Low-Slope Roofing Systems: Built-Up Roofs and Modified Bitumen
The roof design decision is the most important decision in the roofing process. It encompasses a number of important issues that can only be determined at this stage of a roof’s progression. Ensuring proper roof performance is the main issue with roof design. Consideration must be taken in determining proper design, material and application techniques.
It is at the design stage that all aspects of the roof system are devised. The roof assembly generally falls into two component groups: substrate and roofing.
The substrate consists of all components that are protected from the environment. This group would include the roof deck, any vapor retarders and the insulation. Roofing is considered to be all of the components that provide protection for the substrate. This group includes the membrane, flashings, exposed trim, drains and all other items required for a complete roof installation.
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 types of conventional roofing systems used in commercial construction. The type of material used in the system determines the application methods. The types of systems include:
• Built-up roof systems.
• Modified bitumen roof systems.
• Single-ply roof systems.
Built-Up Roof Systems
Built-up roof systems have been on the U.S. commercial roofing market since the late 1800s. They are considered conventional roof systems. The documented use of bituminous products such as coal tar and asphalt as waterproofing products dates back thousands of years.
BUR systems are comprised of multiple plies (three or more) of felt set in an application of heated bitumen (coal tar or asphalt). The membrane system is covered with a surfacing of roof aggregate that is set in an application of a heated bitumen (coal tar or asphalt), smooth surfaced or covered with a mineral cap sheet.
Coal Tar in BUR Systems
There are two types of coal tar used in commercial roof applications. The most common formulation is classified as Type I Coal Tar, which has been used since the late 1800s. In the 1980s the coal tar manufacturers responded to concerns over fumes with a low-fume Type III Coal Tar. The basic difference between these types is that there is less cryosyle in Type III Coal Tar. There have been reported problems with Type III Coal tar and Type I is preferable.
Asphalt Used in BUR Systems
There are four classifications of asphalt used in commercial roof systems.
• Type I (Dead Level) asphalt has a low softening point and is recommended for aggregate surfaced roof systems that have a slope of ½ inch in 12 or less.
• Type II (Flat) asphalt is moderately susceptible to flow and is recommended for aggregate surfaced roof systems that have a slope between ½ inch and 1½ inches in 12 or less.
• Type III (Steep) asphalt is relatively non-susceptible to flow and is recommended for roof systems that have a slope between 1 inch and 3 inches in 12 or less.
• Type IV (Special Steep) asphalt is moderately non-susceptible to flow and is recommended for roof systems that have a slope between 2 inches and 6 inches in 12 or less.
Asphalt and coal tar pitches are often thought of as one and the same. The probable reasons for this are twofold. First, to the naked eye and superficial examination, both materials appear similar in black or brown-black thermoplastic substances of relatively high viscosity at ambient temperatures.
Because of similar or parallel type applications, both materials are sometimes considered to be the same. Both materials are used for roofing and as protective coatings for a number of other industrial applications.
However, the similarity ends with the above observations. Regardless of the fact that these materials fall within the broad definition of “bitumen,” they are dissimilar not only in their origin but also in their chemical makeup. Because of physical and chemical differences, it is reasonable to expect that these materials will differ not only in their service behavior, but also in respect to their gaseous or vapor emissions when the materials are heated during manufacturer or application. It can be expected that such emissions will vary not only in quantity but also in their chemical compositional characteristic.
Petroleum asphalt cements and roofing asphalts are derived from crude petroleum oil by a process not involving cracking or thermal conversation. On the other hand, coal tar, used to manufacture pitches and road tars, is obtained by the high temperature carbonization of bituminous coal.
Chemically, pitch and rag tar - just as the parent material, coal tar - are predominantly composed of highly condensed ring aromatic and heterocyclic hydrocarbons. Petroleum asphalts, on the other hand, contain a much higher proportion of high molecular weight paraffinic and naphthenic hydrocarbons and their derivatives.
The physical or mechanical properties of coal-tar products and petroleum asphalts differ greatly. Generally, for a given viscosity level, asphalts are less susceptible to temperature changes and are significantly more sensitive to shearing forces than coal-tar products.
Asphalts exhibit high heat stability, and, when heated under comparable conditions, the viscosity of asphalts changes considerably less than that of pitches or road tars.
The density or specific gravity of asphalts is substantially lower than density of coal tar products. The solubility characteristics of asphalt are highly different from those of pitches or road tars. Finally, asphalts flash at considerably higher temperatures than comparable coal-tar pitches.
Built-Up Roof Felts
The built-up roof membrane consists of the bitumen (asphalt or coal tar) and reinforcing plies of roof felts. In the built-up roof membrane composition, it is the bitumen that provides the waterproofing protection. The roof felt acts as a stabilizer within the system. There are five basic types of roofing felts used in conventional built-up roof systems. They are:
1. Asphalt-saturated organic felt. This is available in three types:
• Type I or No. 15 Asphalt Felt.
• Type II or No. 30 Asphalt Felt.
• Type III or No. 20 Asphalt Felt.
2. Coal tar-saturated organic felt. This is used in BUR systems with coal tar.
3. Asphalt-impregnated fiberglass felt. This is available in two types:
• Type IV, which has a minimum tensile strength of 44 pounds per inch.
• Type VI, which has a minimum tensile strength of 60 pounds per inch.
4. Coal tar-impregnated fiberglass felt, coated in varying degrees with coal tar. There have been a number of reported problems with these types of felts and there use has diminished.
5. Non-woven polyester felts. These have a high temperature dimensional stability when used in hot bitumen applications. However, they are predominately used in cold process systems.
Organic felts have been the traditional reinforcements for built-up roofing felts for over 100 years. They are typically composed of organic materials, such as jute, animal hairs, shredded fabrics and cellulose. Cellulose fibers were initially comprised of recycled newspapers and sawdust. Due to the insurgence of fiberglass felts, the use of organic felts has diminished in the past 30 years. They are now most commonly used in coal tar systems.
Fiberglass felts gained prominence in the United States within the last 30 years due to concerns with organic felts due to material shortages and technical variations. Fiberglass felts offered cost savings in manufacturing and were better suited for the lightweight substrates that are prominent in the United States. The fiberglass felts also have a more inherent resistance to moisture due to the porosity of the sheets. Porosity of the felts is a concern with coal tar, and this marriage should be avoided.
The use of polyester fibers in bituminous felts has increased in the past two decades. Polyester provides advantages attractive to roll good manufacturers. When used in BUR membranes, polyester provides superior thermal shock resistance, excellent fatigue endurance, puncture resistance and crack bridging capabilities.
There are two types of polyester felts: stitchbonded polyester and spunbonded polyester. Stitchbonded polyester is produced with fibers that are 1 to 3 inches, which are used to form non-woven fiber mats. Special needles are stitched through the mats prior to the addition of the chemical binder that improves the strength of the mats. The mats are nonisotropic, which means they are unequal in all directions. In spunbonded polyester, endless filaments are spun and immediately added to a non-woven mat. The binder may be applied to the mat by needle punched stitching or thermal melting of the filaments together. These types of mats are isotropic, which means that they are equal in all directions.
Modified Bitumen Roof Systems
Modified bitumen roof systems were introduced in the United States commercial roofing market in the late 1970s. The technology for modified bitumen systems was developed in Europe in the 1960s. There are two primary types of modified bitumen systems:
• Atactic polypropylene (APP).
• Styrene-butadiene-styrene (SBS). Atactic polypropylene (APP) technology was developed in Italy out of necessity. The Italians did not have the proper blowing equipment to apply blown asphalt on the roof area. This prohibited them from convening conventional asphalt flux (used in built-up roof systems) into a product that was capable of withstanding the high temperatures reached on a roof system. An additive or modifier in the asphalt was required to provide the properties required for a proper rolled roofing product.
They discovered that a waste by product of polypropylene called amorphous polypropylene when added to asphalt over the proper reinforcement sheet provided a sufficient rolled roofing material. The reinforcement is a heavy spunbonded polyester mat. The polyester mat provided the proper durability and elongation stability necessary for the APP modified asphalt.
APP sheets are applied to the substrate by torch application. The APP membrane sheets can be torch applied because the APP modifiers extend the asphalt without changing its basic characteristics. This allows for a thicker asphalt content to be applied to the base of the membrane sheet. The additional asphalt content of the sheet - when torched - provides the adhesive capacity of the membrane to the substrate. The polymer chemistry of APP modified asphalt consists of 25 percent to 30 percent of polypropylene added to asphalt mixed and blended for four to six hours.
Styrene-butadiene-styrene (SBS) membrane sheets were developed by the French and Germans in the 1960s to be applied in their colder climate regions. SBS asphalt uses 10 percent to 15 percent of a rubber polymer called styrenebutadiene- styrene as an additive to the asphalt modifier.
When the rubber polymer is added to the asphalt, it goes through an inversion phase from a compound into an isometric form. This form of modified asphalt has many of the characteristics of rubber, including its excellent elongation capacities.
When applied over fiberglass reinforcement, SBS membrane sheets have the inert capacity to stretch and recover to their original shape. Tests conducted have indicated that SBS modified asphalt with 10 percent rubber content can be stretched to six times its original length without breaking and has the capacity to fully recover to its original shape. SBS membrane sheets can be applied with hot asphalt or solvent-based and waterborne liquid adhesives.
The blending technology of SBS modified bitumen is critical to the success of the sheet. When the bitumen and synthetic rubber polymer are blended, there is a tendency for high polymer liquids not to mix with each other. This causes a phase separation, which can be clearly observed under a microscope. The SBS blend typically consists of the SBS polymer, asphalt and other low-level fillers, such as limestone (15 percent content). Some manufacturers add substances that inhibit the oxidation of the asphalt. The byproducts are blended together with the reinforcement at 350 to 390 degrees for three to five hours.
There are two types of reinforcements that are primarily used with modified bitumen systems: polyester and fiberglass The use of polyester reinforcements is widespread in SBS modified bitumen systems. The primary benefit of using polyester in SBS systems is for its high elongation capabilities. This is well suited to the performance of the SBS modified bitumen coating. Polyester also exhibits high puncture resistance, good tear strength and the durability to withstand heavy roof traffic. Polyester’s ability to withstand high temperatures has made it the predominant choice for use with APP modified bitumen. Polyester can remain dimensionally stable during the torching application or when applied in hot asphalt.
Fiberglass exhibits superior dimensional stability and tensile strength to polyester. Fiberglass is not affected by heat or tension. When a fire-retardant formulation is applied, Fiberglass can provide excellent fire resistance on SBS systems. Fiberglas has excellent tensile strength and when used in SBS sheets they will resist roof movement until the stress building up in the mat forces it to break. Modified bitumen sheets are manufactured to widths of 36 to 39 inches. The thickness of the modified sheets range from 70 to 170 mils.
APP modified bitumen systems are primarily torch applied. Some APP sheets have a specialized backer that allows for the application of the sheet in a cold adhesive. This is a special condition, and APP sheets are typically not compatible with asphalt adhesives.
There are four application methods for SBS modified bitumen roof systems. They can be applied with torches, set in hot SEBS bitumen, set in cold adhesive or self-adhered.
Next Issue: In part two of his series on low-slope roof systems, the author explores single-ply and EPDM Roof Systems.