Low-slope roofing systems are defined by code as roof systems applied on slopes of 2:12 or less. It is the architect or designer’s responsibility to specify the system that is best suited for the facility. There are three basic types of conventional roofing systems used in commercial construction. We examined built-up roof systems and modified bitumen systems in Volume 3 (See “Low-Slope Roofing Systems Part 1”). This article focuses on single-ply roof systems.
There are two main groups of single-ply roof systems. They are thermosets (mainly comprised of EPDM) and thermoplastics (comprised of several polymer formulations).
EPDM Roof Systems
EPDM systems have been on the United States low-slope roofing market since the 1960s. Their widespread use began in the 1980s and the technology has developed a favorable market share in the industry since that time. The rise in market share correlated with the United States Energy Crisis of the 1970s, which drove up the price of bitumen. During the development boom of the 1980s, roofing labor shortages helped drive the market.
The size of the membrane sheets (typically 20 feet by 100 feet) allowed contractors to install the systems with less manpower and virtually no equipment. These factors quickly contributed to the widespread acceptance of EPDM systems, as they were more economical to install over the labor-intensive, equipmentladen built-up roof systems.
Contractors also found a willing workforce who did not have to contend with the safety risks and odors associated with hot bitumen. Instead of long pants, long shirts and heavy work boots, EPDM applicators could comfortably work in short sleeve shirts, short pants and tennis shoes. Building owners and developers quickly gravitated to the initial cost savings provided by these systems. EPDM is synthesized from ethylene, propylene and small amounts of diene monomer, and these ingredients give the material its name.
The basic rubber polymer (EPDM) is the binder of the sheets and constitutes 50 percent of the total formulation. The formulation also consists of carbon black, antiozonants, antioxidants and other curing agents that account for another 25 percent of the formulation including oils, which act as plasticizers and rubber extenders. The mixed rubber is called a rubber compound.
Carbon black is added to the EPDM polymer to add structure to the composition. Carbon black is the byproduct of oil or natural gas that has combusted under the deficient supply of oxygen. Carbon black adds the reinforcement to the sheet and also adds the color to the membrane. The proper amount of carbon black can double or triple a sheet’s tensile strength.
Oils are added to improve the low temperature flexibility of the rubber materials. Typically, the structure of the carbon black and other reinforc- ing materials make it difficult to blend into the EPDM composition. Oils are added to allow for quicker mixing and to keep the sheet in a flexible condition. The oils used are generally long chain paraffinic oils that are very temperature stable. This helps the EPDM withstand rooftop temperatures from 40 below zero to 180 degrees without losing flexibility.
EPDM sheets are manufactured in widths from 54 inches to 50 feet and in lengths of 50 feet to 200 feet. The standard thicknesses of EPDM are 45 mils (0.045 inch) and 60 mils (0.060 inch). Some manufacturers provide sheets with thicknesses of 75 to 100 mils. Most EPDM sheets are reinforced.
Application Methods
Whereas built-up roof systems and modified bitumen systems are generally applied in a similar manner not withstanding the bituminous product - asphalt or coal tar, adhesive, torch, self-adhered - single-ply roofs are installed in one of three separate application methods. There are three types of application systems for EPDM membrane. They are:
• Loose-laid ballasted.
• Mechanically attached.
• Fully adhered.
Although the repair procedures to the membrane remain consistent, inspection for maintenance purposes and problems may differ with each application method.
Loose-Laid Ballasted
This is the least expensive of all roof applications. The premise is simple. As the name implies, insulation is set over the roof deck in a loose manner. A single layer of EPDM is set over the insulation. The membrane is only adhered at the seams, flashings, and penetrations. Ballast is applied over the membrane to serve as weight for the roof system. It should be noted that Factory Mutual does not recognize this method of application as a proper roof system due to the potential for roof blow offs. Due to the ballast application, inspection of the membrane and locating roof leaks is very difficult and time consuming.
Mechanically Attached
With mechanically attached systems, the EPDM is secured at strips or using the point method. The strip attachment method uses linear bars hidden at each membrane lap with fasteners penetrating the membrane and insulation into the deck. The point method uses stress plates and metal fasteners or wide plates that are fastened to the deck. The EPDM membrane is adhered to the plates.
Fully Adhered
With a fully adhered system, EPDM membrane is adhered to the substrate (deck or insulation) with cold adhesive or contact cement. The full underside of the membrane sheet must be adhered for proper attachment.
Thermoplastic Roof Systems
There are currently six thermoplastic single-ply roof membranes used in the United States. They are:
• Copolymer alloys (CPA).
• Chlorosulfonated polyethylene or Hypalon (CSPE).
• Ethylene interpolymer alloy (EIP).
• Nitrile thermoplastic membrane (NBP).
• Polyvinyl Chloride (PVC).
• Thermoplastic polyolefins (TPO).
Copolymer Alloys
Copolymer alloy (CPA) thermoplastics are often referred to as modified PVCs in reference to the material’s PVC base content. The membrane sheets are plasticized vinyl with base resins that are primarily made of PVC and other blends of polymers.
The applied plasticizers are used to keep the materials in a flexible state. The average elongation of the material is 25 percent to 40 percent. All CPA membrane sheets are reinforced with polyester.
Due to the chlorine content from the PVC formulation, the membrane has inherent fire resistance. However, all of the manufacturers of these membranes add additional fire retardants. Similar to PVC membranes, the chlorine component is considered an environmental hazard. Because chlorine is not an environmentally friendly chemical, future disposal may be complicated.
In the manufacturing process, the formulated polymer is applied to both sides of the non-woven polyester fabric. The membrane is then run through a laminating machine and is formed into a composite sheet. One manufacturer of CPA prefabricates up to 85 percent of the membrane in a manufacturing facility and delivers a completed membrane, with fully adhered seams, to the building site. In this process all seams are adhered in a controlled environment free of dirt, dust, wind and precipitation.
CPA membranes have been on the United States roofing market for over 20 years, which is a sufficient track record to investigate their performance capabilities. They generally have a greater chemical resistance than standard PVCs and EPDMs.
Chlorosulfonated Polyethylene
Chlorosulfonated polyethylene (CSPE) membranes commonly referred to as Hypalon, utilize a synthetic rubber membrane that has been on the United States roofing market for well over 20 years. CSPE is a synthetic rubber that is formulated around prime polymers that are assisted by a blend of pigments and fillers. The main polymer is chlorinated polyethylene (CPE) blended with an occasional chlorosulphonyl group instead of chlorine. The chlorosulphonyl groups are crosslinked into the polyethylene molecules of the adjacent chains, which form a more rigid thermalset structure after curing. The other ingredients include, but are not limited to, CaCO (filler), TiO (for whitening and UV resistance), MgO (for cross linking and color stabilizing), processing aids, antistat and antioxidant compounds, and epoxy based curing agents.
CSPE is classified as a non-vulcanized elastomeric because the material has rubber-like characteristics. These membranes have the capacity to stretch under stress and return to their original state once the stress is removed. The nonvulcanized membrane is processed in an uncured state, curing begins once the material is exposed to the elements of sunlight, heat or moisture. The vulcanization process occurs when the molecular structure becomes permanently fixed. Once the material has cured all seams must be completed using an adhesive. Due to this fact, it is advisable that all flashing work and seams are completed the same day. This type of adhesion method is inferior to heat welding and may not provide proper long-term waterproofing performance. Prior to curing, seams can be completed using a solvent or by hot air welding.
CSPE sheets are manufactured through a calendering process by applying the formulated polymer to a reinforcement sheet. For mechanically fastened systems, the reinforcement sheet is typically a polyester mat. Mineral fiber or felt backing sheets are typically used for fully adhered and ballasted systems.
The fully adhered sheets can be attached to the substrate with either latex adhesive or modified asphalt. Membranes are available in thicknesses of 30 to 60 mils.
One of the primary advantages of these membranes is their ability to accommodate thermal movement and mechanical strains. This is due to the high elasticity of the sheet propagated from the materials conversion from a plastic in an uncured state to an elastomer in the cured state. This change provides the cross-linked material with substantial creep resistance.
The membrane has also exhibited high resistance to ultraviolet radiation and most of the atmospheric pollutants. The membrane surface is self-cleaning through continuous exposure to ultraviolet radiation. It has also proved to be a preferable product for restaurants and food processing facilities because it has superior resistance to grease, mineral fats and oils.
There have been reported problems associated with these membranes. One significant concern is the excess chalking of the membrane surface. This phenomenon predominately occurs when the pigment in the sheet washes away from the surfacing. If the loss of the pigment manifests itself as surface erosion and there is an eventual decrease of mil thickness, this can shorten the performance service life of the system. Another concern associated with these membranes is associated with algae and biological agents. These problems have mainly been reported in warm, humid climates and costal areas. The membrane is also prone to degradation by accelerated weathering.
Symptoms such as surface checking and cracking down to the reinforcement scrim occur when there are physical and chemical changes caused by UV radiation, heat, and water and thermal shock.
It should be pointed out that these failures are primarily caused by formulation problems by individual manufacturers. There is no evidence of inherent polymer failures. In most cases, these issues can be resolved by modifying the formulation through the use of pigments, biocides, and other accepted ingredients. However, it is known that CSPE is adversely affected by chlorinated and halogenated hydrocarbons. The service life of these membranes can be extended with a top surfacing of 100 percent acrylic elastomeric coatings.