If the floor around the toilet is damp, or the cabinet below the tank smells musty, the supply line is one of the first places to check. In St. Louis homes, toilet supply lines are often original to the bathroom or were replaced years ago, and the connections at each end are the parts most likely to weep. This guide walks through a simple check you can do with a flashlight, a dry towel, and a few minutes.

What the supply line does

The supply line is the flexible hose that carries water from the shutoff valve on the wall or floor up to the fill valve inside the tank. It has a nut at each end. The nut at the wall valve and the nut at the tank are both places where water can escape if the seal has worn or the nut has loosened.

Dry the area first

Before you look for a leak, dry everything. Wipe the hose, the nuts, the shutoff valve, and the floor. Wrap a dry paper towel or a folded cloth around each connection. Wait a while and check again. A leak shows up as a damp spot on the towel or a line of water running down the hose. Condensation on a cold tank or a cold pipe can look similar, so the dry-towel test matters.

Check the wall connection

  • Look at the nut where the supply line meets the shutoff valve.
  • Look for crust, white or green residue, or a dark ring on the threads.
  • Run a dry tissue around the nut and see whether it picks up moisture.

Mineral crust is common in areas with hard water, and St. Louis homes are no exception. Crust around a fitting usually means water has been seeping there for some time, even if the drip is slow.

Check the tank connection

The other end of the line connects to the fill valve at the bottom of the tank. Reach behind the tank and feel the nut. Look under the tank for drips. Water that runs down the outside of the tank can be mistaken for a leak at the supply line, so watch where the moisture starts, not just where it pools.

Inspect the hose itself

Look along the full length of the line. Check for bulges, cracks, kinks, and fraying on the outer braid if the line is a braided type. A bulge means the inner hose is failing under pressure. A kink can restrict flow and make the tank fill slowly. Either problem is a reason to replace the line rather than tighten it.

Tightening a loose nut

If a nut is loose and the seal looks intact, a gentle snug turn with a wrench may stop the drip. Do not force it. Overtightening can crack a plastic nut or damage the washer inside. If tightening does not stop the leak, the washer is likely worn, and the connection needs a new seal or a new line.

Shut off water before any work

Close the shutoff valve at the wall or floor before you loosen anything. Flush the toilet to empty the tank, then place a towel under the connection. Confirm the valve is fully closed before you open the nut. If the valve does not turn freely or does not stop the flow, do not force it, because an old valve can fail when turned hard. Plan to replace it separately.

When to call for help

Call a licensed plumber if the shutoff valve drips when closed, if the floor under the toilet feels soft, if you see stains on the ceiling below, or if the wall valve itself is corroded. A supply line replacement is a small job for many homeowners, but a failing valve or subfloor damage needs a professional's eye.

Keep a simple record

Note the date you checked the line, what you found, and whether you tightened anything. A line that has leaked before is more likely to leak again, and a written note helps you decide when to replace it instead of waiting for a stain to appear.