Since the introduction of elastomeric and plastomeric flexible-sheet membranes 20 years ago, advances in technology and application methods have resulted in the industry's growth to encompass more than half of the commercial roofing market.  During this time, there have been product enhancements and modifications, as well as improvements in system design and application technology, resulting in proven in-place performance.  This applies to the types of flexible membrane products and systems described in this article.

Marketplace overview
The flexible-membrane market is divided into membrane types, defined by chemical makeup and by application type or method of installation:

Membrane types
Flexible membranes are known by their primary polymer component makeup.  More broadly, they are defined as:  thermoplastic, a membrane that can be heat formed; or thermoset, a product that has been chemically cross-linked or cured.
Thermoplastic membranes include:

• polyvinylchlorides (PVC)
• co-polymer alloys (CPA, a PVC blend)
• ethylene interpolymer (EIP, a PVC blend)
• acrylonitrile butadiene polymer (NMB, a PVC blend)
• thermoplastic polyolephin.

The last thermoplastic to enter the market-place was the TPO membrane, on the market about three years.  There are several suppliers who have recently introduced TPO membranes.

Thermoplast membranes include:

• ethylene propylene diene monomer (EPDM)
• chlorosulfonated polyethylene (CSPE)

To a roofing contractor, specifier or end user, the primary distinction between thermoplastic and thermoset is the method of joining seams.  Thermoplastics are generally hot-air welded.  Thermosets are seamed with adhesives or tapes.  CSPE, or HypalonÔ, is heat weldable in its uncured state only.

Prefabricated seam tapes and cover tapes are available from most EPDM suppliers and have been shown to improve seam strength.  Seam tapes also have appeal because the amount of adhesive--tape--used is consistent.

EPDM membranes are available in reinforced and unreinforced form; thermoplastic membranes typically are available with a reinforcing fabric within the sheet, which provides for membrane stability and improves tear and tensile strength.

Application types
Flexible membranes are applied in several ways:
Ballasted
In this application, the roof membrane is loose-laid over roof insulation or the roof deck--for example, over lightweight concrete.  The membrane is attached at the perimeter and flashed, seamed, and then held in place by gravel ballast, pavers, or some other type of overburden.

This system offers lower installation cost.  The design restriction is to be certain the roof deck can support the weight of the new system, generally 10-15 pounds per square foot.  This factor should be weighed against the low-cost-installation benefit.
Fully adhered
In this application, the membrane is adhered with contact adhesive to roof installation and attached at the perimeters, then flashed and seamed.  This system is versatile, lightweight, produces high wind resistance, and can be used with most deck types.
Mechanically attached
Today, most mechanically attached flexible membrane roofs are attached with fasteners through metal bars or plates concealed in membrane seams or fasteners placed through the roof membrane and covered with strips of membrane.  While the standardization is not complete, fasteners are now generally placed in parallel rows from three to ten feet on center, depending on the wind uplift and membrane manufacturer.  Fastener spacings and row spacings are determined by wind uplift requirements and are now generally based on laboratory testing or wind uplift calculations.  Mechanically attached systems are lightweight and easy to install but are best suited for steel and wood decks.

Specification considerations
Here are a number of points for consideration when choosing a flexible-membrane system:

• Ballasted flexible membranes have a low initial cost, but ballast can make finding leaks and making repairs more time consuming and expensive and may not be well suited for roofs with heavy or frequent traffic.
• As with all roofing systems, flashing and roof perimeter attachment requires attention.  Manufacturers have developed improved perimeter attachment methods, resulting in improved membrane restraint.  
• Fully adhered roof systems depend on the roof insulation to which they are adhered for wind uplift resistance.  Experiences on roofs with concentrated heavy traffic have shown that some insulation compresses or that insulation facers delaminate, leaving little to hold the roof in place in these areas.  This is especially true for lower-density insulation or insulation that gets wet.  To prevent damage to insulation from roof traffic, consider installing walk pads.
• Mechanically fastened systems also must be carefully designed when installed in high-wind situations.  Much study is underway to identify what constitutes sufficient fastener spacings in those high-wind exposures.
• Due to poor indoor air quality in energy efficient buildings, changes in the design of HVAC systems have boosted the supply of outside air into buildings.  Often, this increase is not compensated for by exhausting an equal volume of air, causing positive interior pressure, and it can cause billowing of the roof membrane, even when there is no wind.  The best solution is for the HVAC designer to provide for exhausting of this air.  If this is an HVAC design expectation, reduce the effect of pressure on the roof membrane by introducing an air barrier at the roof deck level.  The building stays positively pressured, and the effect on the membrane is eased.

The term "conventional" roofing has for the past 20 years referred to built-up roofing systems, inferring that flexible membranes were "unconventional."  This term seems inappropriate and antiquated today, as the roofing industry as a whole has come to see flexible membranes as a conventional and viable roofing mainstay.

The flexible-membrane roofing industry will continue to evolve and mature, and new suppliers will enter the market.  Also, new and improved membranes and systems will be developed.  The high level of flexible-membrane installations over the past 20-plus years are history.  Our experiences have made us wiser, and they have prompted the industry to focus on the continuous improvement of its products and systems.