Pacific Construction Analysts Inc. has investigated and analyzed moisture related claims on recently re-roofed condominium complexes utilizing low slope single-ply “Cool Roofs”, notably in Coastal Southern California. Thermoplastic Polyolefin (TPO) and Polyvinyl Chloride (PVC) have been in use for decades for low slope (flat) roofs, with both single-ply systems being known to provide a durable waterproof membrane for low slope structures. The state of the art for TPO and PVC roofing materials has changed drastically over the years, with each system undergoing alterations to resolve design deficiencies in their initial designs, and today, both PVC and TPO products are vastly superior to their historical predecessors.
Prior to 10-15 years ago, PVC and TPO roofing were almost exclusively used for commercial structures. More recently, management of apartment and condominium complexes have seen the advantages and cost saving properties of PVC and TPO. The use of PVC and TPO is strongly influenced by the California Building Code’s Title 24 energy provisions. The physical properties of PVC and TPO reflect sunlight resulting in a cooler surface temperature than other roofing options such as torch down roll roofing and hot tar built up roofing; both of which dominated residential low slope roofing for decades. Unfortunately, there is a major design setback in PVC and TPO low slope cool roofs; the tendency for interior humidity to condense on the underside of the roofing where the roof lacks an adequately vented and/or insulated attic space, a common condition in low rise, low slope condominium roofs.
When occupants shower, cook, or just simply exhale, water vapor is transmitted into the air. This vapor can pass through ceiling drywall and into the attic space above the unit where it is trapped by the impermeable PVC or TPO liner. On cooler mornings, the temperature differential between the unit interior and surfaces of the cool roof material is such that water vapor condensates on the underside of the roofing material, similar to how water beads on the outside of a glass of ice water on a hot summer day. In evening and morning hours when the exterior temperature cools, a temperature differential is created between the cool exterior air and warm moist interior air. Without proper insultation and/or ventilation, this temperature differential occurs at the underside of the cool roof membrane where humid air is trapped at the impervious roof liner. When the temperature of the single ply membrane drops below the dew point, water beads form on the underside of the roofing which then drip onto the roof sheathing and framing, saturating this material and causing it to fail. This roof sheathing failure can become so extreme that the cost to repair the framing can exceed the cost of the TPO or PVC re-roof. This cycle is exacerbated in Coastal Southern California where the ambient background humidity is high.