Safety is top priority for any reputable contractor. Just as critical as maintaining safety on the job site is to deliver work that will support a safe working or living environment for future occupants.

Sweating slab syndrome (SSS) is a phenomenon that occurs when moisture condenses on an indoor concrete slab. The resulting wet floor can create a variety of hazardous conditions within the building. SSS is most common in buildings that are not climate controlled and contain exposed concrete slabs, for example warehouses, or schools that are unoccupied in summer. Contractors that are aware of this potential issue and how to address it can take steps to mitigate the problem — preventing accident and injury and maintaining their reputation for delivering quality work.


Water on a smooth indoor concrete slab acts as a highly effective lubricant. The entire wet area becomes an extensive slip hazard that may not be readily or immediately visible to the average person. This is especially dangerous in a workplace environment. Slipping workers are prone to injury, and tipping or sliding equipment — such as a forklift sliding out of control across a warehouse floor or off a loading dock — can easily damage property and/or cause injury or death.

Sweating slab syndrome also contributes to undesirable indoor environmental conditions. Where moisture-permeable items such as wooden pallets or cardboard boxes are stored on a moist floor, mold and fungus often grow. This can ruin these items and their contents, and lead to serious air quality issues in the building. 


SSS occurs when the ambient air temperature changes faster than the temperature of the concrete slab. Concrete warms and cools at a much slower rate than air does. When warm, humid air flows over a cool concrete slab, the temperature of the slab can stay significantly lower than the dew point. This will cause moisture to condense from the air onto the floor.

Sweating slab syndrome happens frequently in spring and summer when temperature fluctuations are high: A slab will cool down at night. When doors and windows are opened and let in warm, moist air, moisture can condense rapidly. SSS also happens frequently when a warm front follows a period of cool weather.

Additionally, SSS can take place when salt deposits are present in the concrete. The salt absorbs moisture from the air and/or within the concrete. When the moisture content is high enough, the salt can dissolve. In turn, this creates a saline solution that can form a slick layer on the slab’s surface.

The less permeable the slab, the more likely it is to experience SSS. The use of surface densifiers and sealers may exacerbate sweating slab syndrome. It is also more prevalent where the concrete is dirty, oily and/or heavily trafficked.


Correcting SSS can be tricky business. However, there are many ways a contractor and/or property owner can control it.

There are three general principles to keep in mind to reduce or prevent SSS. These are:

  • Reduce moisture level;
  • Increase and/or control air flow; and
  • Reduce temperature differential between floor and air.

Anything you can do to encourage these conditions will help avoid sweating slab syndrome.

Some effective techniques include:

Moisture barriers: Installing a moisture or vapor barrier, such as a sheet of plastic, under a concrete floor will reduce the amount of moisture the slab absorbs from the ground, and thus lower the general humidity level within the building.

Dehumidifiers: While costly to run, dehumidifiers are an effective way to reduce the moisture level of indoor air.

Building design: The way air flows through a building can contribute to or reduce sweating slab syndrome. For instance, SSS happens frequently in buildings that vent air through roof vents. The resulting chimney effect draws air through windows and doors, which must traverse over the slab before exiting the building through the roof. Where possible, design building ventilation to avoid such situations.

High-speed fans: These can be an effective short-term solution to quickly dry a wet floor — especially where puddling has occurred. However, they present a number of drawbacks, including noise, high-energy use, and trip hazards presented by fan cords strung through work areas.

High-volume-low-speed (HVLS) fans: HVLS fans are rapidly gaining popularity as the most effective solution for sweating slab syndrome in many situations. Large HVLS ceiling fans combat SSS in two ways:

  • They mix stratified layers of air to reduce temperature differentials between ceiling and floor; and
  • They move large volumes of air through the space to effectively increase the evaporation rate from floors and other surfaces.

Because HVLS fans are designed to move large masses of air slowly, they are very energy-efficient and operate quietly without creating disruptive breezes. HVLS fans work well on their own, or may be used in conjunction with HVAC systems and other solutions such as dehumidifiers and high-speed fans. Some are also candidates for smart building integration, and can be networked or controlled automatically or manually through a smartphone or other device.


While it may not be possible to completely eliminate sweating slab syndrome in every situation, implementing preventative measures can greatly reduce the potential for SSS-related damage or injury. Just by staying aware of the issue and communicating it to clients, contractors can proactively prevent accidents and stay in the good graces of their customers.

Infographic provided by Go Fan Yourself

Publication date: 8/22/2018

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