concrete slab has a single damp location. plumbers are stumped.
around last november i removed ceramic tile in my living room. it was installed pretty poorly, which resulted in air gaps between the tile and slab. made removal a breeze, but i got to one spot and there was water pooled up in that air space. not just some drops of water..
at this point i’ve removed 1500 sqft of tile and theres no other damp spots.
taped some plastic down in that area and sure enough, had condensation build up.
under the house is just sand, no vapor barrier.
picture was taken after removing plastic.
this week ive had plumbers out for air tests with microphones, checked the slab temp, meter, etc. was noted that my slab was unusually cold to them, around 61f throughout the house. i keep the thermostat set to 68 and theres just 2 minisplits in bedrooms, none in the living room/kitchen. according to them every slab they look at is around 70-72.
okay, maybe i don’t have a water leak. but they insisted a cracked drain line would not cause water to come up but i wasn’t sure of that. i’ve tried digging some holes around my house and after about 18-24″ theres solid red clay that doesn’t drain water. no clue how much sand/backfill is under my house.
if water can’t drain downward, then it would just saturate the sand and eventually go up, right?
their suggestion was to do a small 6-8″ square cut out, or something big enough to see under and to start digging.
i decided to drill some 1/2″ holes in the slab for an endoscope camera. only thing noticeable was a bit of settling, slab aint sitting on much of anything. used a longer 10″x1/4″ screw to ‘core’ out some sand to feel how damp it was. pretty damp.
did the same in other parts of the home and.. pretty dry.
if my drain line isn’t leaking and neither is my fresh water.. any rhyme or reason for this damp spot?
GBA Detail Library
A collection of one thousand construction details organized by climate and house part
Replies
cs55,
To prove which, if any, pressure line is leaking you will need to fit a water pressure gauge to all the water taps. It might be easiest to get a new short hose that is 3/8 compression to 1/2" ips and find reducers to suit. Attach the water pressure gauge to the hose, hose to cold then hot shutoffs under the sink and elsewhere. Open shut off and see what the pressure is. Go turn off the house main and wait. One or the other may show a slow drop in pressure. If not move on to any toilet shut offs and repeat the steps.
It does sound like a pinhole leak that has etched the sand aside and is actually squirting up against the bottom of the slab. Given how the sand away from the current wet spot show up as dry it must be a very small leak volumetrically speaking.
If you can get a FLIR camera or know someone with the phone attachment FLIR it may be possible to narrow down the coldest spot on the wet area. (any firemen friends?) Hopefully, this would represent the closest guess to guiding a hole cut into the slab.
Cutting into the floor is pretty scary if you don't know what else they may have embedded into the concrete or just laid onto the sand before pouring. Think hard about what else might be in the area.
Don't forget to test the washer lines and the set tub faucet lines. Aside from the washer hook up lines, a real left field possibility is a top side crack/leak in the drain line coming from the washer discharge hose. The sudden burst of water when the washer is discharging could make a leak strong enough to squirt up to the slab, but only intermittently. If the drain line from the laundry is set high and draining toward a lower central collection point, the leak could be at an elbow that got a short glue line. May also be exacerbated by lint build up at the turn.
A really long shot, but I have seen stupider, is a leaking buried saddle valve on a water line meant to come up to the fridge ice and water dispenser. I would still show as a slow leak on the cold water line either at the sink shutoff or maybe the far bathroom toilet/cold line.
And you thought tiling and grouting an entire living room was going to be your only nightmare...
Clay soil won't drain and will tend to channel water towards a low spot. To me it does sound like a drain or supply leak but could also be ground water accumulating there.
Since it is a small area, I would break the concrete and see which issue it is. Water leaks are easy to fix. Ground water pooling is a much bigger job. In either case, you want it dealt with before you refinish the floor.
62F does sound cold. I have an uninsulated basement that only gets heat for the exposed boiler pipes and slab is typically mid 60s in the winter.