Under most kitchen sinks in St. Louis, the cabinet floor is the part of the house nobody checks until something smells wrong. A reverse osmosis unit adds fittings, a storage tank, a drain line, and tubing to that already crowded space. Each of those parts is a place where a slow drip can start, and a slow drip can soak a cabinet base well before it shows up on the floor.
Why the cabinet floor matters
Particleboard and many cabinet bases swell when they stay damp. Once the edges soften, screws lose their grip, the bottom can sag, and the cabinet may start to pull away from the wall. Damp dark corners can also grow mold, which is one more reason to catch moisture early. Summer humidity in St. Louis adds another complication. Cold supply lines can sweat, and that condensation can look like a leak even when nothing is actually failing.
A quick check you can do in one visit
- Empty the cabinet under the sink, and set aside anything stored on the floor.
- Use a flashlight to look at the back wall, the floor corners, and the area around every pipe entry point.
- Press gently on the cabinet floor near the front edge and near the back. Soft or spongy spots are a warning sign.
- Look for water stains, white or dark rings, and flaking paint or laminate on the cabinet sides and bottom.
- Smell the cabinet. A musty or sour odor often means moisture has been sitting for a while.
- Dry the area with a clean towel, then leave the doors open so air can move through.
Telling condensation from a leak
Condensation and leaks can look alike, so it helps to test rather than guess. Dry a section of each supply line and fitting with a clean paper towel, then wrap a fresh dry towel around the same spots. Check the towels after several hours. Condensation usually forms on the outside of cold lines and beads up in a band, and it often returns after the house warms up. A leak tends to show a wet spot on a specific joint, a drip from a connection, or moisture that keeps returning even after the line is dry.
Common trouble spots
- Feed water shutoff valve, where the cold line meets the unit.
- Storage tank valve and its connection, especially where the tubing bends near the cabinet wall.
- Drain saddle or drain fitting, where the wastewater line meets the sink drain.
- Tubing elbows and compression fittings, which can loosen as the tubing shifts over time.
- Faucet base nut and the faucet supply lines underneath the sink.
- Housing mounting bolts, if the bracket or the filter housing has loosened.
What to do when you find moisture
If a fitting is wet, shut off the feed water first, then dry the area completely before retightening anything. Tighten compression nuts gently, since overtightening can crack a fitting. If tubing has a cut, kink, or brittle section, replace that section rather than patching it. If the cabinet floor has softened or stained, the panel usually needs replacement, and the damp wood should not be left in place to dry out on its own. For swollen floors, a licensed plumber or a qualified contractor in the St. Louis area can judge whether the subfloor underneath has been affected.
Keeping the space dry over time
A few habits go a long way. Keep a shallow tray or absorbent pad under the storage tank so a small drip is visible before it spreads. Leave some air gap between stored items and the cabinet floor so moisture can dry out. Store cleaning supplies in sealed containers, and wipe up any spills right away. After heavy rain, a power outage, or a period of high humidity in St. Louis, repeat the quick check described above. Make it part of your regular routine, the same way you would check a water heater or a dishwasher hose.
A short, consistent look under the sink is often enough to catch a problem while it is still small. If the cabinet ever feels soft, smells sour, or shows fresh staining, treat it as a signal to investigate the connections before the damage spreads to the rest of the kitchen.