Bitumens are mixtures (in gaseous, liquid, semi-liquid or solid form) consisting primarily of hydrocarbons. They are thermoplastic, becoming viscous fluids with heat and reverting to more solid material as they cool. Their adhesive and waterproofing properties have been utilized in construction for hundreds of years. The most common of the many forms of bitumens are tars, pitches and asphalts. The roofing industry in Canada uses two forms of bitumens for built-up roofs:
- Asphalt (commonly used)
- Coal tar pitch (rarely used)
(In British Columbia coal tar pitch is not used in new construction, therefore, detailed information and specifications have been omitted.)
Asphalts are dark brown to black bitumens derived from crude petroleum oils. They are produced naturally or by industrial distillation and are usually in a solid or semi-solid state at ambient temperatures.
Distillation of crude petroleum oil removes gasoline and other volatile substances, producing “straight-run asphalt” (used in paving). Roofing asphalt is further refined by blowing air through molten asphalt or asphaltic flux at an elevated temperature (finished blowing temperature or FBT) to produce controlled oxidation. The oxygen reacts with the bitumen, removing a portion of the hydrogen, physically changing the asphalt from a liquid to a gel. This process raises the softening point (SP) producing a “harder”, less temperature sensitive asphalt more suitable for roofing. The final product is known as either oxidized asphalt, air blown asphalt, or roofing asphalt and is available in 45 kg (100 lb) “kegs” or by bulk in heated tanker trucks.
Although both asphalt and coal tar pitch are bituminous hydrocarbons, their chemical properties vary greatly and are incompatible for roofing purposes. Incompatibility can even result between two asphalts or two coal tars which have been obtained from different “crude” sources. For this reason it is best to specify that the bitumen used on a project be obtained from one source. Similarly, felts saturated with one type of bitumen should not be mixed with felts or bitumens of the other.
Incompatibility occurs when a chemical reaction between the bitumens results in one bitumen hardening and the other softening. In basic terms, the oils from one bitumen are “absorbed” into the other bitumen and the two products do not adhere properly. The possible results of this reaction are as follows:
- Asphalts applied over pitches will soften and flow off, leaving the hardened pitch exposed to rapid weathering.
- Coal tar pitch will harden and crack.
- Poor adhesion may result in membrane slippage and poor wind uplift resistance.
It is extremely rare to have coal tar pitch specified on new roofing projects in British Columbia. Most compatibility problems result from roof replacement projects or roofing over existing coal tar pitch roofs. In these cases a separation layer, such as insulation board, is often used to prevent the asphalt system from contacting any traces of the coal tar pitch roof. Consultation with a Professional Roofing Consultant is strongly recommended.
The purpose of bitumens in a built-up roofing system is not to “glue” the plies of felt together, but to fuse with the saturant bitumen in the felts, essentially “welding” the plies together. Therefore, correct application temperature is vital and must be maintained to ensure fusion occurs.
Temperature affects the viscosity, or flow, of bitumen and, thus, the mopping weight. Temperatures that are too high (bitumen that has low viscosity and high flow) can lead to:
- light moppings
- incomplete film coverage
- a potential lack of waterproofing qualities
Temperatures that are too low (bitumen that has high viscosity and insufficient flow) can lead to:
- heavy moppings
- poor adhesion
- potential slippage problems
- high expansion properties
- low tensile strengths
All of these may contribute to roof splits and other problems. Obviously, an optimum viscosity range, and therefore an optimum temperature range, exists at the point of application for achieving complete fusion, optimum wetting and mopping properties, which results in the desirable inter-ply mopping weight.
The EVT is the temperature at which asphalt will attain a target viscosity of 125 centistokes. A tolerance range is added for practical application in the field to accommodate the effects of wind chill, sunshine or ambient temperature. This range is expressed as a temperature, plus or minus 15oC (25oF).
Asphalt cools rapidly once it has been applied, making the roofing felt or membrane application critical. The National Bureau of Standards (U.S.A.) has published the following test results:
|CONDITIONS||SUBSTRATE||TEMPERATURE DROP||DROP INTERVAL|
|Air and Substrate Temperature: +21°C (+70°F)
Wind Speed: 16 km / hr (10 mph) Quantity of Asphalt: 1 kg / meters squared (20 lb / square)
|On Felt||+260°C (+500°F) to +149°C (+300°F)||11 Seconds|
|Over Plywood||+260°C (+500°F) to +149°C (+300°F)||31 Seconds|
|On Fibreglass||+204°C (+400°F) to +149°C (+300°F)||5 Seconds|
|Insulation||+204°C (+400°F) to +149°C (+300°F)||16 Seconds|
Obviously these figures indicate that the asphalt should not be mopped very far ahead of the roll, and that the felt / membrane should be applied promptly.
Although it is very important that the asphalt is sufficiently hot, care must be taken to ensure the asphalt is not overheated. The oxidation process, described earlier, produces asphalts with specific properties for roofing purposes. These properties can be altered if the asphalt is subjected to extreme temperatures for extremely long periods of time. However, ordinary asphalt heating temperatures are not damaging to the material and are required for complete fusion and strong bonding of the plies.
Bitumen heating is subject to two restraints:
- Bitumen should not be heated to or above the actual COC Flash Point (FP). (ANSI / ASTM Test Method D-92, Test for Flash Point by Cleveland Open Cup.)
- Bitumen should not be heated and held above the Finished Blowing Temperature (FBT) for more than four hours.
The following information should be printed on all asphalt packages or bills of lading covering bulk asphalt:
- Softening Point Range (SP): The temperature range of the asphalt determined in accordance with ASTM Standard D-312 and D-36.
- Flash Point (FP): The flash point of the asphalt as determined by ASTM Standard D-92.
- Equiviscous Temperature (EVT): The temperature range, plus or minus 15°C (25°F), at which a viscosity of 125 centistokes is attained.
- Finished Blowing Temperature (FBT): The temperature at which the blowing of the asphalt has been completed.
Four types of asphalt are presently in use for the construction of built-up roof systems:
- Type 1 (Dead Level) asphalt is relatively susceptible to flow at roof temperatures with good adhesive and self healing properties. It is rarely used in British Columbia.
- Type 2 (Flat) asphalt is moderately susceptible to flow at roof temperatures. It is used on roofs with slopes up to 1:12 (1" in 12").
- Type 3 (Steep) asphalt is relatively non-susceptible to flow at roof temperatures. It is used on roofs with slopes exceeding 1:12 (1"in 12") up to 1:4 (3" in 12").
- Type IV (Special Steep) asphalt is relatively non-susceptible to flow at roof temperatures. It is useful in areas where relatively high year-round temperatures are experienced.
Note: Check with the membrane manufacturer's technical representative when selecting or specifying the type of asphalt to be used with a bituminous or modified bituminous flexible membrane system.)
The following chart lists the minimum and maximum softening point temperatures for the various types of asphalt.
|Type||Kind of Asphalt||Minimum||Maximum|
|Type 1||Dead Level||60°C (140°F)||68°C (154°F)|
|Type 2||Flat Asphalt||75°C (167°F)||83°C (181°F)|
|Type 3||Steep Asphalt||90°C (194°F)||98°C (208°F)|
|Type 4||Special Steep Asphalt||99°C (210°F)||107°C (225°F)|
The asphalt materials discussed above should meet or exceed the standards set forth in CSA A123.4-M1979 for Types 1, 2, and 3 or ASTM Standard D312-78 for Type IV.
Asphalt may be adapted, by combination with other materials, to suit varied applications. Only CGSB tested products should be used or specified. These products include:
- Cutback Asphalt Plastic Cement (CGSB 37-GP-5Ma) is composed of asphalt and solvents sometimes mixed with asbestos fibre or other mineral stabilizers to reduce sagging on vertical surfaces. CGSB defines it as being used “for filling and sealing joints in masonry, wood or metal building construction, sealing and coating flashings around buildings, embedding glass, and repairing leaks and cracks of all types and tabbing down strip shingles”. Also referred to as roofing cement or roofing mastic. (To be applied in accordance with CGSB 37-GP-11M.)
- Asphalt Primer (CGSB 37-GP-9Ma) is defined by CGSB as being used “for priming surfaces prior to the application of roofing”, asphalt and solvents combined to form a liquid used to prime surfaces (concrete, wood and metal) receiving direct application of bituminous roof membranes. (To be applied in accordance with 37-GP-15M.)
See also Membrane Surfaces.
In a built-up roofing system the asphalt provides the waterproofing and the felts act as the reinforcement. The felt stabilizes the asphalt, provides the strength to span structural irregularities, and distributes strains over a larger area.
A felt is defined by CSA as “a flexible sheet built-up by the interlocking of fibres (organic or inorganic) by any suitable combination of mechanical work, moisture and heat but without weaving, spinning, or knitting”. Felts for use in roofing are either saturated or saturated and coated with asphalt.
These materials generally fall into one of the following categories:
- Organic felts
- Glass Fibre felts
- Polyester felts
Organic felts are defined by CSA as a “felt fabricated essentially from fibres of vegetable or animal origin.” In Canada, roofing felt is most commonly manufactured from a mixture of wood fibre (cellulose) and recycled paper pulp.
Organic felts for use in built-up roof membranes include:
- No. 15 Perforated Asphalt Felt (conforming to CSA A123.3-M1979) is the most common felt for built-up roofing. It is an asphalt saturated felt and is perforated to prevent gases being trapped underneath the felt during application. Commonly referred to as “fifteen pound felt”.
- No. 30 Asphalt Saturated Felt (conforming to CSA A123.3-M1979) is similar to No. 15 Felt but heavier in weight for a higher breaking strength. It is most commonly used as a separator or base ply mechanically fastened to a wood deck. Commonly referred to as “thirty pound felt”.
- No. 40 Asphalt-Coated Base Sheet, conforming to CSA A123.2-M1979, Type CF, is defined by this CSA standard as “a single thickness of asphalt-saturated felt, coated on one or both sides with asphalt and surfaced with fine mineral matter.” Generally used as the first ply or base sheet in a built-up roof membrane where a stronger material is required to span deck joints or where mechanical fastening of the base ply is required. Commonly referred to as a “forty pound base sheet” or “43 lb. Sheet”.
- No. 50 Smooth Surfaced Roofing, conforming to CSA A123.2-M1979, Type S, is defined by this CSA standard as “a single thickness of asphalt-saturated felt, coated on both sides with asphalt, and surfaced on both sides with fine mineral matter.” It can be used as a heavy-duty base sheet, although it is not common.
- No. 90 Mineral Surfaced Roofing, conforming to CSA A123.2-M1979, Type M, is defined by this CSA standard as a “single thickness of asphalt-saturated felt, coated on both sides with asphalt, and surfaced with mineral granules on the entire weather side, except that a lapping edge not more than 102 mm wide may be left bare of granules and may be either asphalt coated or uncoated.” Commonly referred to as a “ninety pound” or “ninety pound mineral”.
- Wide Selvage Roofing, conforming to CSA A123.2-M1979, Type MC, is similar to No. 90 Mineral Surface Roofing, but it is defined by the CSA standards as surfaced “on the weather side with mineral granules for half the width less 25 mm (1"). The material used as a parting agent shall not hinder proper adhesion in membrane construction.”
Glass Fibre Felts (Type IV and VI)
Glass ply felts (Type IV and VI) consist of a non-woven glass fibre mat impregnated with asphalt. They are called impregnated rather than coated because their surface is not completely coated with asphalt. This provides an open porous sheet through which vapours may be vented during membrane application. These felts may provide higher tensile strengths and do not “wick” moisture into the roof membrane like organic felts sometimes can. Glass fibre felts should meet or exceed the standards set forth in ASTM D-2178, Type IV and VI.
Polyester felts consist of a spun bond polyester reinforcing mat that has been saturated with non-oxidized asphalt or modified asphalt.
Where a built-up roof membrane is applied directly to a wood or plywood deck, a mechanically fastened “base ply” may be used in lieu of a coated base sheet. Only RoofStar-accepted base plies may be used; install according to built-up roofing specifications.