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The Water Below: Water Vapor and Concrete Slabs


September 12, 2012



Poet Rupert Brooke once wrote that good shall come of water. He obviously never dealt with concrete.

One of the lesser known yet more frustrating sources of water damage can occur on a concrete slab on grade.

A concrete slab on grade is a flat area of concrete, poured directly on the earth or water membrane. This is where flooring – wood, carpet, tile, – is laid in residential construction.

Water or water vapor rising out of a concrete slab on grade can cause flooring to lose adhesion, to warp, and to peel.

Beneath continuous flooring, moisture can also cause ripped or separate seams, air bubble efflorescence deposits, and other deterioration.

Other harmful effects include damage to flat electrical cable systems, buckling of carpet or carpet tiles, odors, and fungi growth.

It can even damage the concrete itself and corrode steel reinforcements within the concrete.

Frequently it affects individual units, but it can affect common elements as well.

Where the Problems Originate

Excessive moisture in concrete slabs on grade usually originates from three sources:

-Residual water in or below the slab remaining from the construction process -Naturally occurring ground water from a permanent or seasonal high water table -Irrigation systems, broken pipes, or other recurring, manmade sources

A slab may appear dry but actually may have a detrimental level of moisture vapor passing through it.

Early warning signs of moisture transmission can include carpet, tile, and paint staining; dampness; and musty odors.

Moisture passing through the slab can carry damaging alkaline salts from the ground or concrete itself. It can take from six weeks to six months for slabs to dry out sufficiently and not adversely affect floor coverings and finishes.

The concrete must be dry enough for adhesives to bond property. But there is a catch. Slabs that are placed on earth are never really dry unless a membrane water-proofing layer is installed beneath them.

Some tiles – asphalt and vinyl composition tile, for example – may be applied on earth-based concrete slabs without water proofing materials.

Even those materials should not be installed at least three months after the concrete is placed, and not until the slab is tested and proven to comply with the manufacturer’s requirements.

Other products, such as vinyl and rubber tile and sheer vinyl, should not be used under these conditions.

Sheet vinyl with a moisture-resistant backing and vinyl composition, and rubber and asphalt tile, can be applied on slabs that are in contact with earth and waterproofed membrane.

Sometimes the problems are deeper – contractors must keep any below-grade excavations free of water. This is important to avoid potential reservoirs of moisture that can migrate upward through slabs.

Excessive moisture below concrete slabs can cause structural degradation of soil bearing strengths, and swelling and shrinkage of soil. These can adversely affect a concrete slab on grade.

Vapor retarders are the last defense against sub-slab moisture. Vapor retarders are any material – anything from membranes to airspace – through walls or floors and to prevent condensation within them.

They do have a down side, however – punctured vapor retarders are often a source of slab-on grade moisture problems.

Vapor retarders must be able to withstand the rigors of construction traffic between the time they are installed until the slab is poured. They must also resist attack by moisture, chemicals, and organisms in the soil.

What Can We Do?

If your association is about to undertake a new construction project, identify all floor covering products that will be in contact with slabs on grade.

Do this when selecting the floor covering and developing specifications.

Contact manufacturers or trade associations to determine the substrate moisture conditions require for acceptable performance.

Perform bond and moisture tests on the concrete as recommended by the manufacturer to determine if the surfaces are sufficiently dry and free from harmful substances.

If your association is suffering from an existing problem, the most common solutions are:

  • Installing subdrains at the building perimeter. A subdrain is a drain below grade that carries water away from a building. This is the most effective – and most expensive – solution to concrete slab on grade moisture problems. -** Install vapor retarders to limit moisture movement.** After removing the damaged flooring, and cleaning and drying the damaged area, install a vapor retardant underneath the new floor finish.
  • Making the floor finish vapor permeable. Flooring materials such as glazed or ceramic tile and stone pavers are not adversely affected by moisture.

Preventing Common Water Problems

  1. Slope all ground areas, balconies, and deck surfaces away from your building or unit.
  2. Avoid flat water exterior stucco surfaces. If they exist, install a waterproofing membrance beneath the stucco, or a sheet metal coping or cap on top of it.
  3. Install weatherstripping around exterior windows and doors.
  4. Caulk gabs and cracks around windows, doors, and stucco.
  5. Apply a 100 percent acrylic-based elastomeric coating on vertical stucco surfaces.
  6. Install flashing around window openings and roof penetrations, such as vents, piping, and chimneys.
  7. Regularly inspect flashings around roof and sheet metal.
  8. Keep roof surfaces and weep holes free of debris and clean out gutters and downspouts.
  9. Determine the source of leaks fast. Hire a reputable contractor to do the job right the first time.
  10. Install gutters and downspouts to carry drainage away from the building. Otherwise it may stain exterior walls and damage landscaping.

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