Why Is My Grohe Bathroom Faucet Leaking Under the Sink, and How Do I Fix It?

If you’ve got a Grohe bathroom faucet leaking under sink, take a breath — this is one of the most fixable plumbing problems you’ll ever meet, and you almost certainly don’t need a plumber or a new faucet. Grohe builds genuinely good hardware (their ceramic cartridges are some of the best in the business), so when water shows up in the cabinet, the culprit is usually a connection point *around* the faucet, not the faucet itself. Below, we’ll walk through exactly how to find the leak, what’s most likely causing it, and how to fix each one — in plain language, with real numbers.

Where exactly is the water coming from — the faucet or the connections?

Start here, because the fix depends entirely on the source. Under a sink there are only about five places water can escape, and each one has a distinct fingerprint. Dry every surface completely with a paper towel, then turn the water back on and watch for 60 seconds — the first spot to get wet is your leak.

Here’s how to read what you see:

• Drip at the top, where the faucet tailpiece meets the supply hose: a loose supply-line nut or a dead rubber washer. Most common by far.
• Water tracking down the faucet’s threaded shank from above: the faucet’s base gasket or the deck (top-mount) seal has failed, and water from the sink surface is running down inside.
• Drip at the angle valve (the small shut-off knobs on the wall pipes): a worn valve packing nut or compression fitting.
• Water only when the drain is used, pooling near the P-trap: that’s a drain leak, not a faucet leak — a different (also easy) fix.
• Slow weep from the flexible hose itself, mid-length: a split or corroded braided supply line that needs replacing.

One trick that saves a lot of guessing: wrap a dry paper towel around each joint and leave it a few minutes. The towel that darkens tells you the exact fitting. A leak that shows up only when the faucet is running points to a *pressurized* connection (supply line, cartridge). A leak that’s present even with the faucet off but the valves open points to the angle valves or the supply hose.

Why is my Grohe faucet leaking from the supply line under the sink?

Because the supply-line connection is the single most common failure point, and it’s usually just a loose nut or a tired washer. Grohe bathroom faucets typically ship with either integrated flexible copper/PEX tails or accept standard 3/8″ braided stainless supply hoses. Over months of tiny temperature swings and vibration, those threaded connections back off a hair, or the rubber washer inside the hose nut hardens and stops sealing.

The fix, in order of what to try first:

1. Hand-tighten, then a quarter turn. With the water still on, snug the connection nut by hand, then give it no more than a quarter turn with a basin wrench or adjustable wrench. Overtightening is how people *crack* fittings — stop the moment the drip stops.
2. Shut off, disconnect, inspect the washer. Close both angle valves, open the faucet to release pressure, then unthread the supply nut. Look at the small rubber washer inside — if it’s flattened, cracked, or brittle, replace it (a pack of assorted supply washers is about $4).
3. Replace the whole supply hose. If the braided line shows green corrosion, rust bleed, or a bulge, don’t nurse it — swap it for a new braided stainless hose ($6–$12). They’re rated for years, but they don’t last forever, and a burst supply line is a genuine flood risk.

A quick note on thread sealing: the compression washer or the hose’s built-in rubber gasket does the sealing on these connections — you generally do not need Teflon tape on a supply-hose-to-faucet joint. Save the PTFE tape for tapered pipe threads (like the angle valve inlet), never for the flat-washer faucet connection.

What if the leak is coming from the base of the faucet, not the pipes?

If water is running down the faucet’s central shank and dripping off the mounting nut, the leak is coming from *above* — water is getting past the faucet’s deck seal on top of the sink and traveling down inside the cabinet. This fools a lot of people into thinking they have a supply leak when the real problem is on the counter.

This happens for two reasons:

• The base gasket or plumber’s putty under the faucet has failed, so splash water from the sink deck seeps under the escutcheon and down the shank.
• The faucet has gone loose — the mounting nut underneath backed off, the faucet rocks slightly, and that movement breaks the seal.

To confirm: dry the shank, then splash a cup of water around the base of the faucet on top of the sink (without running the faucet). If the underside gets wet, it’s a deck-seal problem, not plumbing. The fix is to tighten the mounting nut from below — and if the faucet is wobbly, that’s worth fixing on its own, because a loose faucet keeps re-breaking the seal. We’ve got a full walkthrough on how to tighten a wobbly faucet step by step that covers the mounting hardware in detail. If tightening doesn’t do it, pull the faucet, clean off the old gasket, and reseat it with fresh silicone or a new rubber base gasket.

How do I tell the difference between a Grohe leak and a Glacier Bay or generic faucet leak?

Honestly, the diagnosis is nearly identical across brands — the leak points under any sink are the same — but Grohe has a couple of quirks worth knowing. Grohe uses high-quality ceramic disc cartridges and often proprietary cartridge designs, so an *internal* leak (water appearing only when the handle is in a certain position) may require a Grohe-specific replacement cartridge rather than a universal one. For the under-sink connection leaks we’re focused on here, though, the parts are standard.

The troubleshooting logic carries straight over from other brands — if you want to see the same method applied to a different faucet, our guide on the leaking Glacier Bay bathroom faucet follows the exact same shut-off-dry-and-watch approach, and the same is true for the broader how to fix a dripping lever tap walkthrough. Here’s a side-by-side of the most common under-sink leak sources so you can match your symptom to the fix:

How do I actually fix a leaking Grohe supply connection, step by step?

Here’s the full repair for the most common case — a leak at the supply-line connection under the sink. Budget 20–30 minutes and about $10 in parts, worst case. You’ll need an adjustable wrench or basin wrench, a towel, a small bucket or bowl, and possibly a new braided supply hose and washers.

1. Turn off the water. Reach under the sink and close both angle valves (righty-tighty) — the hot on the left, cold on the right. If there are no under-sink valves, shut off the main.
2. Relieve pressure. Open the faucet handle fully until water stops running. This drains the lines so you’re not sprayed when you disconnect.
3. Put a bowl under the connection to catch the cup or so of water still sitting in the hose.
4. Disconnect the supply hose from the faucet tailpiece. Turn the nut counterclockwise. Hold the faucet tailpiece steady with your other hand or a second wrench so you don’t twist the whole faucet.
5. Inspect the washer and threads. Pull out the rubber washer. If it’s cracked, flattened, or brittle, replace it. Wipe the threads clean of any mineral crust or old sealant.
6. Check the hose. Any corrosion, kinking, or a bulge in the braid? Replace the whole hose — don’t reuse a compromised line.
7. Reconnect. Thread the nut back on by hand first (this prevents cross-threading, the #1 cause of new leaks), then snug it with the wrench — firm, plus a quarter turn. Don’t gorilla it.
8. Turn the water back on slowly and watch the joint for two full minutes with a dry paper towel wrapped around it. Dry towel = you’re done.

If you’ve retightened and replaced the washer and it still weeps, the tailpiece threads themselves may be damaged, or the leak is actually migrating down from the base — go back to the base-seal check above. And if the drip only appears when you move the handle, you’re looking at an internal cartridge issue rather than a connection issue; that’s a different repair, and it’s worth reading why your faucet stops working suddenly and how to fix it to understand how the cartridge behaves before you open it up.

Can hard water or old washers cause a Grohe faucet to leak under the sink?

Yes — hard water is a quiet, long-term cause of under-sink leaks, and it works by wrecking your seals. Water with high mineral content (calcium and magnesium) leaves scale on rubber washers and inside compression fittings. Over a year or two, that scale builds up, the rubber loses its flexibility, and a joint that used to seal perfectly starts to weep. If your area has hard water — you’ll see white crust on the aerator and spotting on glass — expect supply washers and cartridge seals to wear faster, maybe every 3–5 years instead of 8–10.

A few things that genuinely help:

• Replace washers proactively. If you’ve got the connection open anyway, just swap in a fresh washer — they cost pennies and save you a repeat visit.
• Don’t overtighten to compensate for scale. Cranking a fitting harder to stop a weep just crushes the washer and damages threads. Clean the scale instead.
• Descale the connections. A little white vinegar on a rag clears mineral buildup from threads and ferrules so a new washer can seat properly.
• Watch the flexible hoses. Hard water accelerates internal corrosion in braided lines; if yours are more than 8 years old and you’re in a hard-water area, replacing them proactively is cheap insurance against a burst.

Because water chemistry and pressure both affect how your fixtures age, it’s worth understanding how water pressure affects faucet flow — abnormally high supply pressure (above ~80 psi) stresses every seal in the system and is a frequent hidden reason connections start leaking in the first place. If your whole house sees leaks and drips across multiple fixtures, get your static pressure checked; a $10 gauge on a hose bib tells you in seconds.

When should I just call a plumber instead of fixing it myself?

Fix it yourself if the leak is at a supply hose, a washer, an angle valve, or the faucet base — those are all beginner-friendly with hand tools. Call a pro if you hit any of these: the angle valve won’t close or spins freely (a failed shut-off is its own repair and often means shutting the house main), the leak is inside the wall or you see water staining drywall, the faucet tailpiece threads are stripped, or you’ve replaced parts twice and it still leaks. There’s no shame in it — a stuck valve or corroded galvanized pipe can turn a five-minute job into an afternoon, and water damage in a cabinet gets expensive fast.

One more judgment call: if your Grohe faucet is more than 12–15 years old, leaking in multiple places, and you’re pricing out a cartridge plus hoses plus washers, it can make more sense to replace the faucet outright than to keep patching it. Grohe faucets are worth keeping when the body is sound, but there’s a point where a fresh install is the better value.

FAQ

Why is my Grohe bathroom faucet only leaking under the sink when the water is running?

A leak that appears only under pressure — when the faucet is on — points to a pressurized connection: the supply hose nut, the faucet tailpiece, or an internal cartridge. Start by drying and watching the supply-line connections while the water runs. If it stays dry there but drips only when you move the handle, the cartridge is the likely cause.

Do I need special Grohe parts to fix an under-sink leak?

Usually no. Supply hoses, rubber washers, and angle valves are all industry-standard 3/8″ parts you can buy anywhere. The one exception is the internal cartridge — Grohe uses proprietary ceramic cartridges, so if the leak is internal (handle-position dependent), you’ll want a genuine Grohe replacement matched to your model number, which is often printed under the handle or in your original paperwork.

How much does it cost to fix a Grohe faucet leaking under the sink?

If it’s a connection leak, expect $0–$12 in parts — a new braided supply hose runs $6–$12 and washers are a few dollars. A replacement Grohe cartridge is typically $20–$45. A plumber’s visit for the same job usually runs $100–$250 depending on your area, which is why the DIY route makes sense for these simple fixes.

Can I use plumber’s tape (PTFE) on the supply hose connection?

No — the supply hose seals with a flat rubber washer, and adding tape there can actually interfere with the seal. Reserve PTFE tape for tapered threaded joints like the angle valve inlet. If a washer-sealed connection leaks, the answer is a fresh washer or a slight retighten, not tape.

My Grohe faucet is loose and leaking at the base — are those related?

Very often, yes. A loose faucet rocks with use, and that movement breaks the deck seal, letting water seep down the shank into the cabinet. Tightening the mounting nut underneath usually fixes both problems at once. If the wobble persists, reseat the faucet with a fresh base gasket or silicone so the seal can’t keep breaking.

How do I know if it’s a faucet leak or actually a drain leak under my sink?

Run the faucet with the drain stopper closed and the sink filling — if you get water under the cabinet, it’s a supply/faucet leak. Then let the sink drain and watch the P-trap — if water only appears while draining, it’s a drain-side leak (a loose slip nut or worn trap washer), which is a separate, equally easy fix.

A quick word on doing this right

The reason under-sink leaks feel scary is that they’re hidden — you don’t see them until there’s a puddle or a musty smell. But mechanically, they’re simple: water escapes at a joint, and joints are made to come apart and reseal. Work methodically, don’t overtighten, and replace cheap rubber parts rather than nursing them, and you’ll fix the vast majority of Grohe under-sink leaks in under half an hour.

Author note: This guide was written by the KES Faucet product and support team, drawing on thousands of real customer repair questions about leaking bathroom and kitchen faucets. KES designs and manufactures faucets, shower systems, and bathroom fixtures sold worldwide, and our fixtures are built and tested to standard plumbing certifications (including cUPC/NSF-type performance and pressure testing) so seals and cartridges hold up over years of daily use. We stand behind our products with manufacturer warranty coverage — and when we recommend a repair, it’s the same advice we’d give a customer calling our own support line. Always match replacement cartridges and parts to your specific faucet model, and when in doubt about your water shut-off, consult a licensed plumber.