While it has been raining seemingly every day now for the past month here in Atlanta, we've been to the basements of other houses still under construction in the same subdivision and none of them have the problem we're having, so it's just our house.
And the problem is, we're seeing water on the basement floor. It's worst after a heavy rain, of course, but regardless we get wet floors. It's mostly just at the base of two of the walls, and in one corner the water actually pools up, though mostly its just dampness on the floor along the walls. The floor at the middle of the basement is dry as a bone - it's just where the wall meets the floor, coming out maybe 2 or 3 inches from the wall.
There's no dampness on the walls, mind you, JUST the floors.
There are no visible cracks anywhere in the floor.
I have a weather gauge down there that showed the humidity at 80% at one point. The builder came and put a dehumidifier in the basement, but after a week of running 24/7, the humidity is down to just 72%.
About a week ago we saw (yellow) mold growing on stuff we had stashed down there, so we've spent the past week trying to clean up what we could and tossing the rest.
The foundation has the black stuff (tar?) on the outside, and like I said, it's not coming in via the walls anyways. The windows and door are sealed correctly, so that's not the source of the problem, and anyways the areas most affected are those furthest away from the windows and door.
The grading on the house appears ok, although I don't have instruments to measure it - I'm just eyeballing it.
The downspouts were just the standard ones that didn't have any extensions on them, but after we complained the first time the builder added some piping to the downspouts so they emptied out about four feet from the foundation. It appears that did little to nothing about this problem.
The builder also ran a snake through the foundation drain (which yes, it is exposed to the surface at both ends) and there was nothing clogging the drain.
We've checked all around on the second floor ceilings for leaks and there's nothing visible. Nothing visible in the attic either, so there's no *obvious* leakage coming in from the top of the house.
The builder doesn't have any more ideas (at least not that he's sharing with us), and we certainly don't know what else could be the problem, unless there just so happens to be a spring under our foundation (I know one guy to whom that actually happened!).
While it's tempting to just throw down a sealant on the basement floor and forget about it, that only keeps the water from getting to the top of the basement slab, it doesn't stop it from permeating the entire rest of the slab just short of the surface.