Kitchen islands have come a long way from the "permanent
table with doors" that were common in older kitchens.
Today’s kitchen island is often an appliance warehouse,
housing cooktops, ranges, trash compactors, sinks, and
dishwashers. But contractors need to be aware that when a sink
is added to the island mix, the designer’s dream can
become a plumber’s nightmare. Running the supply lines to
an island sink is seldom a problem; it’s equipping the
sink with a code-approved drainpipe that often presents the
Plumbing vents are an essential part of a
properly designed drainage system. The vent does two things: It
allows makeup air to enter the system as the sink drains and
enables sewer gases to escape outdoors. A plumbing vent must be
located within an acceptable distance of the island sink. The
maximum allowable distance will vary depending on your local
code, but generally falls in a range of 8 to 10 feet.
This critical distance can present a problem for island
sinks. Plumbing vents are usually installed inside a wall, but
because an island sink drain may already be about 61/2 feet
from the nearest wall, the 8-to-10-foot distance often falls
within a very narrow band of wall area (see Figure 1).
1. Most plumbing codes require that an island
sink be no farther than 8 to 10 feet from the nearest
plumbing vent. This may limit the location of the vent
pipe to a small wall area.
Whether you’re bidding a kitchen job from a
designer’s set of plans or designing the kitchen
yourself, it pays to verify the venting location early in the
process. Retrofitting a vent after failing an inspection is
both costly and embarrassing.
There are a number of piping configurations that can be used
to drain and vent an island sink. I’ve listed a few of
the more popular types below.
S-Trap Not an Option
S-traps may seem like the perfect solution (Figure 2), but take
my advice and cross them off your list.
2. S-traps are illegal because a large discharge
of water from another fixture can siphon off the trap
Modern plumbing codes do not allow the use of S-traps, and
for good reason: A large amount of water discharged through the
drainage system can suck the liquid trap seal out of the
S-trap, thus allowing sewer gases to enter the living space.
Avoid the use of S-traps — even if there is no code
enforcement in your area.
Single-Fixture Wet Vent
Some inspectors will allow the use of a single-fixture,
double-sized wet vent for island sinks (Figure 3).
3. A double-sized single-fixture wet vent
provides enough makeup air to allow the drain to flow