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The key to any good green roof project is a good waterproofing membrane. There are many types of waterproofing membrane, and it is important to understand how they work and how they are installed to provide the appropriate green roof assembly relative to the membrane chosen. Below are explanations of the leading roofing membrane systems that have been installed under green roofs. Consult with your membrane manufacturer for information relative to the membrane specifications. Contact us so we may recommend green roof system details specific to the roofing membrane chosen.
PVC sheets are produced by calendering, spread coating, or extruding, and they typically are reinforced with polyester or glass-fiber mats or scrim. They contain plasticizers and stabilizers as well as other additives to impart flexibility and achieve other desired physical properties. Some membranes are available with non-woven fleece backing adhered to the underside of a sheet. Sheet widths range from 6 to 12 feet wide and are typically 45 to 90 millimeters thick. Seams are sealed by heat or chemical welding.
PVC membranes are produced in numerous colors, though gray and white are the most common. They can be installed fully adhered, mechanically attached or ballasted. Most PVC membranes do not receive surfacings.
Polymer-modified bitumen and modified bitumen (MB) sheet membranes were developed in Europe in the early 1960s and have been in use in the US since the mid-1970s. Polymer-modified roof membranes are composed of reinforcing fabrics that serve as carriers for the hot polymer-modified bitumen as it is manufactured into a roll material. MB roof system membranes are composed of multiple layers, much like built-up roof (BUR) membranes. MB roof systems are typically installed as a two-ply system and almost always fully adhered.
There are two types of MB roofing membranes.
SBS polymer-modified bitumen membranes commonly are installed in hot moppings of asphalt (similar to BUR systems) or cold adhesive. Some SBS modified membranes are self-adhering; that is, they contain an adhesive backing.
APP polymer-modified bitumen membranes typically are heat-welded or torch-applied. Consumers should be cautioned that the National Roofing Contractors Association (NRCA) does not recommend torch-applying a modified bitumen membrane sheet directly to a wood deck. Generally, APP modifiers impart a "plasticized" quality to asphalt, and SBS modifiers impart a "rubberized" quality to asphalt. MB membranes and EPDM, a thermoset membrane, often are confused by consumers because of colloquialisms used by roofing contractors. MB and EPDM membranes are sometimes called "rubber roofs."
Surfacings for MB membranes include aggregate surfacing, mineral surfacing, metal foil-laminate surfacing and smooth liquid-applied surfacing. A roof system composed of a built up roof membrane with 2 plies or 3 plies and a polymer-modified bitumen membrane cap sheet is commonly referred to as "hybrid" system. NRCA considers this type to be a polymer modified bitumen membrane system.
TPO membranes are produced by calendering with lamination, extrusion with lamination, or extrusion-coating techniques. TPO sheets are a blend of polypropylene and ethylene propylene polymers and usually are reinforced with polyester. TPO sheets contain colorant, flame retardants, UV absorbers and other proprietary substances to achieve desired physical properties. Sheet widths range from 6 to 12 feet wide and are typically 40 mm to 100 mm thick. Seams are sealed by heat welding with hot air. TPO membranes are commonly white and can be installed fully adhered, mechanically attached or ballasted. Most TPO membranes do not receive surfacings.
Thermoset membranes incorporate principal polymers that are chemically cross-linked or vulcanized. Membranes that are vulcanized also may be referred to as "cured." One characteristic of true thermoset polymers is that once they are cured, they only can be bonded to similar materials with adhesives. The most common thermoset roof membrane is EPDM. EPDM principally is composed of two compounds, ethylene and propylene, that are derived from oil and natural gas. EPDM membrane sheets widths range from 7.5 to 50 feet wide and are typically 45 mm and 60 mm thick. Seams are sealed using liquid adhesives or specially formulated tape. The membranes are commonly black, but white is available and can be installed fully adhered, mechanically attached (using batten bars), or ballasted. Most EPDM membranes do not receive surfacings.
Hot-fluid-applied, rubberized-asphalt membranes have a long track record of performance in waterproofing applications, but often face considerable challenges during construction, such as unanticipated existing conditions and unique details. While construction challenges are not uncommon, their exact nature is difficult if not impossible to predict during design. Active construction administration, with frequent monitoring through site visits, along with a sophisticated quality control program orchestrated by a professional waterproofing contractor, is critical to successfully address construction challenges in order to avoid conditions that can reduce the durability of the waterproofing system.
Polymethyl methacrylate (PMMA)
Polymethyl methacrylate (PMMA) is a cold, liquid-applied roofing system. The product is capable of performing well under adverse conditions. PMMA membranes are extremely tough, resist physical damage, and can be used where frequent abuse could occur. These membranes resist both static and dynamic loads. They withstand roof equipment being placed directly on them. If extreme roof traffic or loads, such as vehicles, are expected, an optional heavy-duty wearing course can be applied. PMMA roofing systems are particularly well suited for roofs that have many penetrations, including irregularly shaped penetrations like I-beams, H-beams and conduits. PMMA membranes can be installed in temperatures as low as 23°F. PMMA's rapid cure rate allows projects to be completed and made rain-proof quickly. Cured PMMA membranes are dry to the touch and can withstand rain in less than an hour after installation. Traffic can resume almost immediately.
PMMA is a thermoplastic with characteristics that make it suitable for a roofing system. PMMA molecules are hydrophobic, which means that the roofing membrane repels water droplets. This makes PMMA particularly suitable for roofs where ponding water would otherwise be a problem. Many flat or near-flat roofs allow water to stand on the surface for long periods of time. Standing water causes accelerated aging and damage of most roofing membranes, and roofing warranties commonly exclude damage caused by standing water. PMMA roofing systems aren't subject to such exclusions. Hydrophobicity also makes it a good choice for inverted or green roofing system designs. While many roofing materials are susceptible to damage caused by chemical contaminants found on some industrial roofs, PMMA membranes resist most acids and aqueous alkalis, aliphatic hydrocarbons, and nonpolar solvents. Vegetable and animal fats have little effect on them.
Generally, PMMA flashings are installed as a sandwich of polyester fleece reinforcement between two layers of PMMA polymer encapsulation. The PMMA will adhere to most common building materials, including concrete, masonry, metals, and plastic. Its quick curing and ease of application provide a very rapid seamless, reinforced, waterproof termination without the need for counter-flashing. A PMMA roofing membrane can be used for new construction, replacement, or re-covering an existing roofing system. Because PMMA is compatible with almost any roofing substrate, it can be used to overlay the existing roofing membrane. It's compatible with single-ply, modified bitumen and some types of built-up roof, reducing disruption to occupants, processes in the building, and risk of in-progress leaks.
Photo by Tommy Hileman.