Undermount Kitchen Sink Installation
Installing an undermount kitchen sink can be tricky. If you are getting new countertops, such as granite, the installers will often cut the sub-top so that it supports the weight of the sink, and then the granite lays on top of the sink for a nice installation. However, if you are replacing an undermount sink or repairing one that has come loose from the counter, there is a trick that works very well.
An undermount sink is typically held to the underside of the counter with adhesive, and some mounting clips. The adhesive is typically a two-part epoxy to hold the clips and sink in place. Silicone caulking also goes around the perimeter of the sink
The trick is to get the sink to pull tightly to the underside of the new countertop. If your countertop is granite, make sure you clean the underside of the counter as dust will often cause the epoxy to fail. When the installer cuts and polishes the opening for your sink, this creates a slurry of dust that will often find a home under your counter...right where your new sink mounts.
Getting your sink to mate to the underside of the countertop is not difficult, but will take some creativity. Use a 2 x 4 stud and lay it across the counter to span the opening. At this point you have a choice. You can use some adjustable bar clamps and place one end through each drain hole in the sink and the other end over the 2 x 4 and cinch them snuggly so that the sink mates perfectly with the hole in your countertop. If you don't have bar clamps, you can use rope instead. You will make a loop out of the rope and you will need another short piece of a 2 x 4. The shorter 2 x 4 will go through the drain hole in the sink and the rope loop will go over both 2 x 4's so that you are left with the rope going around the 2 x 4's and suspending the sink in the air through the drain(s). Then you can use another piece of scrap lumber and insert it above the 2 x 4 laying across the opening in the countertop but below the rope. Twist this scrap piece of lumber repeatedly and the rope will twist causing the sink to rise up until it meets the underside of the countertop. Then you can clamp the scrap lumber until everything is dry and secure, usually 24 hours.