8.1 Built-Up Roofing (BUR) Systems: Asphalt vs. Coal Tar Pitch, Organic/Glass Felts & Surfacings

Built-Up Roofing (BUR) Assemblies, Bitumen Chemistry & Surfacings

Built-Up Roofing (BUR) is the oldest continuous commercial and low-slope roofing technology utilized across North America, with a track record of more than a century. Built-up roofing is governed by IBC Section 1507.10 (IRC R905.9 for dwellings). Its material standards are in Table 1507.10.2. BUR is the benchmark for redundant, multi-ply waterproofing. Unlike single-ply membranes that rely on a single elastomeric or thermoplastic sheet, a BUR assembly functions as a composite laminated plate: multiple layers of bitumen-saturated reinforcing fabric (plies) are laminated together in the field with continuous moppings of hot liquefied bitumen, crowned by a protective surfacing layer. In Louisiana's extreme subtropical climate—characterized by intense ultraviolet (UV) solar radiation, rapid thermal shocks from afternoon thunderstorms, torrential tropical downpours, and high hurricane wind-uplift pressures—understanding the thermodynamic properties of bitumens, reinforcing felts, and aggregate embedment is essential for licensed roofing professionals.

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1. Principles of Built-Up Roofing & Composite Action

The fundamental engineering philosophy of Built-Up Roofing is redundancy through lamination. If an exterior mechanical puncture or hail strike breaches the top layer of a four-ply BUR assembly, three underlying waterproof bitumen membranes and reinforcing plies remain intact to prevent moisture intrusion into the building envelope.

  ┌────────────────────────────────────────────────────────┐  ◄── Embedded Gravel / Slag Aggregate (UV & Fire Protection)
  ├────────────────────────────────────────────────────────┤  ◄── Flood Coat (about 60 lb/sq asphalt, 75 coal tar)
  ├════════════════════════════════════════════════════════┤  ◄── Ply Sheet #4 (ASTM D2178 Type IV or VI Glass Felt)
  ├────────────────────────────────────────────────────────┤  ◄── Interply Mopping of Bitumen (20–25 lbs/sq)
  ├════════════════════════════════════════════════════════┤  ◄── Ply Sheet #3 (ASTM D2178 Type IV or VI Glass Felt)
  ├────────────────────────────────────────────────────────┤  ◄── Interply Mopping of Bitumen (20–25 lbs/sq)
  ├════════════════════════════════════════════════════════┤  ◄── Ply Sheet #2 (ASTM D2178 Type IV or VI Glass Felt)
  ├────────────────────────────────────────────────────────┤  ◄── Interply Mopping of Bitumen (20–25 lbs/sq)
  ├════════════════════════════════════════════════════════┤  ◄── Ply Sheet #1 / Base Sheet (ASTM D2178 or D2626)
  ├────────────────────────────────────────────────────────┤  ◄── Substrate Mopping or Mechanical Fastening
  └────────────────────────────────────────────────────────┘  ◄── Approved Cover Board / Rigid Insulation Substrate

Composite Structural Mechanics

In a BUR system, the bitumen (asphalt or coal tar pitch) provides the waterproofing agent and adhesive matrix, while the reinforcing fabric (fiberglass or organic felt) provides tensile strength, dimensional stability, and tear resistance. Unreinforced bitumen is brittle in cold temperatures, flows under extreme heat, and cracks when subjected to building structural movement. Conversely, dry reinforcing felts are porous and permit water penetration. When laminated together, the liquid bitumen encapsulates the fibers, creating a monolithic, structural diaphragm that distributes thermal expansion, building settlement, and wind shear forces across the entire roof plane.

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2. Bitumen Chemistry & Classifications: Asphalt vs. Coal Tar Pitch

Bitumens utilized in BUR assemblies originate from two distinct chemical sources: petroleum crude oil refining (asphalt) and bituminous coal coking (coal tar pitch). Each possesses drastically different chemical structures, temperature behaviors, and application constraints.

                                  ┌──────────────────────────────────────────────┐
                                  │    Bitumen Types in Built-Up Roofing (BUR)   │
                                  └──────────────────────┬───────────────────────┘
                                                         │
         ┌───────────────────────────────────────────────┴───────────────────────────────────────────────┐
         │                                                                                               │
┌────────▼──────────────────────────────────────────────┐       ┌────────────────────────────────────────▼──────────────────────────────┐
│         Petroleum Asphalt (ASTM D312)                 │       │           Coal Tar Pitch (ASTM D450 Type I)                   │
├───────────────────────────────────────────────────────┤       ├───────────────────────────────────────────────────────────────────────┤
│ • Refined from crude petroleum vacuum distillation    │       │ • Byproduct of metallurgical coking of bituminous coal                │
│ • Blown with hot air to adjust softening points       │       │ • Highly condensed aromatic ring molecular structure                  │
│ • Graded into Types I, II, III, and IV               │       │ • Exceptional resistance to standing ponding water                    │
│ • Slopes from dead-level up to vertical parapet walls │       │ • Cold-flow self-healing properties in warm temperatures              │
│ • Sensitive to continuous ponding water degradation   │       │ • Limited to very low slopes (code minimum design slope 1/8:12)        │
└───────────────────────────────────────────────────────┘       └───────────────────────────────────────────────────────────────────────┘

A. Roofing Asphalt (ASTM D312)

Roofing asphalt is produced by vacuum distillation of crude petroleum, followed by an industrial oxidation process known as "air blowing." Air is bubbled through molten petroleum flux at temperatures between 450°F and 500°F. The oxygen reacts with asphalt molecules, cross-linking hydrocarbon chains, raising the softening point, and increasing the material's viscosity and toughness.

Under ASTM D312 (Standard Specification for Asphalt Used in Roofing), roofing asphalt is categorized into four distinct grades based on its softening point (measured by the Ring-and-Ball method under ASTM D36):

1. ASTM D312 Type I (Dead Level): softening point about 135°F to 151°F. Soft and self-healing; for dead-level and very low slopes. Rarely used in hot climates such as Louisiana's.
2. ASTM D312 Type II (Flat): softening point about 158°F to 176°F; for low slopes.
3. ASTM D312 Type III (Steep): softening point about 185°F to 205°F. The general-purpose grade for most slopes a BUR sees, commonly used up to about 3:12, and used for base flashings.
4. ASTM D312 Type IV (Special Steep): softening point about 210°F to 225°F. Very stiff, for steeper slopes (commonly up to about 6:12) and hot climates.

The higher the softening point, the less the asphalt flows on a slope or in heat, and the less it self-heals. The roof system manufacturer's specification gives the exact type and slope limits.

B. Equiviscous Temperature (EVT) and Kettle Thermodynamics

To achieve proper lamination and uniform mopping thickness, asphalt must be applied at its exact Equiviscous Temperature (EVT). The EVT is the precise temperature at which a specific batch of bitumen achieves the optimal fluid viscosity for application:

• Hand Mopping EVT: The temperature at which the bitumen reaches a liquid viscosity of 125 centipoise (0.125 Pa·s).
• Mechanical Spreader / Jet-Shooter EVT: The temperature at which the bitumen reaches a viscosity of 75 centipoise (0.075 Pa·s).
• Application Tolerance Window: The roofing contractor must apply the bitumen within a strict tolerance window of EVT ± 25°F (±14°C).

Critical Jobsite Rule: If asphalt is applied below EVT, it becomes too viscous and cool, resulting in heavy, uneven moppings, improper ply adhesion, void entrapment, and premature delamination. Conversely, if asphalt is heated excessively above EVT, it applies too thinly (starving the plies), drains away from high spots, and cooks off light volatile aromatic oils in the kettle. Overheating also breaks down the bitumen and brings it closer to its flash point, a serious fire hazard. Never heat bitumen to or above the maximum heating temperature or flash point shown on the manufacturer's label or data sheet, and keep kettle thermometers working.

C. Coal Tar Pitch (ASTM D450 Type I)

Coal tar pitch is a dense, black resinous byproduct obtained during the high-temperature carbonization (coking) of bituminous coal to produce metallurgical coke for steel manufacturing. Governed by ASTM D450 (Standard Specification for Coal-Tar Pitch Used in Roofing, Dampproofing, and Waterproofing), Type I coal tar pitch differs fundamentally from petroleum asphalt:

1. Aromatic Molecular Structure: Coal tar consists almost entirely of closed-ring aromatic hydrocarbons. This structure makes it highly resistant to water. Coal tar tolerates long-term ponding better than asphalt, which is one reason the IBC allows it at a lower slope.
2. Low Softening Point & Cold-Flow Self-Healing: Coal tar pitch has a low softening point of 126°F to 140°F (52°C to 60°C). In the presence of sunlight and summer ambient temperatures, coal tar exhibits pronounced "cold flow." If a crack or micro-fissure develops in the membrane due to structural movement or thermal contraction, the pitch softens, slowly flows across the fissure, and self-heals the membrane under gravity.
3. Slope Limitations: The IBC minimum design slope for coal-tar built-up roofs is 1/8:12 (1507.10.1), half the 1/4:12 required for asphalt BUR, because coal tar tolerates ponding. Its cold flow means it migrates downhill on steeper slopes, so manufacturers limit coal-tar systems to very low maximum slopes (commonly about 1/4:12 to 1/2:12, with back-nailing on the steeper end). Follow the system specification.
4. Chemical Incompatibility: Petroleum asphalt and coal tar pitch are chemically incompatible. If liquid asphalt and coal tar pitch come into direct contact, the volatile flux oils within the coal tar dissolve and liquefy the asphalt, while the asphalt strips oils from the coal tar, causing both bitumens to curdle, blister, and fail. They must never be mixed in a jobsite kettle, nor can an asphalt roof be directly recovered with coal tar without a complete physical separation barrier.

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3. Reinforcing Fabrics: Organic Felts vs. Porous Fiberglass Mats

The internal skeleton of a BUR assembly is provided by factory-manufactured reinforcing plies. The choice of reinforcement dictates the roof's tensile strength, porosity, moisture resistance, and lifespan.

Evolution from Organic Felts (ASTM D2626) to Fiberglass Mats (ASTM D2178)

Historically, BUR assemblies relied on organic felts manufactured from rag paper pulp and recycled cellulose wood fibers (ASTM D226 / ASTM D2626). Organic felts absorb ambient moisture from high-humidity air. When hot asphalt is mopped over damp organic felt, the trapped water instantly flashes into steam, forming large vapor pockets beneath the plies known as roof blisters. Over time, organic fibers rot, mold, and break down under moisture intrusion.

Modern commercial BUR specifications overwhelmingly mandate inorganic glass fiber felts governed by ASTM D2178 (Standard Specification for Asphalt-Impregnated Glass Fiber Felt Used in Roofing and Waterproofing):

• Inorganic Composition: Glass fibers are completely rot-proof, chemically inert, and moisture-resistant.
• Engineered Porosity: ASTM D2178 fiberglass felts feature millions of microscopic pores. When hot asphalt is applied and the felt is broomed into the bitumen, the porous structure allows hot bitumen to bleed directly through the sheet while allowing air and moisture vapors to escape freely before the bitumen sets. This porous bleed-through creates a solid, blister-free monolithic lamination between plies.

ASTM D2178 Ply Specifications: Type IV vs. Type VI

Interply Mopping Rates & Mechanical Installation

Proper interply mopping requires precise volume control. Bitumen is applied across the roof substrate using heavy cotton or fiberglass mops, mechanical mop carts, or heated jet-shooters:

• Asphalt Interply Mopping Rate: NRCA guidance is about 25 pounds of hot asphalt per square (100 sq ft), ±15%, between each ply. Mopping must be continuous across the entire sheet width, leaving zero dry spots, streaks, or unmopped gaps (referred to in the trade as "holidays").
• Coal Tar Pitch Interply Mopping Rate: Similar or slightly heavier moppings of coal-tar pitch between plies, as the system specification states.
• Brooming-In Technique: Immediately behind the mopping applicator, each roll of felt must be rolled into the hot bitumen and firmly "broomed-in" using soft-bristle squeegees or specialized brooms. This action presses out air pockets, embeds the glass fibers completely into the molten bitumen, and forces bleed-through without crushing the sheet.

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4. Surfacing Systems: Aggregate, Cap Sheets & Liquid Coatings

Bitumen is an organic hydrocarbon that degrades rapidly when exposed to ultraviolet (UV) solar radiation. Direct solar exposure causes photo-oxidation: the flexible maltene oils within the bitumen break down and evaporate, transforming the bitumen into hard, brittle asphaltenes that crack into a distinctive "alligatoring" pattern. To prevent premature weathering, a protective surfacing must be applied over the final ply.

┌────────────────────────────────────────────────────────────────────────┐
│                     BUR SURFACING SYSTEM OPTIONS                       │
├────────────────────────────────────────────────────────────────────────┤
│  1. AGGREGATE EMBEDDED IN FLOOD COAT (Standard Commercial Heavyweight) │
│     • About 60 lbs/sq Hot Asphalt (about 75 lbs/sq Coal Tar)           │
│     • 400 lbs/sq Clean Gravel or 300 lbs/sq Crushed Slag               │
│     • Functions: UV shield, listed fire rating, impact protection      │
├────────────────────────────────────────────────────────────────────────┤
│  2. MINERAL-SURFACED CAP SHEET (ASTM D3909 Glass Cap Sheet)           │
│     • Factory-applied ceramic granules on asphalt-coated glass mat     │
│     • Lightweight alternative; eliminates loose aggregate              │
├────────────────────────────────────────────────────────────────────────┤
│  3. REFLECTIVE LIQUID COATINGS (Cool Roof Formulations)                │
│     • ASTM D2824 Aluminum-pigmented asphalt coating                    │
│     • High-solids elastomeric acrylic or silicone cool roof coatings    │
└────────────────────────────────────────────────────────────────────────┘

1. Aggregate Embedded in Flood Coat (ASTM D1863 Gravel & Slag)

Aggregate surfacing is the traditional, heavyweight defense for commercial BUR assemblies:

• Bitumen Flood Coat: A continuous, heavy flood coat of hot bitumen is poured and spread over the finished plies. NRCA guidance is about 60 pounds per square for asphalt and about 75 pounds per square for coal-tar pitch. Follow the system specification.
• Aggregate Embedment: While the flood coat remains molten and fluid, clean, dry, non-porous aggregate conforming to ASTM D1863 (Standard Specification for Mineral Aggregate Used on Built-Up Roofs) is mechanically spread and embedded:
  • Washed River Gravel: Minimum 400 pounds per square (4 lbs/sq ft).
  • Crushed Blast-Furnace Slag: Minimum 300 pounds per square (3 lbs/sq ft), which covers equivalent area at lower weight due to its porous, angular structure.
• Functions: The embedded aggregate blocks UV, protects the membrane from foot traffic and hail, and contributes to the fire rating of listed aggregate-surfaced assemblies. IBC 1504.9 requires parapets on aggregate-surfaced roofs, of heights set by Table 1504.9 for wind speed, exposure, and roof height, so wind does not blow the gravel off.

2. Mineral-Surfaced Cap Sheets (ASTM D3909)

On decks that cannot carry the weight of 400 lbs/sq of gravel, or on slopes too steep to hold aggregate in the flood coat, contractors specify a mineral-surfaced cap sheet. Governed by ASTM D3909, this product consists of a heavy fiberglass mat coated on both sides with oxidized asphalt and surfaced on the weather side with dense, ceramic-coated mineral granules (identical to those used on asphalt shingles). The cap sheet is mopped into a final layer of hot Type III or Type IV asphalt with the side and end laps the manufacturer specifies.

3. Reflective Aluminum & Elastomeric Liquid Coatings

To reduce rooftop heat gain and meet owner or energy-program cool-roof targets:

• Aluminum-Pigmented Asphalt Coatings (ASTM D2824): Formulated with pure aluminum flakes suspended in an asphalt solvent vehicle. Upon application, the leafing aluminum flakes rise to the surface, creating an opaque metallic shield that reflects 50% to 65% of solar radiation, dramatically lowering membrane surface temperatures.
• High-Solids Acrylic / Silicone Elastomers: Liquid-applied over smooth-surfaced BUR plies to reach high Solar Reflectance Index (SRI) values for cool-roof programs.

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5. Comprehensive BUR Component & Bitumen Specification Matrix