I don't believe that putting down the vapor barrier and adding concrete would work, just because then you have two separate concrete pads, that are not attached in any way and this may cause cracking, even more so if the first pad is already damaged and is not a nice flat pad. In addition to this the moisture would build up MORE in the bottom pad causing further degrading of this pad, which is the foundation for the second pad and this could cause the new pad to crack or sink and just cause problems in general.
Also this is going to reduce your basement height by a couple inches. I have an older house and know that you don't usually have high clearance in your basement. You may not want to lose that couple inches.
I think they make products you put over the concrete to reduce moisture evaporating into the air. I am not sure on this though. Maybe this would be something you would want to look into.
We also have stone foundation walls and alot of moisture comes through the walls too. This is something you may have to consider also.
-renee

Where is the dampness coming from? Is it condensation from warmer moist air from upstairs dropping water in the cool basement? Is it coming through the walls or floor? Have you checked the drainage and landscaping around the outside of the house? Are your gutters working properly?

Find the source of the dampness before throwing money at the problem. The floor is the least likely source unless you live in a swamp.

Sharon sd, thanks for your reply. You're right, the floor is not the only source of the problem. The drainage on one side of the house is creating a problem. I also plan to add gutters where necessary and redo a walkway that has settled and drains water toward the foundation. And yes, moisture is also coming through the walls, especially on the side with the drainage issues. It's also possible that warmer air from upstairs is part of the problem. So my goal is to tackle multiple sources of the problem.

The water table is very high, so to some extent I'm fighting a losing battle with the limestone walls. I'm guessing that the floor is part of the problem because I see darker/damp sections on it, and because I thought there ought to be a vapor barrier under it.

I'd be thrilled, though, if it were the case that dealing with the landscaping/gutter issue and maybe treating the walls somehow (what does one do to old limestone walls?) could be sufficient.

My old house (built around 1850) has a concrete slab floor in the basement and rock walls. We live in a swamp area, except we live in the highest point in the neighborhood ! (HAHA! those smart people!)
When we bought the house I carefully went around it and made sure the grading was appropriate and fixed it where it wasn't. The only time we have problems now is when a gutter clogs. Then we go out and fix it and we are just fine. The gutters really do their job at keeping the basement dry however you need to keep up their maintanance. Once after a big rainstorm we found a nice river in the basement. We found that one of the gutter downspouts had clogged and all the water was just running into a huge hole the water had drilled next to the foundation. We fixed everything up and havent' had the same problem since.

We also have a dehumidifier and a sump pump (that never kicks on but is there for emergencies! )
We will NEVER be able to store things like fabric or paper in our basement. That is just the way it is, however it sure is perfect for wine and canned goods!!!
-renee

I asked my husband if you could pour concrete over concrete ,hes been a builder of his 2 construction co.for over 30 years.He said yes.But that wont end moisture problem.You need to find where its comming from

New concrete can have trouble sticking to old. Some tile people paint old concrete with sonnebourne liquid latex before adding another layer of concrete. I don't know if this would help in your scenario.

Thanks for the continued advice.

Actually I thought that there would have to be a layer of some type of material (class 5 aggregate?) over the old concrete, as well as a vapor barrier. Then the new floor would be on top of the aggregate (plus vapor barrier). And the vapor barrier would address moisture that rises through the floor, although of course I would also have to address other points of moisture.

I'm lucky to have very high ceilings in this space, so losing a few inches of headroom won't be a problem.

I have had wet spots under anything laying on the floor of my converted garage. I have already pulled up $1000,00 worth of carpet in hopes of saving it. I do not experience this wetness, moisture, or condensation until after nice size rains. I do not have gutters but I am afraid that installing them will not fix the problem. I am wondering if it is a moisture problem where moisture is coming into the room from under the wall which used to be the garage door. The garage floor is flush with the driveway right on the other side of the wall. I have considered totally replacing the concrete floor or bricking the outside wall.
Do you have any suggestions?