a small nut is fastened down on each post to hold the clip in place. The epoxy will cure hard enough to fasten the clips onto the posts in approximately 10 minutes, but the clips still need a full 24 hours to dry before the counter is mounted into place.