How To Install Undermount Kitchen Sinks
Understanding how to install undermount sinks is an
extremely important part of creating a high quality countertop. This article
covers undermount kitchen sinks. Small bathroom or bar sinks are easier – they
typically can be mounted directly to the concrete using the hardware provided
with the sink. Kitchen sinks are too heavy to mount directly to the concrete,
so special preparations must be made.
Order of Installation
It’s very important to install an undermount kitchen sink
before the countertop slabs are set down. Once the countertop slabs are set
down, you usually will not be able to fit the sink through the hole, and you
won’t be able to properly seal between the sink and the underside of the
1. First install the sink supports. This article explains 3
different ways to create sink supports.
2. Then set the sink onto the support frame.
3. Apply a generous bead of silicone or siliconized acrylic
kitchen and bath caulk around the top of the sink’s flange.
4. Install the adjacent countertop slabs over the sink,
sandwiching it between the countertop and the cabinet frame.
Types of Sink Supports
For concrete countertops, the concrete should not bear the
weight of the sink. Instead, you build a support out of plywood, and the
cabinet frame together with the support bear the weight of the sink.
The first type of support is a commercial sink bracket sold
by granite supply companies. These brackets are designed to be installed onto
the cabinet frame. Follow the manufacturer’s instructions.
Custom Wood Frame
The second type of support is a custom made wood frame that
mounts to the cabinet walls. The frame supports the sink by its flanges. Always
leave some wiggle room between the sink bowl and the cradle so that you can
align the sink with the countertop hole.
Custom Plywood Shelf
The third type of support is a sheet of ¾” plywood shelf
(yellow, below) that’s cut to fit inside the cabinet. The plywood has a hole
cut into it that the undermount sink drops into. The hole in the plywood must
be large enough to allow for the sink to be moved to align it with the hole in
the countertop. Sometimes it’s useful to rout a roundover along the inside edge
of the hole. In addition, it’s important to cut holes for all the plumbing and
faucet fixtures that mount to the countertop. The hole in the plywood must be
large enough to allow for tools to access the faucet and plumbing hardware.
It is very important to create a watertight seal between the
underside of the countertop and the sink. Take care when applying the silicone
caulk along the flange of the sink that it is generous enough to create a
watertight seal, but not so much that it is messy.
Bare concrete will react with silicone to produce a
nasty-looking, oatmeal-like consistency that will ooze out of the sink joint.
Therefore, you must seal the underside of your sink openings so that only
sealed concrete touches the silicone.