Built-Up Roofing PDF

# Built-Up Roofing ## Chapter 5 ### Today's low-slope roofing technology offers several basic choices to the contractor, architect, and building owner: * single-ply * overlapped and heat-sealed * modified bitumen roofing (MBR) applied as one-, two-, or three-ply systems * built-up roofing (BUR...

# Built-Up Roofing ## Chapter 5 ### Today's low-slope roofing technology offers several basic choices to the contractor, architect, and building owner: * single-ply * overlapped and heat-sealed * modified bitumen roofing (MBR) applied as one-, two-, or three-ply systems * built-up roofing (BUR) utilizing bitumen asphalt or coal tar as the waterproofing and adhesive agents. Each system offers advantages for specific applications. As stated in Chapter 1, BUR is by far the oldest of the modern low-slope methods and is still a popular commercial application (Fig. 5-1). BUR consists of layers of reinforcing felt sandwiched between bitumens. Asphalt and coal tar are the bitumens of choice. These thermoplastics become more fluid with heat and revert to a more solid state as they cool. They are versatile waterproofing materials. ### Welding the Plies The plies are welded when heated, mopped bitumen melts, and then fuses with, the saturant bitumen in the roofing felts. Therefore, correct application temperatures are vital to the creation of a quality roof membrane. A high bitumen temperature must be maintained to create the welding process. ### Properties of Bitumen The properties of bitumen are such that bringing it to a high temperature for a long period of time can reduce the softening point of asphalt and raise the softening point of coal-tar bitumen. Bitumens can be kept at high temperatures for short periods of time without damage. In fact, they must be heated to high temperatures in order to achieve complete fusion and strongly bond the plies. ### Manufacturing Asphalt Asphalt is made up of three elements: saturants, asphaltenes, and resins. * Saturants are light oils that make the asphalt soft and flexible so that it expands and contracts with the application surface. * Asphaltenes provide the body, rigidity, and strength. * Resins bond the saturants and asphaltenes and furnish the asphalt with its resilience. The crude oil source contributes to the quality of the finished asphalt product just as much as the manufacturing processes do. Different crudes impart distinct characteristics. For example, some California crudes have relatively low saturants and flash points, typically 460° to 525°F. The flash point is the temperature at which the asphalt's light oils begin to evaporate and can be ignited. As the flash point increases, the chance of combustion and the vaporization of light oils both decrease. The result is a harder flux that is better suited for paving applications than for roofing applications. The best roofing-grade asphalts are produced from the softer crude sources, or from blends that incorporate a sufficient percentage of soft material to meet industry standards and provide the anticipated service life. After the crudes are selected, the process of blending and blowing the crude-oil fluxes determines the characteristics of the finished asphalt product. Manipulating these two processes produces the following for grades of asphalt. * **Type I (dead level)**: This grade of asphalt is relatively susceptible to flow at roof temperatures and has good adhesive and self-healing properties. It generally is used in aggregate-surfaced roofs on slopes up to ½ inch per 12 inches. * **Type II (flat)**: Type II asphalt is moderately susceptible to flow at roof temperatures. It generally is used on roofs with slopes between ½ inch per 12 inches and 1½ inches per 12 inches. * **Type III (steep)**: Steep asphalt is relatively nonsusceptible to flow at roof temperatures. It generally is used on roofs with a slope between 1 and 3 inches per 12 inches. * **Type IV (special steep)**: This asphalt is relatively nonsusceptible to flow at roof temperatures. It is useful in areas where high year-round temperatures are experienced. It generally is used on roofs with a slope between 2 and 6 inches per 12 inches. In 1977, the National Roofing Contractors Association (NRCA) adopted the Equiviscous Temperature (EVT) concept. This followed the determination that the flow characteristic, or viscosity, of roofing bitumens at the point of application to the substrate is an important factor in the construction of BUR membranes. EVT was then defined as the temperature at which the viscosity of a bitumen is sufficient to support the proper adhesion, waterproofing, and application rate. If the EVT information is not furnished by the manufacturer, use the temperature given in Table 5-1 as a guide. ### Manufacturing Coal Tar Coal tar is a byproduct of the coking of bituminous coal during steel making. Coal tar offers superior resistance to the elements. Its chemical structure consists of closed, ring-like molecules that make it highly stable and resistant to aging, chemical breakdown, and moisture. Coal tar is known for its resistance to ponded water. Another advantage of coal tar is its cold flow property, or the ability to self-heal cracks and fissures. Traditionally, coal tar's flowability has kept it from being a choice product for roofs with more than minimal slope. As a liquid solid, it just does not stay put. BUR technology developed partly in response to this fact. Several thin layers of bitumen, alternating with felt plies, form a strong, flexible, waterproof membrane that stays in place, at least on gently sloping roofs. Three types of coal tar are presently used for BUR, dampproofing, and waterproofing systems: * **Type I coal-tar pitch**: used in BUR systems with coal-tar saturated felts. * **Type II coal-tar pitch**: used in dampproofing and membrane waterproofing systems for below-grade and plaza deck waterproofing. In these types of constructions, the bituminous materials are combined with plies of coal-tar-saturated felts and fabrics. * **Type III coal-tar bitumen**: used in BUR systems, has a slightly higher softening point than Type I coal-tar pitch. Coal-tar products are described by the American Society for Testing and Materials (ASTM) Standard D-450, Types I and III, Coal Tar Pitch Used in Roofing, Dampproofing and Waterproofing. Types I and III are suitable for use in the construction of coal-tar BUR, the difference being that Type III has less volatile components than Type I. The EVT concept is equally applicable to coal-tar products, although the recommended viscosity base is considerably lower for the coal-tar products than for roofing asphalts. If the information is not furnished by the supplier, the following temperature ranges are appropriate for both mechanical spreader and mopping application techniques: * Coal-tar pitch (Type I), approximately 360°F, plus or minus 25°F * Coal-tar bitumen (Type III), approximately 375°F, plus or minus 25°F ### Selecting Felts and Base Sheets Several materials are used as reinforcing sheets and base sheets in BUR construction. | Material | Description | | :---: | :---: | | Saturated Roofing Felts | Either asphalt or coal-tar bitumen can be used as a saturant for organic felts. Organic felts are manufactured from the fiber of paper, wood, rags, or a combination of the three. Prior to saturation, organic felts are described by a number that indicates the weight of the felt. For example, an unsaturated organic felt might be designated as 27-gauge. This means that the unsaturated product weighs approximately 27 pounds per 480 square feet of paper measure and is a nominal 27 mils in thickness. The saturated product is generally referred to as a nominal number. For example, No. 15 in a No. 15 asphalt-saturated organic felt describes the nominal, or approximate, weight in pounds of one ply of No. 15 asphalt-saturated organic felt needed to cover one roof square, or 100 square feet. | | Saturated and Coated Felts | Saturated and coated felts can be manufactured from either organic or inorganic materials. Generally, they are factory-coated on both sides and are surfaced on one or both sides with very fine mineral sand or other release agents to prevent adhesion inside the roll prior to application. | | Impregnated Felts | These felts are generally lighter in weight than organic felts. They are termed impregnated, rather than coated, because the surface is not completely covered with asphalt. These felts usually provide an open, porous sheet through which vapors can be vented during membrane application. | | Prepared Materials | Prepared roofing materials are saturated and coated felts with talc, mica, sand, or ceramic granules incorporated into their weather surfaces. This provides both weather protection and decoration. Prepared roofing materials can be manufactured from organic or glass-fiber base felts or a combination thereof. A commonly used prepared roofing material is the reinforced flashing membrane, which consists of a glass-fiber base felt that is laminated with cotton or glass-fiber fabric and coated with asphalt. Prepared roofing materials are usually packaged in rolls that are one roof square, or 100 square feet, in size. | | Asphalt-Saturated Organic Felts | No. 30 Asphalt-Saturated Organic Felts: These medium-duty, unperforated felts are used as the underlayment for shingle, slate, and tile work and, in some cases, as the base ply in BUR systems. | | Asphalt-Saturated and Coated Organic-Felt Base Sheets | No. 40 Asphalt-Saturated and Coated Organic-Felt Base Sheets: Generally, these materials are used as the first ply or base layer for some BUR membranes. They are also used for temporary roofing or temporary flashings and as vapor retarders beneath insulation. These sheets are designed to prevent the intrusion of moisture from below the roof and to minimize the occurrence of ridges and wrinkle cracking. They also are incorporated where the designer requires a tougher or thicker base to help span joints in the deck or substrate immediately below the roof membrane, and where mechanical fastening of a base ply is required. | | Asphalt Glass-Fiber Mat Sheet | This material is composed of a glass-fiber mat that is impregnated and coated on both sides with asphalt. It is available in perforated or unperforated forms and is designed for use as a base ply in the construction of BUR membranes. | | Venting Asphalt-Saturated and Coated Inorganic Sheets | Several manufacturers make a special, heavy-gauge felt to vent certain roof conditions. The venting characteristic of the felt is assisted by mineral granules on the underside of the felt. These venting sheets are, in some cases, specified for use over damp or concrete substrates as an underlayment or base ply. The sheets can be nailed in place and/or spot-mopped with hot bitumen. If a vented base ply does not have a buttonhole attachment and is to be mopped in place, spot mopping is recommended, preferably with a mechanical spot mopper. This prevents solid mopping or strip mopping of the asphalt under the base sheet, which restricts the venting properties of the base sheet. If employed in a BUR system, these sheets provide for the venting of moisture vapor pressure by lateral, or horizontal, movement. | - **No. 15 asphalt-saturated organic felts** are manufactured as: * perforated felts, which are ply marked for use in two-, three-, or four-layer applications. * unperforated felts, which are generally used as the underlay-ment of shingles and for general building paper usage. - **No. 15 coal-tar saturated organic felts** are ply marked for two-, three-, or four-ply applications and are commonly used in waterproofing specifications and in coal-tar BUR specifications. - **Asphalt-impregnated glass-fiber felts** are used in the construction of BUR membranes. - **Asphalt-impregnated glass-fiber felts (Type IV)** are slightly heavier than the Type III asphalt-impregnated glass-fiber felts. They have a breaking strength in both longitudinal and transverse directions that is nearly twice that of Type III asphalt-impregnated glass-fiber felts. ### Caps and Flashings Several caps and flashings are available for use in BUR applications. * **Wide-Selvage Asphalt Cap:** This roll roofing material is composed of an asphalt-saturated felt that is coated on approximately one-half of the weather side. The asphalt-coated surface is then treated with colored mineral granules. It is generally used for two-layer applications over a base ply or as succeeding sheets over the initial plies of a BUR system. * **No. 90 Mineral-Surfaced Roll Material:** This product is manufactured either with no selvage or with a 2-inch selvage, depending on the manufacturer. It is supplied with various ceramic-colored granule surfacings, and is used as an organic roll roofing material and specified for a variety of flashing applications. * **No. 50 Smooth-Surfaced Material:** This roll asphalt product is used in some roof membrane specifications as a heavy-duty base sheet. It is also useful in the construction of temporary roofs and for flashing applications. * **No. 60 Smooth-Surfaced Material:** This roll roofing product is an organic felt that is saturated with a mineral-stabilized asphalt compound and surfaced with talc or mica. It is used when inexpensive roll roofing is desired, as well as for flashings and temporary roofs. * **Inorganic Cap Sheet:** This roll material is composed of a heavy-weight inorganic felt that is saturated and coated on both sides with asphalt and then surfaced on the weather side with various colored mineral granules. It is used as a top, or cap, surface over other BUR plies, for flashings, for granular-surfaced lightweight roof system constructions, and for decorative purposes. * **Reinforced Bituminous Flashing:** This material is a heavy saturated felt that is reinforced with glass fiber. The material is surfaced with fine mineral matter. ==End of OCR for page 1==