Community Managers!
TPO and PVC Roofing and Other “Cool Roof” Products on Multi-Family Flat Roof Applications

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.
The reverse of this phenomenon is true and is often observed by homeowners on newly re-roofed complexes. In the early morning when the cold single ply membrane is exposed to the now warming outside air, condensation occurs on the top of the membrane. This condition is often so substantial that homeowners report roof gutter downspouts dripping and running with water on an otherwise clear morning. 

The National Roofing Contractors Association (NRCA) has been making significant waves in the roofing industry regarding the unintended consequences of water vapor buildup and its resultant damage in areas with either high background humidity, or significant temperature variation between the interior and exterior of a building. In Southern California coastal multi-family applications, it is imperative to install various components to mitigate the condensation that will occur as the result of installing cool roofs. Failure to properly insulate and ventilate single-ply cool roof systems in coastal Southern California can have catastrophic results to the roof sheathing and framing. 

In recent years, roofing manufacturers and the NRCA have stressed to roofers that the potential for cool roof condensation must be evaluated when “designing” a roofing replacement system. Several manufacturers now recommend third-party evaluation of the building to determine the likelihood of condensation and the proper remedies. Neither the NRCA nor the roofing manufacturers provide specific design details on how to remedy the condensation issue, and most roof manufacturer warranties specifically exclude responsibility for condensation related issues.  Products are available such as thick insulating boards that are installed under the single-ply system to create a thermal break between the interior and exterior surfaces so that the interior moisture never cools so much that it hits its dew point, or temperature where condensation occurs.

Condominium and apartment complex managers must be cognizant of the consequences of condensation with single ply roofing systems such as TPO and PVC. Unfortunately, many roofing companies often do not follow up with the manufacturers regarding the proper use of certain roofing projects. The cost for condensation control with insulation and ventilation will add a significant sum to a TPO or PVC bid, as much as 30-50%. Roofers who bid the project properly with insulation boards and ventilation ports will likely be higher than competitors who ignore these key factors. In an industry where low bid is often king, one must consider if the low bid is adequately protecting the longevity of the buildings at hand. We strongly recommend a third-party architect, engineer, or construction consultant be retained during the specification process to develop a site-specific plan for the handling of the condensation issue. Third-party quality control is equally important.
Pacific Construction Analysts:
Pacific Construction Analysts Inc. (PCA) is a California licensed General Engineering and Building Contractor (Lic. No. 833399 A/B) specializing in construction management, development services, construction forensics, and the evaluation of distressed properties and structures. Our staff has extensive experience construction managing a wide range of projects throughout California including roofing, exterior restoration, structural repair, commercial tenant improvement, and new construction.  A major segment of our work is assisting homeowner associations with major construction, renovation, and repair projects. We work with associations to alleviate association boards and community managers from the daily duties and tasks managing major ongoing projects, as well as provide a high level of construction expertise working on behalf of the association.

Pacific Construction Analysts Inc.
28600 Mercedes Street, Suite 101
Temecula CA, 92590
(951) 527-5359