Why is My Bathroom Sink Leaking Underneath? What to Do About It
It often starts with a damp whiff when you open the vanity for a spare loo roll. You notice water pooling and forming a small puddle, then discover your bathroom sink leaking underneath. Not exactly an ideal situation.
So, why is water leaking?
In most situations, you’ll find there is a simple reason for your leaking sink problem. It might be a loose joint on the sink drain, a tired washer in the P-trap, or a weep from the water supply lines. Addressing any leak beneath the sink as soon as possible is the smart way to avoid more extensive and costly repairs in the future.
Let’s explore the common reasons why this happens, how to fix it, and some preventative measures to minimise the risk of it happening again.
How a Bathroom Sink’s Plumbing System Works
Before we look further into why you might find your bathroom sink leaking underneath, let’s first review how the bathroom sink works.
Under the vanity, the bathroom sink is composed of a few simple parts that must seal together. The sink basin feeds into a drain assembly made up of the:
- Drain flange (sink flange)
- Drain body
- Rubber gasket or washer, and
- Slip nuts to the P-trap and drain pipe
Around the flange, plumber’s putty forms a watertight seal. Hot and cold water supply lines run from shut-off valves to the tap and are tightened with small compression or retaining nuts. The pivot rod operates the plug.
When water flows from leaking taps, any loose connections, cracked putty, or faulty components can cause your bathroom sink to leak underneath. Therefore, it’s important to check each joint for moisture or stains.
Drain-side: Sink Drain Leaks and Failing Seals
Finding your bathroom sink leaking underneath? The issue likely starts where the sink meets the drain.
The drain flange (sink flange) compresses a rubber gasket and plumber’s putty against the sink. You’ll soon get drips if the putty is cracked, the gasket perishes, or the flange is misaligned.
Loose slip nuts on the drain body, P-trap or drain pipe will weep, especially if over-tightening distorts the washers. Typical clues are damp only when the plug’s in and the odd whiff of sewer gas.
The fix: clean and re-bed the flange with fresh putty. Replace any worn-out washers, then tighten by hand plus a gentle quarter-turn with an adjustable spanner for a watertight seal.
Supply-side: Water Supply Lines and Valves
A steady drip from the water supply lines is a common cause of a bathroom sink leaking underneath.
Check the flexi hoses from the shut-off valves to the tap tails: look for corrosion on braided sleeves, damp ferrules or a loose connection at the compression nuts. A leaking valve spindle (packing) can track water down the pipe and mimic leaking pipes at the base.
Test cold water and hot water separately, but make sure to wipe joints with paper towels to spot fresh moisture. Gently hand-tighten compression or retaining nuts, then use an adjustable spanner to apply a small turn (make sure you avoid over-tightening).
Use plumber’s tape only on threaded male fittings, not compression seats. Replace cracked hoses or faulty valves promptly to prevent further damage, or call a plumber.
How a Plumber Will Repair Common Leak Points
If you’ve found your bathroom sink leaking underneath, you will need a licensed plumber on board. Here’s how they’ll address the problem. They will:
- Lay paper towels on the base of the cabinet. Run the cold water tap, followed by the hot, and note where drips appear.
- Close the shut-off valves to isolate the tap.
- Hand-tighten slip nuts and retaining nuts on the P-trap, drain body and sink drain; add a gentle quarter-turn with an adjustable spanner (do not over-tighten).
- Loosen the trap, clean the joints, check the rubber gasket/washer and reassemble for a watertight seal.
- If allowed by the manufacturer’s instructions, snug the drain flange locknut or refresh the plumber’s putty.
- Use plumber’s tape on threaded tailpieces only.
- Reopen valves and test.
If you’re thinking, “I’m pretty handy with the DIY; I can fix my bathroom sink leaking underneath myself!”, know that it’s illegal for an unlicensed person to carry out any plumbing work in South Australia.
Calling for professional assistance to fix a leaking tap is the best way to avoid further damage and any associated plumbing problems.
Faults, Risks and When to Call a Pro
If a leaking tap dripping once per second can waste more than 12000L of water per year, imagine how much water you could lose from a bathroom sink leaking underneath. Persistent drips usually suggest there is a faulty part or damage you can’t see.
Hairline cracks in the drain pipe or P-trap, a split in the flexi supply lines, or a warped drain body can all defeat a seal and lead to your bathroom sink leaking underneath. You might also notice a sulphur-like whiff of sewer gas, swollen cabinetry or spreading water stains. These are all early signs of water damage and expensive repairs ahead.
To keep further damage to a minimum, turn the shut-off valves to isolate the tap, don’t use the sink and place a bucket underneath it. Then call a licensed plumber, as this is not a DIY job. They can assess related plumbing in your home’s plumbing system, replace worn-out parts and provide you with a reliable, watertight repair.
Seal the Deal: Keep Your Vanity Dry
From quick checks to careful resealing, small steps go a long way to prevent leaks.
Keep an eye on the drain assembly and give the P-trap and drain pipe a periodic once-over. Replace tired rubber gaskets, refresh plumber’s putty when it cracks and tighten retaining nuts gently for a watertight seal.
Inspect water supply lines and shut off valves for weeping and clear a clogged drain before water forces past worn parts. Build preventative maintenance into your regular routine.
If a bathroom sink leaking underneath keeps coming back, isolate the supply and call a professional plumber to prevent expensive repairs later.
Please note: Thank you for reading our blog “Why is My Bathroom Sink Leaking Underneath?” This information is provided for advice purposes only. Regulations differ from state to state, so please consult your local authorities or an industry professional before proceeding with any work. See our Terms & Conditions here.