Low-Slope Coal-Tar Pitch BUR vs Asphalt BUR
Why this matters
Built-up roofing is still specified on hospitals, schools, and federal buildings where service life is measured in decades. The two bitumens are not interchangeable. Coal-tar pitch self-heals and stands up to standing water but is restricted to dead-flat decks and triggers a different set of OSHA exposure controls. Asphalt BUR runs on a wider slope range and a wider crew skill base but oxidizes faster under ponded water. Pulling a core that is half coal-tar and half asphalt and treating it as one assembly is how warranties get voided and how re-cover decisions get made on bad data.
Bitumen selection by slope
ASTM D450 governs asphalt for roofing. Type I (dead-level, 0 to 1/2 in per ft) and Type III (steep, 1/2 to 3 in per ft) are the two most commonly stocked. Coal-tar pitch is governed by ASTM D450 Type III pitch and ASTM D4022 for mineral-modified pitch. Pitch is limited to slopes of 1/4 in per ft maximum because it cold-flows. Asphalt Type III tolerates slopes up to 3 in per ft.
EVT (equiviscous temperature) windows are tight. Per the Asphalt Roofing Manufacturers Association, asphalt EVT is the mopping temperature plus or minus 25 F, typically 425 to 475 F at the mop. Coal-tar pitch EVT runs cooler, 375 to 425 F. Exceeding the flash point (asphalt around 500 to 550 F; pitch around 400 to 450 F) destroys the volatiles that bond the ply and is a kettle-fire trigger.
Ply construction and felt compatibility
ASTM D2178 Type IV and Type VI glass-felt plies are the modern standard for asphalt BUR. Organic felts (ASTM D226) are still permitted on pitch but largely obsolete. Four-ply construction with three plies of Type IV felt and one cap sheet (ASTM D6163 SBS-modified) is the FM Global baseline for FM 1-29 wind-uplift ratings.
Coal-tar pitch is built with coal-tar saturated felt (ASTM D227) only. Asphalt-saturated felt in a pitch system is a chemical incompatibility that delaminates within two to five years. Conversely, pitch felt in an asphalt mop will not bond because the pitch saturant blocks the asphalt from wetting the glass mat.
Wind uplift and FM ratings
FM Global property loss prevention data sheet FM 1-29 establishes the wind-uplift test classes. A FM 1-90 rating requires 90 psf uplift resistance and is the minimum for most commercial perimeters. FM 1-29 also dictates fastener pull-out tests (minimum 300 lb per fastener on steel deck per the data sheet) and tapered-insulation slope requirements (minimum 1/4 in per ft on new construction per IBC 1507.10).
FM 1-49 covers perimeter flashing, which is where most BUR failures actually start. Base flashing must terminate at least 8 in above the finished roof per the NRCA Roofing Manual and be reglet-set or counter-flashed, not surface-nailed.
Coal-tar exposure controls
Coal-tar pitch volatiles (CTPVs) are classified as a Group 1 human carcinogen by IARC. OSHA 29 CFR 1910.1000 Table Z-1 sets the PEL at 0.2 mg/m3 as an 8-hour TWA for the benzene-soluble fraction. A pitch crew needs:
- Half-face APR with organic-vapor and P100 cartridges minimum; full-face if kettle proximity exceeds the action level.
- Tyvek-style coveralls. Pitch causes severe phototoxic dermatitis ("pitch burn") under UV exposure within hours.
- Eye-flush stations at the kettle and at the roof edge.
A pitch crew that works in shorts and short sleeves in summer sun will burn within one shift. The phototoxic reaction is not an allergy; it is a chemical interaction with UV-A and presents as second-degree facial and arm burns. Long sleeves, long pants, and face shields are not optional.
Coring for re-cover suitability
IBC 1511.3.1 allows a re-cover (second roof) only when the existing roof is not water-saturated and not already a second roof. Pull a minimum of one 4 in diameter core per 10,000 sq ft, with extras at every visible blister, ridge, and ponded area. Document each core with plies counted, bitumen type identified by smell and color (pitch is olive-green to black with a creosote odor; asphalt is jet-black with a petroleum odor), and felt type confirmed.
Moisture content above 1 percent by weight (ASTM D4263 plastic-sheet method as a field screen, oven-dry weight as the lab confirmation) disqualifies the deck from re-cover. Wet insulation must be torn off.
Detailing standards
The NRCA Roofing Manual: Membrane Roof Systems is the construction-grade source. SMACNA Architectural Sheet Metal Manual governs counter-flashing, coping, and gravel-stop fabrication. Both pitch and asphalt BUR require:
- A minimum 4-ply membrane turn-up at the base flashing, sealed with a stripping ply.
- Cant strips at every vertical-to-horizontal transition, minimum 4 in face dimension.
- Gravel-surface ballast at 400 lb per 100 sq ft minimum on asphalt BUR; pitch is typically smooth-surface or light gravel because the pitch self-heals.
References
- ASTM D450, Standard Specification for Coal-Tar Pitch Used in Roofing, Dampproofing, and Waterproofing
- ASTM D6163, Standard Specification for Styrene Butadiene Styrene (SBS) Modified Bituminous Sheet Materials Using Glass Fiber Reinforcements
- FM Global Property Loss Prevention Data Sheet 1-29, Roof Deck Securement and Above-Deck Roof Components
- FM Global Property Loss Prevention Data Sheet 1-49, Perimeter Flashing
- IBC 2021 Section 1507.10 (BUR roof coverings) and 1511.3.1 (recover restrictions)
- OSHA 29 CFR 1910.1000 Table Z-1, Air Contaminants
- NRCA Roofing Manual: Membrane Roof Systems, current edition
- SMACNA Architectural Sheet Metal Manual, 7th Edition