How to Prevent Water Damage by Checking Under Sink Supply Lines

When we think about home maintenance, our minds often jump to the big, expensive projects like replacing a roof or installing a new water heater. However, some of the most critical work happens quietly inside your cabinets.

Looking for more guidance? Take a look at our Maintenance overview.

The area under your kitchen and bathroom sinks is easy to ignore. You likely store cleaning supplies or toiletries there and rarely look at the plumbing itself. But the supply lines—the flexible tubes connecting the water source to your faucet—are a common point of failure.

This isn't about becoming a plumber. It is about a five-minute inspection that saves you from the stress and expense of warped cabinets, ruined flooring, and mold remediation.

Why This Task Matters

Your home's plumbing system is under constant pressure. While copper or PEX pipes behind your walls are sturdy, the connections under your sink are more vulnerable. Supply lines are often made of braided stainless steel or polymer, and they have a finite lifespan.

If you ignore these connections, you risk a slow, silent leak. These leaks often start as a tiny drip that goes unnoticed for weeks or months. By the time you see water pooling on the floor, the bottom of your cabinet may already be rotted out, or mold may have taken hold in the drywall behind it.

What this inspection protects:

Cabinetry: Prevents particle board and wood from swelling and warping.
Flooring: Protects subfloors and finish materials (tile, hardwood, vinyl) from water damage.
Indoor Air Quality: Stops mold growth before it begins.

Step-by-Step Inspection Checklist

You do not need tools for this inspection, just a flashlight and a paper towel.

Clear the area: Remove all bottles, sponges, and boxes from under the sink. This is a good time to toss expired products, but the main goal is to get a clear view of the plumbing.
Identify the parts: Look for the two flexible tubes (hot and cold) connecting the valves coming out of the wall to the bottom of the faucet.
The Paper Towel Test: Take a dry paper towel and wipe it along the length of each supply line, paying close attention to the connections at the top (faucet) and bottom (valve).
Inspect for corrosion: Shine your flashlight on the metal nuts and valves. Look for white, crusty buildup (mineral deposits) or green discoloration (corrosion). This often indicates a very slow "weeping" leak.
Check the valves: Gently turn the shut-off valves (the oval or round handles) slightly to ensure they aren't seized. If they are stuck, do not force them. Just note it for future reference.
Feel the cabinet floor: Press on the bottom of the cabinet. It should feel firm. If it feels spongy or shows water stains, you may have had a past leak or have a current one.

How Often Should You Check?

We recommend performing this inspection every six months.

A good way to remember this is to pair it with another recurring task, such as changing your HVAC filter or checking your smoke detector batteries. If you live in an older home with original plumbing fixtures, you might consider checking quarterly.

Signs You've Waited Too Long

If you spot these issues, the problem has likely moved past "prevention" and into "repair":

Pooled water: Standing water in the cabinet means a leak is active and significant.
Musty smell: A persistent damp odor usually indicates mold or mildew is already growing.
Warped cabinet bottom: If the shelf under the sink bows downward or the laminate is peeling, it has absorbed moisture over a long period.
Stuck valves: If you cannot turn the water off at the sink during an emergency, you will have to shut off the main water to the house.

The Cost of Neglect vs. Maintenance

The financial difference between proactive checking and reactive repair is substantial.

Cost of Maintenance: $0 for the inspection. If you find a worn supply line early, a replacement hose typically costs between $10 and $20 at a hardware store.
Cost of Neglect: If a supply line bursts or leaks unnoticed, costs escalate quickly. Replacing a rotted vanity cabinet can cost $500 to $2,000. If water damages the subfloor or ceiling below, repairs can easily exceed $5,000, not including the cost of drying out the home and treating for mold.

Safety Boundaries: When to Call a Pro

This inspection is safe for any homeowner to perform. However, knowing your limits is part of being a responsible homeowner.

Safe to try yourself:

Tightening a slightly loose nut with a wrench (give it a quarter turn).
Replacing a standard braided supply line if you are comfortable using basic tools and the valve shuts off completely.

Time to call a professional:

The shut-off valve won't close: If the valve is seized or spins without stopping the water, you need a plumber to replace the valve. Do not attempt to unscrew a supply line if the water is not fully off.
Corroded pipes coming from the wall: If the pipe entering the cabinet from the wall looks heavily rusted or fragile, do not wrench on it. You risk breaking the pipe inside the wall.

How to Build This Into Your Home Maintenance Schedule

Routine is the enemy of anxiety. When you know you have a schedule, you don't have to worry about what you're forgetting.

We suggest creating a "Wet Room Walkthrough" day twice a year. On this day, walk through every room with water access—kitchen, bathrooms, laundry room—and perform this visual check.

If you use Casa to organize your home care, you can log the age of your supply lines. Most manufacturers suggest replacing flexible supply lines every 5 to 10 years, even if they aren't leaking yet. Tracking this installation date helps you replace them on your terms, rather than waiting for a failure.