Technical Details: BUR Systems - Back in Black
Chances are that if you know who George Harrison was and what his contribution to culture was you are aware of built-up roof systems. Whereas, if you know who Snooki is and why she matters you probably have never seen a built-up roof system applied.
Built-up roof (BUR) systems have been on the United States commercial roofing market since the late 1800s. They were long considered conventional roof systems. The documented use of bituminous products such as coal tar and asphalt as waterproofing products dates back thousands of years. In the past two decades the use of BUR systems has continually declined. Coal-tar BUR systems now comprise less than 1 percent of the market, while asphalt BUR systems struggle to reach double-digit percentages on an annual basis.
The decline of BUR use has spawned the first generation of roofers that have advanced in their careers without ever applying a BUR system. It is increasingly evident that BUR systems are on the same path as land telephone lines and VCRs.
While their application rates continue to decrease, the longevity of these systems cannot be argued. So while roofing contractors may not have to apply as many of these systems in the future, they will encounter them for repairs and maintenance for decades. In order to properly repair these systems, the new generation of contractors will require a crash course in BUR systems that previous generations learned firsthand.
Built-up roof systems are comprised of multiple plies (three or more) of felt set in an application of a 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 used in built-up roof 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 manufactures 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 built-up roof systems: There are four classifications of asphalts 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 1/2:12 or less.
• Type II (Flat) asphalt is moderately susceptible to flow and is recommended for aggregate surfaced roof systems that have a slope between 1/2:12 and 1 1/2:12 or less.
• Type III (Steep) asphalt is relatively non-susceptible to flow and is recommended for roof systems that have a slope between 1:12 and 3:12 or less.
• Type IV (Special Steep) asphalt is moderately non-susceptible to flow and is recommended for roof systems that have a slope between 2:12 and 6:12 twelve or less.
Asphalt vs. Coal Tar
Asphalt and coal tar pitches are often considered synonymously. The probable reasons for this are twofold. First, to the naked eye and superficial examination, both materials appear similar: they are 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 manufacture 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 or rag tar — just as their parent material, coal-tar — are predominantly composed of 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. Rhetorically, 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 several basic types of roofing felts used in conventional built-up roof systems, they include:
• Asphalt saturated organic felt is available in three types:
1. Type I or No. 15 Asphalt Felt.
2. Type II or No. 30 Asphalt Felt.
3. Type III or No. 20 Asphalt Felt.
• Coal tar saturated organic felt is used in BUR systems with coal tar.
• Asphalt-impregnated fiberglass felt is available in two types:
1. Type IV, which has a minimum tensile strength of 44 lbs/inch.
2. Type VI, which has a minimum tensile strength of 60 lbs/inch.
• Coal tar- impregnated fiberglass felts are coated in varying degrees with coal tar.
• Non-woven polyester felts 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 more than 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.
With the rise 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 three decades due in part to concerns with material shortages and technical variations. Fiberglass felts offered cost savings in manufacturing and were better suited for the lightweight substrates that are common 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.
Silica sand is the prominent component of fiberglass felts and is combined with a number of selected elements and oxides.
Distress Signs of BUR Systems
Look for the following problems during maintenance inspections:
• Blisters are the most frequent distress condition of BUR systems. They can occur due to moisture in the system or from air voids created by improper adhesion of the bitumen and felt.
• Splits occur when the membrane experiences tensile failure. Thermal movement, substrate strain or structural strain typically causes splits in BUR systems. Improper attachment of a roof component can also contribute to this condition.
• Membrane ridges are typically 1/2 inch to 4 inches in height and depict a tent-like appearance. They are common with BUR systems constructed of fiberglass felt and coal tar bitumen.
• Foot traffic, dropped tools, hail and debris cause punctures. The puncture area must be examined for moisture intrusion prior to repair. All wet materials must be replaced.