Residential sink drain plumbing commonly is made from two
types of piping: the "permanent" drain lines that poke through the
wall (or floor, sometimes) and slip-joint pipes that connect
the fixture drain to the end of the permanent pipe. Today the
permanent pipes are typically white-colored PVC or black ABS; before
the 1970's drain plumbing was mostly metal: copper, galvanized
steel, or big heavy cast iron. The permanent pipes are installed
during the "rough-in" phase of construction, before the walls and
ceilings are covered with drywall or plaster. Later, after the wall
and floor surfaces have been installed, the "finish plumbing" is
installed. For drain lines, this finish plumbing is usually done
with slip-joint pipes and fittings, which can be installed and
removed with simple tools. Some builders use permanently-glued PVC
piping for the finish drain plumbing, which can be cheaper but can't
be easily removed for changes (or cleaning).
The connection between the permanent drain lines and the
slip-joint piping is normally accomplished with a slip-joint
adapter that is fastened to the end of the drain pipe.
Kitchen sink drains normally use 1-1/2" diameter drain
pipes. There are also 1-1/4" diameter drain piping which is
mostly used for bathroom sinks and other low-volume drains. Years
ago all slip-joint drain pipes were made from brass, which was usually chrome
plated on the outside. Brass has a tendency to corrode and leak. PVC
plastic, while seemingly cheap and weak is a much longer lasting
material, truly corrosion-proof, for a lower price. Perhaps the
drawback of PVC is the possibility of over-tightening the threaded
fittings and ruining the threads. PVC slip-joint fittings are also
more likely to become loose (and leak) if they get bumped too
much, so they may need to be tightened occasionally.
The starting point for this task is the sink
basket. The smaller diameter threads are for the 1-1/2"
diameter drain pipe.
The sink drain kit includes fittings for two
basins and a "T" junction.
The "S" trap fit includes the two-component trap
and a piece of straight drain tube.
Note that there are some big differences between this PVC pipe
and the PVC pipe that is glued together for building drain lines.
- This pipe uses slip-joint connections, which allow the
pipe to be adjusted in and out.
- The outside diameter is the nominal size (the stated or
the the named size), where the glued-together PVC pipe the
nominal size indicates the inside diameter. This means
that the slip-joint drain fittings will slide inside the
glued-up drain pipe, and there are threaded-to-glued adapters
where this happens.
This metal nut connects the drain tail-piece to
the sink basket. This short tail-piece came with the kit, but
I also bought longer tail-pieces because I prefer them.
The longer tail-piece actually came as one part that is sawed in
half to make two tail-pieces.
I assembled the drain parts that I figured I
would need. This layout shows the form that the pipes would
follow... if the sink drain line goes downwards through the
Note: This S-trap arrangement would not
meet today's plumbing codes. Read the warning below.
Most newer houses use a horizontal drain line that runs into the
wall (and then turns downward, eventually). Horizontal drains are
easier because there is no need to drill holes in the bottom of the
sink base cabinet.
When a horizontal drain pipe is present (which is more common
than vertical) then a P-trap would be used instead of an S-trap.
The metal nut (which came with the sink basket)
was installed over the tail-piece, and the plastic washer was
inserted. I'm not sure if the plastic washer is needed with
PVC pipe, but it certainly is needed with metal pipe.
This assembly was attached to the sink basket. I
used pipe thread compound to help seal the joint and allow for
easy removal later.
Below the tail-piece is an elbow which leads to
the other basin.
At the other basin there is a "T" junction
fitting, which leads into the "S" trap, which leads
into the building drain. All of these pipe are connected with
slip-joint fittings, which can be removed easily for repairs
In fact, the ease of dis-assembly is perhaps the best feature of
PVC drain fittings. If a sink gets clogged, I try the usual drain
cleaner products first, and if that does not work then I just
disconnect the fittings, take them outdoors and blast them with the
garden hose to dislodge any blockage.
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The procedure in this article was a repair to an
old house that may not have met today's plumbing codes,
because the connection to the vent may have been too far
from the sink and the trap.
The procedure shown here would not meet plumbing codes
in most places in the United States and Canada, where a
"P"-trap that connects to a vented vertical drain line would
normally be required. Water in an "S"-trap can easily be
siphoned out by lack of proper venting
A reader pointed out that an S-trap should only used to
replace an existing S-trap. Note that this improper
plumbing could be flagged by a home inspector when a house
is put up for sale.
The owner of the house shown in this
replaced the drain pipes exactly as they were before the
kitchen cabinets were replaced, so the venting situation was
not made worse.
The purpose of this article is to discuss the methods of
working with PVC drain pipe and fittings. For exact code
requirements, we recommend consulting a licensed plumber or
your local plumbing inspector.
Read Replacing Sink
Drain Plumbing for information about adding a vent where
none existed before.
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Lavatory Faucet and Drain.
Some Thoughts On Plumbing And Drain Repairs:
I have repaired many sink drains that leaked, in residential and
hotel kitchens. From the perspective of an engineer, if we can
build systems that hold thousands of PSI without leaking, then
making a non-pressurized drain water-tight should be simple. What
makes systems secure enough to withstand high levels of fluid
Pipes and fittings that are physically strong enough to
prevent bursting. This is not a problem with any modern drain
Pipes and fittings that resist the corrosive action of
liquids they contain. PVC is virtually corrosion-proof. Brass is
not. Brass will corrode over time, and slight acidity of the water
can accelerate this.
The connections between parts have precise mating surfaces.
I believe this is the culprit behind many leaks. If pipes are not
aligned properly, because someone tried to force a straight pipe
into a slight bend, there is a serious risk that a leak will occur.
(I've seen this too often.) Consider this analogy: all automotive
engines have incredibly precise mating surfaces between the engine
block and the cylinder head, carefully machined to within a few
ten-thousands of an inch. These connections are designed to maintain
fluid-tightness at several hundred PSI. That is the last place for
The connections between parts must be clamped securely
together, otherwise no amount of precision will do. Again,
consider the automotive analogy: the numerous bolts that clamp the
cylinder head to the engine block are very tight, perhaps 100
foot-pounds, more than the lug nuts that hold the wheels on the car.
Most of the sink leaks that I have seen were simply caused by
slip-joint nuts that had come loose.
Able to withstand the abuse of normal activity. Under-sink
drain pipes are going to get bumped from people storing items there.
Children and rental-property tenants are more likely to abuse things
around the home. I believe the casual bumping of drain plumbing
explains why they often become loose over time, and this is worse
with PVC than with metal, because PVC fittings cannot withstand as
One other tip to help prevent leaks is to apply a little pipe
thread sealant (pipe dope) to the slip-joint washers. This gooey
material will fill small imperfections in the mating surfaces.
If all else fails, I clean the area around the leak and apply a bead
- Sharp Knife
- Large Channel-Lock Pliers
- S-Trap Kit, 1½"
- Kitchen Sink Drain Kit, 1½"
- Pipe Thread Compound