I just moved into my house 6 months ago and all was well, but about two Weeks ago I noticed a random wet spot at the bottom of my stairs about three feet from the end of the kitchen. I thought nothing of it until it stayed wet all day and was gone the next. Since then it has happened two more times. Always a 10 inch wet spot, always the same area.I live in a two story house with finished basement. It's wood frame on slab. Only 12 years old. The ceiling above is always dry and the ceiling in the basement below is as well. There is a vent only feet from the spots but I'm not sure how to tell if it's to do with the a/c or not. It would appear as though it runs in a linear like direction from the kitchen sink to the main coming in from the house and also the drainage. Is there a way to figure out just what is causing these spots without tearing my floor/basement ceiling apart? It is winter so the a/c isn't on and this wasn't a problem before now...

What does the moisture smell like? Does it have a "sewer" smell. Can you post pics of the spot and the nearby "vent"?

Unfortunately I don't know how to post pictures on this site. It's a vent near the floor, approx 3' long 8" tall. The moisture has no noticeable sewer order. It only smells like wet carpet and is clear in color

What you probably have is a clean water,(Sink drain), leak, or a Supply Line is cracked.
So the best thing you can do is to DRY the carpet (an air mover works best for this). Once carpet is dry, try using only the kitchen sink until you begin spotting these wet spots. MY guess is its either the main drain from the kitchen or the supply lines to the kitchen faucet may need replaced.

What you probably have is a clean water,(Sink drain), leak, or a Supply Line is cracked.
So the best thing you can do is to DRY the carpet (an air mover works best for this). Once carpet is dry, try using only the kitchen sink until you begin spotting these wet spots. MY guess is its either the main drain from the kitchen or the supply lines to the kitchen faucet may need replaced.


I will try this once the carpet has dried. Perhaps a trial and error with all the water sources in the house. The only doubts I have about this is that both times I've noticed it, it was early morning when no water had been running over night

That may be a needed tip. From what you have said I can guess I few things. One guess is a brace may be failing. (most likely a 3" brace around the vent). This will be located somewhere off the edge of your sink under the floor.
IS YOUR BASEMENT FINISHED OR CAN YOU SEE THE RAFTERS AND PIPING?

both times I've noticed it, it was early morning when no water had been running over night

Do you by any chance have a high efficiency furnace in the attic?

That may be a needed tip. From what you have said I can guess i few things. One guess is a brace may be failing. (most likely a 3" brace around the vent). This will be located somewhere off the edge of your sink under the floor.
IS YOUR BASEMENT FINISHED OR CAN YOU SEE THE RAFTERS AND PIPING?


It's a finished basement. There's a crawl space, but it ends about 6 feet from the spot. There isn't any piping in the crawl space leading to the spot though

OK well only things I can see being the probable cause would be either from the attic, (maybe a furnace or high efficiency furnace is the attic), or try ripping floor where the spot is located. Please let me know what the conclusion ends up being! Have a good day!