Effective Roof Maintenance for Concrete Roof Decks

The roofing industry has struggled to come up with a practical and consistent test to determine the moisture content of a concrete roof deck. With that in mind, Joe Schwetz - director of technical services for Sika Roofing - composed an article for Interface magazine in which he delves into the inherent problems with the roof maintenance of concrete decks and offers informative case studies to highlight some issues.

Moisture Leads to Nightmarish Roof Maintenance Issues

roof maintenance detail of concrete roof

Over the years, the construction industry has been aware of moisture issues from freshly placed concrete, as well as the ability of concrete to absorb and hold great amounts of water. Over time, this water may migrate into the roof system, saturating the insulation and cover boards, causing adhered systems to become disbonded, or increasing the risk of corrosion to metal components – to put it simply, that moisture can cause a nightmare when it comes to roof maintenance.

Many articles have been written discussing the issues of moisture and concrete. These articles identify some of the reasons and issues related to the moisture in concrete, and why problems appear to be more prevalent than in the past, such as eliminating vapor retarders (especially ones that are adhered to the concrete deck) and the practice of keeping concrete forms in place, which are typically sheet-metal form decks.

Common Ways Excess Water Is Generated

  • Mixing and pouring new concrete decks/slabs.
  • Interior finish work, including water-based construction materials such as paint, plaster and drywall application, and heating the interior with propane or oil burners.
  • Concrete decks exposed to standing water from various sources, like exposure to long-term leakage into existing roofs.
  • Rain, snow and other sources.

How to Prevent Water Retention

Designers of projects that include concrete decks – either new pours or existing slabs – should strongly consider including in their roofing specifications an adequate, bonded vapor barrier on the top side of the deck to prevent any water that may be retained in the concrete from migrating into the roofing system and condensing over time. Consideration should also be given to minimize the use of organic materials and/or moisture-sensitive products within roofing systems.

Although surface dryness can generally easily be determined, the remaining free moisture that is within the concrete slab cannot readily be assessed. Until such time as a viable moisture test method is found, the decision of when a concrete deck may be roofed should include the project designer, the general contractor, the concrete contractor and their suppliers, as they will have more knowledge of the concrete mix and moisture release rates. The designer and GC should also have the best knowledge of the potential water/moisture migration and potential vapor pressures, based on the concrete specifications and the project environment, including the building microclimate, such as heated interior, additional high-moisture interior components, and other factors that may affect the moisture drive out of the concrete. This design and management group should communicate with the roofing specifier and roofing contractor when they can safely proceed with the installation of the roof assembly.

According to Schwetz, moisture in concrete decks will continue to cause issues with roof maintenance and roof installations thanks in large part to accelerated construction schedules and the increased frequency of adhering insulation directly to concrete decks, among other things