THOMPSON’S TAKE: What Happens When You Have a Roof Over a Roof?
Most are aware that it is common practice in the residential market to apply new roof shingles over existing shingles when replacing the roof of a home. This process is called reroofing or adding an overlay. Many homeowners will choose this option as it avoids the higher cost of a complete tear-off and replacement. The old adage “you are just kicking the can down the road” applies here. Many municipalities allow two and sometimes three “roofs” on a home. Of course, the additional weight on the structure must be considered when choosing the reroofing option.
You might not be aware that this reroofing strategy has also been used on commercial and institutional buildings for many of the same reasons stated above. However, some building owners that have used this reroof strategy on large flat roofs have run into some unique problems that one would not see in the residential application. There are basically two options that are typically used:
A new membrane was installed directly over the existing membrane.
A layer of insulation was placed over the existing leaking roof membrane and then a new membrane was installed over the new insulation. Adding additional insulation was an attempt by the building owner to perhaps comply with new energy codes or simply to, in theory, increase the “R-value” of the roof thereby reducing heating and cooling costs. R-value is the capacity to resist heat flow through a material such as insulation. The higher the R-value the greater the insulating power.
The issue with this reroofing strategy on the surface is obvious. If the roof was leaking, then one can fairly assume that the roof insulation is saturated in several areas. The two reroof strategies described above effectively trap the moisture within the saturated insulation in the roofing system. This results in adversely impacting the thermal conductivity of the roof system. Moisture can be driven into the building depending on indoor and outdoor temperatures. This may result in humidity and odor problems. Conversely, moisture can be driven out of the wet insulation causing moderate to severe blistering in the new membrane. Additionally, perimeter flashing and roof penetrations are difficult to overlay properly and installing contractors rarely get them right. Lack of attention to these details compromises the waterproof integrity of the new roof.
If you suspect you have a building with these types of problems one recommended strategy would be to conduct a thorough roof scan. The scan will show trapped moisture as well as any new leakage. Then take several roof cores completely to the roof deck to determine definitively if you have this type of roof system.
EagleHawk provides a variety of services that provide value to Facility Managers to help prioritize their work. The EagleHawk team can conduct infrared inspections of roofs and underground steam heating systems to identify problem areas. Their highly trained technical team will provide a meaningful technical report to help Facility Managers identify problem areas and make those difficult choices.
This blog is part of a series in which I’ll provide my take about a variety of subjects regarding facility management, maintenance and repairs to help provide insight into common challenges that facilities managers face and how to hopefully best resolve them.
