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Question DetailsAsked on 6/11/2013

House is 10 years and on concrete slab. How does one determine where the drain leak is under concrete slab?

The smell of mold is in one of rooms in home. It is not water coming into home, but seems like a drain that is leaking. Some days are worse than others, while some there is no smell.

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OK - troubleshooting time, based on what I have seen as causes. Check for following, in approximate order of ease of checking out and likelihood:

1) teenagers. I have had TWO neighbors looking for water/sewer leaks because of teenagers and wet towels. In one case, their teenage daughter was throwing wet bath and hair drying towels in the corner of her basement bedroom, which led him to needlessly dig up his sewer pipe at the foundation exit point, thinking it was leaking there. He got the bonus of doing it twice, because the backfilling caused an actual break a few feet away, whcich then caused an actual backup ! In another case, the kids were tossing soaking wet clothes, swim trunks and towels in the corner of the laundry room after using the swimming pool and having water fights - in that case inch by inch inspection of the downstairs discovered that the water from these had seeped under the wall and rotted the carpet in the adjacent room and the base of the wall.

2) floor drain that has gone dry - takes from a few months to years to evaporate, depending on humidity. Unused floor drains evaporate the water in the trap (which can also go stagnant), which is what keeps sewer gases from coming up into the house. Floor drain will be in a slightly depressed area of the basement floor, with a removeable metal cast iron usually) grate over it. Usually one in downstairs bath or closet, and also in garage floor comonly. Pour a gallon or so of hot water with a bit of liquid dish soap frothed up in it - preferably scented type. A portion (a cup or so) stays in the trap in the drain as a seal against sewer gases. Do this at least yearly, more often if problem is recurrent.

3) Unused shower, tub, toilet, sink, washtub, etc - solution ditto to above. Also, if toilet is only rarely used, water in toilet tank may have gone stagnant. Make a point to run water in all drains and flush toilet at least monthly.

4) drain for washing machine getting plugged with lint, causing drain overflow behind washing machine when it is pumping out

5) overflowing dehumidifier, either stand-alone or as part of the HVAC system. I have several times seen homeowners who did not know the water tray needs frequent emptying if it does not have a drain pipe connected to it. Also, the drain pan can start to stink if not cleaned with soap and bleach at least monthly.

6) outdoor watering hitting house wall, causing leak, or ponding along foundation causing water infiltration through the foundation or window wells

7) stagnant water in outdoor window wells

8) sump pump sump - if you have no inflow to the sump during certain seasons, the water in the sump can go stagnant. Carefully, so as to not get any on the electrical switch or motor, you need to gradually pour in about 5 gallons of water (NO soap) so the pump runs and pumps out the old water. If it has actually gone scuzzy, skim that off FIRST before running pump.

9) inadequate ventilation in room - will make for musty smell, eventually turning to mold on the walls. Most common in hot humid weather when humid outside air comes in and contacts cold pipes and cold concrete or block foundation walls, condensing as droplets of water. If exposed cold water pipes have condensation on them, or concrete or cinder block walls or steel items feel at all damp, then you need to improve room ventilation.

10) leaking sewer pipe cleanout - check walls for a sewer pipe cleanout (usually cast iron or black plastic, about 4-6 inches in diameter, with a square or hex head or recess for a wrench to fit on/in) sticking out of the wall, usually pretty much directly under bathrooms, and also commonly at one end of exposed sewer pipes in basement ceiling. The seal on the threads can go bad over time, so you could be getting very minor leakage.

11) leaks at or near furnace or hot water heater, leaking under wall into room with problem - look for leaks on all exposed pipes in the house, including garage and under sinks, dishwasher, and clothes washer

12) mildewy carpet - smell the carpet. Moisture from the ground migrates up through the concrete slab, so if carpet was put down without a vapor barrier, it can mold the carpet, especially in summertime. If you smell mold or mildew in the carpet, peell back the worst smelling edge and look for mold on the bottom or on the concrete, and smell the back of the carpet. If smelly, peel up carpet and look to see if water or lime staining on the concrete - if yes, then groundwater comingn in. If no significant staining, then probably just ground moisture evaporating through the concrete, so you need a vapor barrier and new carpet.

13) sewer under the slab or outside starting to partially back up under high-flow conditions, causing backup into floor drains (or if none, lowest elevation sink, tub, shower, or toilet). Starts off with gurgling sound as air is forced out of the drain as the sewer pipe backs up with water when washing machine is pumping out, then when shower or tub is draining, eventually with flushing and other water use as the clog gets worse. Usually it will start coming up out of the lowest drain or sink during washing machine pumpout or full bathtub emptying before showering or toilet flushing affects it much.

14) leaking water or sewer pipe in wall - check for mold on the drywall, especially at the top and bottom. Get your nose up close and personal with the wall surfaces that have pipes in them and smell for mold or mildewy smell. If moldy at top of wall then is probably a leak from upstairs pipes, or rarely a roof leak seeping down. If at bottom only, probably leaking in that wall. A water leak, unless REALLY small like just a very slow drip, you can usually detect by getting a metal-headed (not cheap plastic) stethoscope at your local pharmacy (about $10) and listening to pipes when ALL water flow is off (including furnace) and hot water heater is NOT heating. Start with furnace and hot water heater so you know what the sound of the pilot light is like, then check basement pipes for water flow sound in the pipes, then check walls and ceilings where pipes are running near the smelly room for a slight hissing sound in the wall where the leak is. Of course, this works only for water leaks - very slow drips,, and sewer leaks make no noise - you have to tear into drywall to track them down.

15) leaking sewer pipe under slab - may cause above symptoms in 12) above, but usually goes down not up. Hard to diagnose except that smell will be of raw sewage or hydrogen sulfide gas smell. A cleaning out with a scraper rooter (not just a blockage-clearing auger) followed by a COLOR camera run by a sewer and drain contractor will find any significant breakage or joint offset, but a small joint leak or very minor crack will not show up that way.

Answered 5 years ago by LCD


Once you locate and solve the waer source for your mold problem, here is a link to one of the most popular questions on Angie's List and a bunch of answers on how to take care of basement mold.

Answered 5 years ago by LCD

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