Hidden leaks in finished basements: where to look first

If your basement smells musty this month and didn't in August, take it seriously. With the furnace running and the windows shut, odors that a summer breeze used to clear out now hang in the air, and damp drywall or carpet pad is a common source. In a finished basement, most of the plumbing is behind walls, under carpet or above a ceiling, so a slow drip can run for months before anything shows on the surface.

Here's where we'd look first, room by room, with what to check with your eyes and what to put your hands on.

Start at the water meter

Before you go hunting, find out whether water is moving at all. Turn off every faucet, make sure the washer, dishwasher and ice maker aren't running, and don't flush anything. Then look at your water meter, whether it's in the basement or in a pit near the street. Most meters have a small leak indicator, often a little triangle or dial, that turns when any water passes through. If it moves with everything off, there's a leak on the pressurized side somewhere in the house. For a second check, write down the reading, go an hour or two without using water, and read it again.

If the meter stays still, a musty smell may be coming from a drain line or toilet seal, which only leaks when something is in use, or from water coming in from outside. Keep that in mind as you walk the rooms.

Bring a good flashlight. A moisture meter from a hardware store helps, but the back of your hand works too: damp drywall usually feels cooler than the wall around it.

The utility room

This is usually the one room where the pipes are out in the open, so it's the easiest place to start.

• Main shutoff valve. Look at the packing nut where the handle meets the valve body for mineral crust or a drip. While you're there, make sure you know which way it turns.
• Pressure reducing valve, if you have one near the main. A crusty or wet valve body is worth having looked at.
• Water heater. Check the cold and hot connections on top, the drain valve near the bottom, and the end of the discharge pipe that runs down from the temperature and pressure relief valve. Water at the end of that pipe means the valve is opening or failing. Don't cap or plug it.
• The floor around the tank and any drain pan under it. Rust streaks or a ring of scale mean water has been there before.
• Furnace humidifier supply line. Many humidifiers are fed by a thin tube tapped into a nearby cold water pipe, and those small fittings drip. November is when the humidifier starts working again.
• Furnace condensate line. High-efficiency furnaces produce water and send it through a small plastic tube to a floor drain or pump. A loose or clogged line can wet the floor or the wall beside it.
• Water softener or filter, if you have one: the connections, the bypass valve and the brine tank.

The basement bathroom

Basement bathrooms are often added after the house is built, and the lines feeding them run through finished walls. Start at the fixtures and work outward.

Toilet

Press gently on the floor around the base. Soft flooring, a toilet that rocks, or discoloration at the caulk line can point to a failing wax ring. Feel the supply line and shutoff valve behind the toilet, and run a dry paper towel around the tank bolts under the tank.

Sink vanity

Empty the cabinet and shine a light on its floor. Warping, staining or soft particleboard means a leak has been going for a while. Check both supply lines, the shutoff valves and the drain trap, then run water for a minute and look again.

Shower or tub

The shower valve sits inside the wall, so check the other side of it. If there's an access panel, open it and look with a flashlight while someone runs the shower. If there isn't, feel the drywall on the far side and look at the baseboard there for swelling or bubbled paint. Cracked grout and gaps in the caulk let water into the wall too, especially at the bottom corners.

If the bathroom sits below the main sewer line, it may drain to a sewage ejector pump in a sealed basin, often in a closet or under the stairs. A sewer smell near that basin points to a seal or venting problem rather than a supply leak, but it still needs attention.

The laundry area

A washing machine fills at full house pressure, and its hoses sit behind the machine where nobody looks.

• Pull the washer out far enough to see the hoses. Rubber hoses that are bulging, cracked or stiff should be replaced. Braided stainless hoses hold up better, but the fittings at each end can still drip.
• Look at the valves in the wall box. Crust on the valve stems or a stain at the bottom of the box points to a slow leak, and that box opens straight into the wall cavity.
• Check the drain standpipe. If it ever overflows during the drain cycle, water runs down inside the wall.
• If there's a utility sink, check its supply lines and drain the same way you did the vanity.
• Feel the drywall and carpet next to the laundry area, not only the floor under the machine.

Under the stairs

The space under the basement stairs usually becomes storage, which is exactly why leaks there go unnoticed. It's also a common route for pipes heading to the main floor, and it often sits below an upstairs kitchen or bathroom.

Move the boxes and look at the bottoms of any cardboard. Water rings or soft, wavy cardboard tell you more than the floor might. Check the underside of the stairs and the nearby ceiling for brown stains or sagging. If carpet runs into the space, press on it along the walls where the tack strip is, since a damp pad often shows up there first. If any part of the basement has a drop ceiling, lift a tile or two below the upstairs kitchen and bathrooms and look at the pipes with your flashlight.

Plumbing leak or water from outside?

Not every damp spot is a plumbing problem. Water from outside tends to show up along foundation walls or where the wall meets the slab, usually after rain or snowmelt. A plumbing leak tends to sit near a fixture or along a pipe route and doesn't follow the weather. If the meter test showed movement, or the wet spot is on a bathroom, laundry or utility wall, start with the plumbing.

When to call

If the meter moves and you can't find where, or you've found a wet wall but not the source, that's what our leak detection service is for. The goal is to pin down the source before anyone starts opening finished walls. Fixes like a dripping supply line, a worn washer valve or a toilet that needs to be reset fall under our residential plumbing work. Estimates are free, and the job is quoted before work starts.

If water is actively coming through a wall or ceiling, shut off the main and call us at 720-916-7720. We answer around the clock, including nights, weekends and holidays.