An under-sink leak is one of four specific joints. Pinpoint which and
the fix is obvious.
You open the cabinet under your sink to grab a dish sponge and you
find the sponge is wet, the cabinet floor is soft in one spot, and a
slow drip is falling from somewhere in the tangle of pipes above. Or you
notice a water stain on the ceiling of the room below the kitchen. Or
the under-sink space has been damp for weeks and the cabinet bottom has
started to swell.
Under-sink leaks are common, cheap to fix, and one of the single best
introductions to residential plumbing repair a homeowner can attempt.
The tools are minimal, the parts are cheap, and every step happens in a
contained space.
The trick is identifying exactly where the leak comes from. Water
runs to the lowest point, so the drip you see on the cabinet floor is
rarely where the leak actually originated. In the under-sink area there
are four places water can leak from:
The P-trap slip-nuts (the large white or chrome
nuts on the U-shaped section of drain pipe). Loose, or the beveled
washers inside are worn.
The supply-line connections (at the shutoff valve
below and at the faucet above). Over-tightened, under-tightened, or the
rubber washers are compressed out of shape.
The shutoff valve body (the small valve handle
between the supply line and the wall). Weeping from the stem when the
valve is turned, or seeping around the compression nut.
The drain tailpiece (the vertical pipe from the
bottom of the sink drain down to the P-trap). Cracked PVC, loose
slip-nut at the top, or bad connection to the disposal (for kitchen
sinks with disposals).
Four leak sources under the sink: P-trap
slip-nuts, supply lines, shutoff valve, drain tailpiece.
This book walks the diagnosis and fix for each in order of
probability.
What this book covers
Finding the actual leak source (not just where water is
visible).
Fixing each of the four leak types.
Tools and parts (under $20 in total for most repairs).
Leak-testing — the part most homeowners skip that leaves a re-leak
Sunday morning.
What this book doesn’t cover
Main water line leaks (inside walls, between the
water main and the sink). Specialist territory.
Leaks inside the sink itself (cracked basin,
leaking drain flange at the top of the drain). Sink replacement.
Leaks from the dishwasher hose or disposal —
adjacent topics covered briefly in Chapter 7 but not the focus.
Slab leaks — if the wet cabinet bottom is tracking
back to water coming up through the floor, not down from above, that’s a
different problem entirely.
What you’ll be able to do
by the end
Stop an under-sink leak in 30–90 minutes, for under $20 in parts,
using basic hand tools. Pass a 24-hour leak test with confidence.
