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By: Ed Pon

October 28, 2020

Roofing materials have evolved for thousands of years. It all started with some form of steep slope roofing that comprised of shingles, thatch, terra cotta tile, etc. The principle for each roofing material configuration is the same.  Small overlapping units of water-resistant material are secured to a rigid structural substrate to shed water from the top of structures.

Traditional and ancient architectural styles are greatly defined by the roofing technology that was available at the time.  Ceramic terra cotta tiles were installed on the temple pediments of Ancient Greece and Rome as well as the sweeping pitched roofs or pagodas of ancient Asian architecture in the Forbidden City of China. Primitive thatch roofs were common in places such as Polynesia and the rural English countryside. Coal tar, a by-product developed from coal mining and asphalt roofing, a product developed from the petroleum industry, had been a revolutionary material for the common low slope roof systems in modern architecture.  The rectilinear forms of the International Style architecture such as the Bauhaus were made possible with technology that developed from coal tar pitch.

In the mid 20th Century modified asphalt roofing products became commonplace with sheet membrane materials; the advanced industrialization and manufacturing processes furthered the development of manufactured single-ply membrane technologies such as EPDM (ethylene propylene diene monomer), PVC (polyvinyl chloride) and TPO (thermalplastic polyolefin); otherwise, these materials are also known as synthetic rubber (EPDM) and thermoplastics (PVC and TPO).  Although these membrane systems had the advantages of integrated reinforcement and consistent minimum material thicknesses, they all have seams where the sheet membranes are overlapped with heat welds for the thermal plastics and adhesive for the synthetic rubber membrane. A seamless system was next in the evolutionary cycle.

By the late 20th to early 21st century there was the burgeoning development of “plastics” (think Dustin Hoffman in “the Graduate”):  polyethylene, polypropylene, polyurethane, polyester, poly methyl methacrylate and poly urea are all variations on a theme.  These materials were developed to be installed as fluid applied systems. Solvent-based materials require the release of solvents to cure. Water based products similarly require the evaporation of water to cure. And chemical cure requires the combination of two components to cure. The one thing they all have in common is that they are all fluid-applied membranes which create seam-less systems.

As we look at concrete substrates in the New York City commercial urban environment, all roofing/waterproofing systems should meet industry performance standards and comply with current regulations. The following list of required criteria should be met for a quality system application:

  1. The membrane should be durable
  2. The membrane should not be flammable or require the use of an open flame
  3. The membrane should be odor free and not emit high VOCs (volatile organic compounds)
  4. The membrane should easily conform to the many irregular shapes a roof surface
  5. The membrane should adhere tenaciously to primarily concrete substrates and fully across the roof surface, horizontally and vertically
  6. The membrane should be cost effective
  7. The membrane should be easy to apply
  8. The membrane should be durable, abrasion, puncture and impact resistant throughout a wide temperature range
  9. The membrane should set up or cure quickly even in low temperatures
  10. The membrane should be UV stable
  11. The membrane should be dimensionally stable when applied to a hard surface
  12. The membrane should not be chemically reactive and must tolerate standing water

For concrete applications, polyurea or polyurea hybrid fulfills the 12 previously listed criteria. 

Polyurea or polyurea hybrid is somewhat of a commodity product that has been used throughout the world for decades; and although is it good, it is not perfect. It has its limitations like anything.  The fluid applied system is easy to apply with a  spray nozzle. An $80,000 specialized mixer and applicator machine that controls temperature, pressure and proportion of the two chemical components that are each pumped through heated 2” diameter. On large jobs with expansive square feet, the polyurea machine, power generator, air compressor, drums of component chemicals and hundreds of feet of hose are included on a trailer or box truck; the hose must reach from the trailer to the roof.  There is a 400 ft hose limit (smaller machines or machine on carts are available for smaller jobs.)

Polyurea or polyurea hybrid is a chemically cured product that requires the precise mixing of two components that are heated, mixed and sprayed under pressure through a gun that is operated by one technician. The proportioning and other variables must be finely adjusted according to the ambient temperature and relative humidity.  If the settings are off, the material may not bond or properly cure. Fortunately, it will be obvious if the settings are off as the material will not set up properly. The polyurea applicator machines will shut off if the mixing ratio of components is not proper.

When applied as specified, the 100 mil thickness membrane tenaciously adheres to masonry surface and sets up within 4-15 seconds to a glossy, tough, membrane finish.  In fact, if properly adhered, you need a jack hammer to remove it.

One manufacturer developed a unique turnkey option to combine the material and installation costs by offering rental of the special machine and the skilled operator to tweak the machine. The applicator provided by the manufacturer have has helped ensure a better rate of quality control/quality assurance.  Once dialed in, the installation process can permit that one machine and a two-man crew with machine technician can spray about 5000 sf a day.

Most polyurea membranes are UV stable but the warranted system does require an epoxy primer and a polyurethane top or wear coat.  The system is typically warranted for 30 years and is comparative in cost to similar resin-based membranes, when compared on larger scale projects.

Another major advantage over other resin-based systems is that there is no noxious chemical odors. This is especially important in a congested urban environment where there is a heightened sensitivity to potentially toxic chemicals.  Spray applied polyurea or hybrid polyurea can even be installed indoors with the windows closed.

This system can be an ideal membrane for concrete substrates in plaza or green roof applications; the membrane typically does not require a root barrier and it is well-suited for standing water.

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