We are having the exact same problem with a new slab which is below grade on one side. It is my understanding that this makes it "below grade". The calcium chloride test read 5.9 -- I don't know what that means except the flooring people said that was too high. The installer's hand-held reader comes out at at least 20%. I know the foundation has a French drain system as well as another 6 inch pipe all around. In 4 places I duct taped down 2 foot squares of plastic and after 3 days there is no moisture under the plastic. The flooring store says you can't glue down or float over this much dampness. They don't even think laminate would do. We are at a loss as to what to do. Can anyone help? (My dream floor was tile around the outside of the wooden floor. Naturally, the tile is already laid.) carole

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 had a recent leak that ruined my floating laminate that was installed on a slab. I have a structural engineer coming Friday to give me advice on how to properly fix the leak. I will also do a french drain outside. Even with these fixes, I want to do something 'just in case'. I am thinking of installing sleepers on the concrete, then a plywood subfloor, then wood laminate. It seems to me that this would give some air flow under the floor, for any moisture issues, and IF I ever got more water under there, it would have space to evaporate. Any advice? And do they make plastic sleepers?

PPF: All materials have a coefficient of linear thermal expansion. Coefficient of linear thermal expansion for concrete is roughly the same as for iron which is why it (rebar) works well with concrete. Expansion/contraction is significant for long linear surfaces such as the large concrete slab with the crack OPs thread. Though there is a much more to material science, especially concrete and types of concrete, hopefully the following excerpt provides an adequate explanation. THERMAL EXPANSION AND CONTRACTION Materials expand or contract when subjected to changes in temperature. Most materials expand when they are heated, and contract when they are cooled. When free to deform, concrete will expand or contract due to fluctuations in temperature. The size of the concrete structure whether it is a bridge, a highway, or a building does not make it immune to the effects of temperature. The expansion and contraction with changes in temperature occur regardless 0of the structure’s cross-sectional area. Concrete expands slightly as temperature rises and contracts as temperature falls. Temperature changes may be caused by environmental conditions or by cement hydration (the exothermic chemical process in which the cement reacts with the water in a mixture of concrete to create the calcium silicate hydrate binder and other compounds). An average value for the coefficient of thermal expansion of concrete is about 10 millionths per degree Celsius (10x10-6/C), although values ranging from 7 to 12 millionths per degree Celsius have been observed. This amounts to a length change of 1.7 centimeters for every 30.5 meters of concrete subjected to a rise or fall of 38 degrees Celsius. Thermal expansion and contraction of concrete varies primarily with aggregate type (shale, limestone, siliceous gravel, granite), cementitious material content, water cement ratio, temperature range, concrete age, and ambient relative humidity. Of these factors, aggregate type has the greatest influence on the expansion and contraction of concrete. Severe problems develop in massive structures where heat cannot be dissipated. Thermal contraction on the concrete’s surface without a corresponding change in its interior temperature will cause a thermal differential and potentially lead to cracking. Temperature changes that result in shortening will crack concrete members that are held in place or restrained by another part of the structure, internal reinforcement or by the ground. Forexample, a long restrained concrete section is allowed to drop in temperature. As the temperature drops, the concrete tends to shorten, but cannot as it is restrained along its base length. This causes the concrete to be stressed, and eventually crack. Joints are the most effective way to control cracking. If a sizable section of concrete is not provided with properly spaced joints to accommodate temperature movement, the concrete will crack in a regular pattern related to the temperature and restraint directory. Control joints are grooved, formed, or sawed into sidewalks, driveways, pavements, floors, and walls so that cracking will occur in these joints rather than in a random manner. Contraction joints provide for movement in the plane of a slab or wall, and induce cracking caused by thermal shrinkage at preselected locations. One of the most economical methods for making a contraction joint is by simply sawing a continuous cut in the top of the slab with a masonry saw.