Mystery water in basement - I'm stumped

Short version: Water is appearing from underneath a built-in bookshelf/wall in the basement. Could it be coming from, perhaps, a crack in the floor?

Director’s Cut: Around 15 years ago I built a wall to sub-divide a basement space. Then I made and attached a bookshelf to one side of the wall. The unit is around 10 feet wide and 18 inches deep, floor-to-ceiling.

Yesterday I noticed a small amount of water leaking out from underneath. I figured one of the overhead pipes was dripping. Upon further inspection, I cannot find any dampness from above; it actually seems to be rising up from underneath the bookshelf. It is a significant amount - the drywall on the reverse side is moist for about an inch up from the floor.

It sure looks like I’m going to have to do some demo to find the source of the leak.

My question: is it possible it could be coming from below? What would cause this? I am not sure if I drove fasteners into the concrete floor or not, but if so, could this be something to consider? How thick are basement concrete floors, anyway?

Any thoughts appreciated.

I would assume the minimum thickness of a residential concrete floor to be 4", but it could be more depending upon soil conditions in your area.

Moisture will move through concrete however ponding on a concrete floor would be rare, but not unheard of.

Did you put down PT lumber as the floor sleeper the wall was built on ? If not I would be concerned about that.

Also what is your ventilation ? If you have living space in a basement you need decent ventilation to move out the moisture laden air.

I did not use pressure treated lumber, unfortunately. Ventilation could be better, I have a dehumidifier that I could hook up. But this is too much, and too localized, to be caused just by humiditiy.

I’ve investigated a bit further. I am now convinced the water is coming from above, not below (based on the pattern of wetness). I think it is somehow dripping from a water pipe into the wall and pooling at the bottom.

There are water pipes directly above, but they are bone dry near as I can tell (access is very limited). I’m going to have to dig deeper.

Turn on the dehumidifier now…

That water could have dripped down and accumulated a while ago. The pipes above and the top part of the shelves and wall may have dried but the moisture is lingering at the bottom. Water from leaks or condensation can run down pipes for some distance, the low point on the pipe may be right over your bookshelf and that’s where it will finally drip down.

Found the source of the leak. TriPolar nailed it. Now to figure out how to repair it without tearing a wall down.

Thanks everyone.

Put a plastic sheet down at the soil are above the wall.

That way you can divide off the its water from the building above, or water coming along the pipes. maybe the plastic sheet is enough to hold enough water for the intermitten cause to not be a problem worth fixing.

Perhaps the water is coming from somewhere else, eg only in strong wind, or only if water users upstairs spill the water, eg in the bathroom, or when washing the floor ?

Nope, I located the actual drip; it is coming from a point where a galvanized pipe is joined to a copper pipe.

The problem is that it is in a very difficult location to work with.

i had a similar problem recently - had a crusty old gate valve that was leaking and in a bad spot to access to actually work on.

so i went either side of it, to places where i could access easily, and just cut and joined in a new length of pipe with at these locations. it was easy to feed the pipe through the tough to access spot.

a bit out of the box and specific, but thought I’d mention on the off chance this line of thinking may help you.

Water can seep from the ground through concrete, even thick concrete. Concrete alone is not waterproof. Modern homes have extensive systems to shunt groundwater away from basements and waterproof the foundation.

I say think big. Take this opportunity to rebuild the wall with a mechanism that makes the bookcase spin around and open up the secret room behind. (Note: You may have to wall off the room behind to make it secret).

Controlled by a candle in a wall sconce (“Put the candle back…”).

This might be helpful:

Joining copper and galvanized pipe

Years ago I bought a house with a garage converted (very badly) into a living room. I ripped out the rotted carpet and applied a couple of coats of sealant to the concrete slab beneath.

It’s extremely common for water to seep up through concrete, which is why there are many products designed to counter it. Check out any large hardware store.

Glad the OP found the source, good luck with that pipe.

Had this happen to me about a year ago.

Turns out the pump that took the air conditioner condensation and brought it to the drain had stopped working.

Because of the nature of my basement floor(very old house), it all pooled in a spot that was pretty far from the pump. Happened to see this stunt in action one day, that’s how I figured it out.

Yep, that’s a common one too. Seen that more than once. The pump could fail, or the pan can leak, or the drain pipe can get clogged…