The Rundown on Low-Slope Roofing

March 1, 2009

Find out what you need to know about all kinds of low-slope roofs

Typically, low-slope roof assemblies are composed of three interrelated components: the roof membrane, roof insulation, and the roof deck. The roof system is defined as the roof membrane (including surfacing) and the roof insulation. The roof assembly is defined as the roof deck, along with the roof membrane and roof insulation. Low-slope roofs are often referred to as flat roofs. Roof assemblies with a pitch (slope) of less than 3:12 are considered low-slope roofs. Generally speaking, roof assemblies are designed in two basic configurations: compact (“warm”) roofs, and ventilated (“cold”) roofs. Referencing The National Roofing Contractors Association’s Roofing & Waterproofing Manual (5th Edition), compact and ventilated roofs are defined as follows:

  • Compact or warm roof designs are configured with each component placed immediately on top of the preceding component. The insulation is placed directly on top of the deck (or vapor retarder), and the membrane is applied directly on top of rigid insulation. The name compact is given to these systems because each component is in immediate contact with the adjacent component, and the assembly is thus compact – with no space provided for ventilating the roof assembly.
  • Ventilated or cold roof designs locate the insulation below the deck, allowing for a ventilation space. This space or cavity for ventilation typically occurs above the insulation and below the deck. In ventilated roof designs, the temperature of the membrane and the deck remains close to the outside air temperature, which, in many climates, is typically colder than the temperature inside the building for most months of the year.

Enemy No. 1: Weather

According to the Rosemont, IL-based National Roofing Contractors Association, even roofs have enemies, including everyday weather. Learn how the elements affect your roofing system.

Sun. Heat and ultraviolet rays cause roofing materials to deteriorate over time. Deterioration can occur faster on the sides facing west or south.

Rain. When water gets underneath roofing materials, it can work its way to the roof deck and cause the roof structure to rot. Extra moisture encourages mildew and rot elsewhere in a building.

Wind. High winds can lift the edges of roofing materials (including shingles) and force water and debris underneath them.

Snow and ice. Melting snow often refreezes at a roof’s overhang, where the surface is cooler, forming an ice dam. This blocks proper drainage into the gutter. Water backs up under roofing materials and seeps into the interior. During the early melt stages, gutters and downspouts can be the first to fill with ice and be damaged beyond repair, or even torn off of a building.

There are several factors that need to be considered when deciding which roofing system most completely meets the needs of a particular facility. Some of these conditions are general; others are specific to the building in question. Basic descriptions of various low-slope roofing systems are as follows:

PVC roofing system. Polyvinyl chloride (PVC) roofing membranes have been produced and marketed for more than 40 years. Production began in Germany in the 1950s, with major commercial production beginning in the early 1970s. The products evolved over the years. PVC sheets range in thickness, from 45 to 90 mils. The success of some PVC membranes is due to thickness, better quality in blending and manufacturing, and reinforcement. The reinforcement is either glass fiber or polyester.

PVC roofing membranes are considered thermoplastic materials. Because of the material’s chemical nature, some thermoplastic membranes may be seamed by heat welding (hot air as opposed to glue), seaming with tape products, or solvent welding. The seam is almost indestructible when properly made, and does not fail when under water for extended periods of time. Ponding water exclusions aren’t part of the warranty. Many reinforced PVC roofing membranes perform properly with a life of 30 years or longer.

PVC roofing membrane sheets are produced by calendaring, spread coating, or extrusion, and are typically reinforced with a fabric mat or scrim. PVC sheets contain plasticizing additives to impart flexibility to the membranes, which are incompatible with bituminous membranes, such as asphalt and coal tar. Separator sheets, or felt-backed or specially formulated membranes, are required when incompatible products, including polystyrene insulations, are present.

Some PVC membranes are available with factory-applied, non-woven fleece backing adhered to the underside of the sheet. Some PVC membranes are available with a factory-applied adhesive backing and release film adhered to the underside of the sheet (referred to as the adhered peel and stick system). PVC membranes can be produced in numerous colors, although gray and white are the most common.

PVC membranes may be installed in four general configurations: adhered, mechanically attached, ballasted, or as a protected roof membrane assembly. PVC membrane systems often utilize PVC-coated metal for perimeter terminations (edge metal, gravel stop, parapet cap, etc.) and certain flashing details. In this application, the PVC roof membrane is bonded to the PVC-coated metal by hot-air welding or using a solvent. Various metals may also be used, but they require adhesive to bond to the PVC membrane.

EPDM roofing system. EPDM is an elastomeric compound synthesized from ethylene, propylene, and a small amount of diene monomer; it’s a synthetic rubber material that can be formulated with a great deal of flexibility for use in roofing. EPDM membranes exhibit a high degree of ozone, ultraviolet, weathering, and abrasion resistance, and have good low-temperature flexibility. EPDM’s properties of resilience, tensile strength, elongation, and hardness are largely retained in aging tests at elevated temperatures. EPDM falls into the thermoset membrane category. Thermoset membranes are those whose principal polymers are chemically cross-linked. This chemical cross-linkage is commonly referred to as vulcanization or curing.

EPDM has been used as a roofing material in the United States since the early 1960s. EPDM sheets range in thickness, from 30 to 90 mils, both reinforced and non-reinforced, and are usually black or white in color (sheets formulated with titanium dioxide produce a white membrane); they can also be coated to create an aesthetically pleasing appearance. EPDM is the most often installed single-ply roofing membrane system, accounting for about 40 percent of the commercial roofing market.

The seams of EPDM roofing systems must be adhered. Early on, EPDM systems experienced seam problems (primarily as a result of poor field cleaning of the seams and adhesive degradation). Changes in the surface preparation of sheets, new adhesive formulation, and the development of tape adhesives have greatly increased the performance of EPDM seams.

EPDM is reported to resist some acids, alkalis, and oxygenated solvents, such as ketones, ester, and alcohols; however, EPDM is susceptible to degradation from aromatic, halogenated, and aliphatic solvents, as well as animal and vegetable oils, and some other oils and greases. Epichlorohydrin (ECH) can be formulated to resist hydrocarbons, solvents, and some greases and oils. ECH is typically used with EPDM membranes wherever damaging substances may be discharged onto the roof. ECH membranes are typically seamed using the manufacturer’s adhesive.

EPDM membranes may be installed in four general configurations: adhered, mechanically attached, ballasted, or as a protected roof membrane assembly.

TPO roofing system. Thermoplastic olefin (TPO) membranes are produced from polypropylene and ethylene-propylene polymers, and may include flame retardants, pigments, UV absorbers, or other ingredients as part of their formulations. TPO sheets range in thickness, from 45 to 80 mils. TPO is considered the new membrane, with claims of recyclability and environmentally friendly materials. Currently, little is known about TPO’s long-term performance.

TPO-based products have been used in various applications, including the automobile industry, since the 1980s. In 1989, TPO-based membrane appeared in the roofing industry as a non-reinforced sheet. In 1993, the original non-reinforced TPO membrane was replaced with membranes containing reinforcing fabric. Since that time, the TPO single-ply roofing market has grown significantly, and some industry reports state that TPO is the fastest-growing segment of the U.S. single-ply roofing industry.

State-of-the-art polymer manufacturing technology has produced TPO membranes that are flexible at low temperatures without the use of polymeric or liquid plasticizers. Unlike other popular thermoplastic roofing membranes, TPO polymer doesn’t contain chlorine, and no chlorine-containing ingredients are added during sheet production. This lack of chlorine has allowed TPO marketers to tout their membrane as an environmentally safe, green product.

TPO membranes are being marketed as the new-generation membranes that combine attributes of two of today’s popular single-ply membranes (EPDM and PVC) without these materials’ associated drawbacks.

TPO membranes may be installed in four general configurations: adhered, mechanically attached, ballasted, or as a protected roof membrane assembly. TPO membranes can be produced in numerous colors.

Built-up roofing system. The built-up roof (BUR) has been the traditional roofing system for flat roofs in the United States for approximately 100 years. BUR consists of multiple layers of roofing felt (ply sheets) applied in shingle fashion with a waterproofing material (interply adhesive) to form a 2-, 3-, 4-, or 5-ply layer membrane over which a coating, surfacing (gravel), or cap sheet is applied to protect the membrane.

Originally the first BUR roofs were made utilizing coal tar pitch as the interply adhesive, which was heated to a liquid state. The roofing plies were organic (rag) felts. Coal-tar pitch roofs were called “self-healing roofs” due to coal tar’s tendency to flow when hot (good, because it flows and seals cracks; bad, because it flows and clogs drains, causes stains on buildings, and, in cold temperatures, becomes brittle and cracks).

After WWII, the abundance of petroleum was responsible for asphalt replacing coal-tar pitch as the interply adhesive and waterproofing agent. With the evolution of asphalt-based built-up roofing, fiber glass roofing felts were introduced, which are stronger than organic felts. Fiber glass felts were responsible for the industry shift toward fewer numbers of plies. This concept was primarily aimed at reducing the labor component involved with BUR installation.

Both asphalt and coal-tar pitch are hot-applied at high temperatures, which is critical to the success of the system. The acceptable temperature range for installation of these materials is called the equiviscous temperature. Basically, it’s the temperature range of the bitumen where it’s hot enough to adequately bond the plies together, as well as provide proper interply waterproofing characteristics.

This temperature range is still the most common problem associated with hot-applied BURs. With a hot-applied BUR system, an asphalt kettle or tanker is required to remain on-site all day, which can pose certain safety and odor problems within occupied facilities.

In the early 1970s, several manufacturers started developing modified asphalt products, which led to cold-process built-up roofs. Basically, the interply adhesive is cold applied (spray or squeegee), thereby eliminating kettles and tankers. The backbone of the cold-process roofing system includes the use of four layers of a trilaminate-reinforced (polyester/fiberglass/polyester) roofing ply sheet. This key component of the roofing assembly provides strength, waterproofing, and stability to the cold-process roofing system. The reinforced composite roofing ply sheet is set in cold-process interply adhesive. Gravel surfacing is then set in a protective flood coat. This proposed roof system has many layers of protection and, installed properly, is extremely durable and long lasting.

Modified bituminous roofing system. Polymer-modified bituminous roofing membranes were developed in Europe in the 1960s, and were introduced to the U.S. market during the “single-ply revolution” of the mid- to late-1970s. Because the product was introduced at nearly the same time as conventional single-ply membranes, polymer-modified bitumen membranes and single-ply membranes were often classified together, largely because they were viewed as different from built-up membrane systems.

However, history has shown that these “prefabricated” roofing sheets should be used as multiple-ply systems. Most manufacturers now require at least a base sheet below their modified sheets. In the 1980s, these products became more widely accepted in the U.S. market, and they became their own membrane type, separate from single-ply membranes. In the late 1980s and early 1990s, the use of polymer-modified membrane roof systems grew significantly, including two roof system configurations: polymer-modified bitumen roof systems and the top layer in multiple-ply built-up roof membrane (or hybrid) systems. These systems account for approximately 17 percent of all commercial roofs.

James M. Russo is president at Boston-based Russo Barr Associates Inc..

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