How to remove hydrostatic pressure from basement floor so we can finish part of the basement?

Question

We purchased a 3000 square foot ranch that was built in 1977. We would like to finish about 1000 sf of the basement. It has poured concrete walls and a concrete floor. Currently, we have two sump pumps, one at each corner of the house. They were put in separately and do not connect. We do not have standing water, just issues when we leave things sitting on the concrete floor. A cardboard box will result with some mold underneath. Carpet tile with a rubber backing will cause some molding, efflorescence, and lime deposits to appear. The efflorescence shows up in random spots throughout the basement whether they are covered with something or not. Sometimes the basement smells musty, and sometimes not. We run a dehumidifier.

We were looking at using a subfloor system such as DRIcore interlocking tiles or Delta - FL to lay under an interlocking floating waterproof flooring material. We purchased a few pieces of the DRIcore to test an area and after a couple of weeks, we removed the pieces and the concrete floor underneath was darker and damp.

We were thinking of having our landscaper dig down to find the exterior drainage tile at the corner of the house to verify that it exists and to see if it is clogged. But then what?

We called a waterproofing contractor and he recommended finishing the interior perimeter drain tile in the basement to connect the two sump pumps. We are waiting on his bid.

Not sure what to do first or where to spend the money. Will finishing the interior perimeter drain tile work? Shall we cross the drain tile at the center of the basement? What will this do to the structural integrity of the concrete floor? We would appreciate any advice that you all may have.

Answer 1

Do your sump pumps run frequently during wet weather? Does water leak through any cracks in the walls or floor? Do you get puddling or standing water in any area of the basement during heavy rains? Do you have an extremely high water table in the area?

If not, I suspect your problem is normal ground moisture not necessarily drainage water or the water table. The developer that built the house or contractor that poured the floor is responsible for not putting down a vapor barrier.

I would propose that amongst the other things you are investigating, you have a small section (maybe 1' x 1') of flooring saw cut and removed to look underneath the floor. This could be done in an area that will not be finished and could be a DIY project costing you less than $100. Or hired done for a couple hundred dollars. There should be a layer of heavy plastic under the concrete floor. (You could also leave this open to see if water puddles in the hole.)

If your basement floor does NOT have a vapor barrier under it, I propose all these other things will come to naught. If you are getting moisture leeching through the concrete floor then I don't think perimeter drainage is your problem.

The best solution would then be removing as much of the basement floor as possible and have the concrete replaced with a heavy plastic vapor barrier underneath.

Ultimately, if you finish the basement without entirely solving the moisture issue, I would recommend you never use carpeting. Paint on the basement floor may not hold. Therefore, check into just staining the concrete and leaving it after that if you want some color to the floor. This will still allow it to breathe. Throw rugs or anything else that is absorbent, on the floor will promote mold. You will need a dehumidifier running in most weather except for the very driest weather.

Good luck!

Answer 2

In the 70's it was not standard practice to put plastic under basement slab. Even in the 80's they were reluctant because they claimed it increased the cure time etc. That being said you most likely do not have any vapor barrier under the slab. I assume since there's 2 pumps they do run intermittently during high water but you say you never had standing water. So this tells me there is a perimeter drains system under the slab draining to the pumps that is still functioning. Cutting to tie the sumps together, in my opinion only adds reliability if one pump fails. My fix for this would be DC backup pumps and alarms. I would water proof the walls and floor. 6mil poly the floor install a pressure treated joist floor and underfloor dehumidifier system. Foam insulation and walls studs. Make it comfy and a great living space. Goodluck GBowman