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Wet slab floor question

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  • Wet slab floor question

    We finally sold our house and are planning to move some time next year to our second house across the state. Last summer while doing some fence repair we had some heavy t-storms and lots of rain and I noticed the concrete slab was getting very damp - no water, but the concrete was definitely soaking up runoff. This also explains why that area of the down stairs has a mildew smell. It is in a desert area and I have never been in the house during a rain - it is a rental, normally. My wife's father built the house and she grew up there but doesn't remember there being water anywhere, and adjacent rooms don't have that problem.

    Anyway - that room is to be my next workshop and I'd like to find some way to keep the machines from rusting shut. Anyone got any ideas on what treatments are available?

  • #2
    Close the door.
    Humid outside air is condensing on the cold floor.



    • #3
      Need to tape a 3' x 3' piece of plastic to the floor. If the area under the plastic stays dry, the problem is humidity. If wet, the problem is moisture coming up through the slab.

      Sounds like coming up through the slab. First step is to work on gutters and such outside the house to redirect water away from the foundation.



      • #4
        Originally posted by Doozer View Post
        Close the door.
        Humid outside air is condensing on the cold floor.


        It has a door that is always closed - all the downstairs rooms have that. This room also has a water heater (electric, no overflow or leaks and recall this problem only happens during rain).


        • #5
          If you are getting runoff the simple solution is to redirect it somewhere else. It sounds like the slab has no vapor barrier, so the secondary solution is to use a dehumidifier in the areas that have excessive moisture. Most of the newer ones will circulate air and only run when you hit a preset moisture level. I like to use a 10% beach solution to kill stuff growing in the basement, it is pretty effective and very cheap. If you have large amounts of mold or other stuff growing you may need something else. Mike


          • #6
            Get a dehumidifier that you can run 24x7 and have it drain away (into a sink or whatever)
            and then DryLoc the floors and walls.


            • #7
              If it is coming upwards and you were thinking of sealing it that will just redirect it up the walls.
              If you find you still need to 'seal' the floor you would want a false floor that you can force ventillate underneath to blow out that mosture. That might be 1 inch concrete non porous non rotting spacers with plastic membrane and timber floor on top using smallish solid areas under the machines.
              One thing to establish is what your water table is - dig a 3 ft deep hole 3 ft from the outside and monitor the water level through the seasons. One solution might be a trench around the outside, drainage pipe to sump and a pump when needed.


              • #8
                +1 for the dehumidifier. During the summer, the basement can reach 99% humidity (as read on the RH gauge). I've had a dehumidifier installed for close to 10 years now. During the summer, it runs 24/7 and empties into a dehumidifier pump which pumps the water up and out of a window so I don't have to empty the tank. It, too, is auto on/off. In the winter, the dehumidifier cycles about one a month. Haven't had a speck of rust on the machinery since the system was installed.

                More tools than sense.


                • #9
                  Its interesting, im rebuilding my house at the moment and floor slab construction in the UK is a challenge, excavate, then 6" hardcore, compacted, then 2" sand, followed by 1200 gauge radon barrier then 120 mm foam slabs, another 1200 gauge damp proof membrane, topped with 6 inches of C40 concrete with fibre reinforcing, steel reinforcing on certain ground, luckily i didn't need it as i hit rock, not so lucky digging it out. On top of all that 3" of sand cement floor screed! It costs as much to get to ground floor level as it does to build half a house
                  Use a liquid damp proofer, kind of 2 pack stuff


                  • #10
                    If the moisture is coming from the underneath (vs. condensation), French drain at the foundation footing level would be my choice.

                    If you have basement or the shop is below the ground level, make sure you also apply insulation boards on the concrete wall of the foundation to minimize condensation issues due to high temperature gradient. Of course, you'll need the insulation only if you have basement/shop space below the ground level. I don't remember the brand name, but I used a very good board with vertical drainage grooves designed specifically for this purpose. It's more expensive, but worth every penny.

                    I went through this problem a couple of years ago with my new garage that was below the ground level. The f-n builder didn't put any drain tile. I needed to research the issue and realized that anything but this type of outside drain will be a BandAid (and an expensive one too).

                    But, first, make sure that rain and sump pump water is directed away from the walls (correct grading, trenches, properly placed downspouts).
                    Last edited by MichaelP; 11-19-2013, 01:15 AM.
                    WI/IL border, USA


                    • #11
                      And make sure that all downspout drains can flow. I had a moisture problem that I solved by pulling a wad of leaves and crap out of the hole in the ground. Water could literally fill the down spouts and start draining out of the junctions, meaning it's flowing right down the outside of the basement walls. A properly installed barrier would help prevent this, but there's no guarantee such a thing exists, or is even installed properly. Best to make sure that drain water can actually flow away from the house through the drainage system.

                      One problem I had with one corner of the house is that even though I unblocked the drain from gutter to the ground level pipe, I still had moisture permeating the foundation from time to time. For that one corner I ran the down spout into a different pipe that ran away from the house for about 8 or 10 feet- it laid on the surface with a bit of an angle to it and just drained directly onto the grass. That solved it. Later on I dug a channel and buried it, with the end of the pipe sitting in the middle of some drain rock. Put some dirt over that and grass is growing there now.

                      Many years ago I had a problem with the basement floor being damp. I came to realize that the water level in the sump pit was high. I didn't know I had a sump pit until I found it one day. I can't be sure, but I think this went away when I had my back yard landscaped. There used to be a sort of low area near the house, and I had the operator more or less even out the back yard, while giving it some slope away from the house. This was in combination with preparing the ground for the cement slab where the garden shed now resides. It seems that I was not experiencing a high water table, but a ground layout that let rainwater use the house foundation as its drain. If it could not drain as fast as the rain came down, the sump level would rise.

                      I had a good one one day. I could hear water pipes making some noise, which was unusual when there was no water taps open, the toilet tank was full and shut off, etc. I thought that maybe my neighbor had hooked to my outside taps, but that was not the case. The basement wall on one side was damp. Then I got my water bill- ten times higher than normal. I decided to check it from the outside. I seldom went into the front yard, but this time I was sloshing in water. I slogged over to where the water meter was, and it was totally flooded. Turns out I was lucky- the break was on the city side of the junction between the meter and my incoming water main. I didn't have to pay for the repair, but I did have to fight the bill.
                      Last edited by darryl; 11-19-2013, 02:25 AM.
                      I seldom do anything within the scope of logical reason and calculated cost/benefit, etc- I'm following my passion-