The History of Built-Up Roofing (Tar and Gravel) and Why It Dominated for a Century

Quick answer: Built-up roofing (BUR) — commonly called “tar and gravel” roofing — is a multi-layer commercial flat-roof system made from alternating plies of asphalt- or coal-tar-saturated felt fused together with hot bitumen, topped with a protective gravel or slag surfacing layer. It traces back to composition roofing techniques documented as early as 1800 in Washington D.C., was commercialized by the Warren brothers in Cincinnati during the 1840s, and became the dominant American commercial flat-roofing system for the better part of a century — reaching over 90% market share of flat roofing by the 1960s — before losing significant ground starting in the 1970s to single-ply membrane alternatives like EPDM and, later, TPO.

The Early History: From Pine Tar Canvas to Commercial Coal Tar Roofing

Built-up roofing’s origins trace back further than most people expect: the National Park Service’s Technical Preservation Services documentation records pine tar applied over canvas as a roofing method at the Octagon House in Washington, D.C. around 1800, a roof that reportedly lasted 17 years — genuinely impressive durability for such an early composition roofing technique. The material that would eventually become recognizable as modern BUR took real commercial shape in the 1840s, when the Warren brothers commercialized the technique in Cincinnati, substituting cheaper, more readily available coal tar for the original pine pitch — a cost-driven substitution that made composition roofing considerably more commercially viable for widespread use beyond individual custom applications.

How Multi-Ply Built-Up Roofing Is Actually Constructed

A built-up roof consists of multiple layers of felt saturated with bitumen (either coal tar or asphalt, depending on the era and specific system), fused together using hot bitumen applied at roughly 20 to 25 pounds per 100 square feet, or in some systems using cold adhesive instead of hot application. A mechanically fastened base sheet anchors the whole system to the roof deck, and the entire assembly is typically topped with a gravel or slag surfacing layer weighing roughly 400 to 500 pounds per roofing square — a substantial protective layer that shields the underlying bitumen and felt plies from direct UV exposure and physical damage. The genuinely distinctive characteristic of this construction method is that the multiple felt plies fuse together into what’s effectively one continuous, monolithic waterproof barrier, rather than functioning as separate, independently sealed layers.

Why BUR Dominated Commercial Flat Roofing for Nearly a Century

Built-up roofing’s multi-decade dominance rested on several genuinely compelling practical advantages: its redundant, multi-ply construction meant no single point of failure could compromise the entire roof system, since even if one ply developed a flaw, the surrounding plies and the fused, monolithic nature of the overall assembly provided real backup protection. BUR also largely eliminated the fastener-related leak points that plague some other roofing system types, since the bulk of the system’s surface area involves fused bitumen and felt rather than numerous individual mechanical fasteners penetrating the membrane. Beyond these performance advantages, BUR benefited enormously from decades of accumulated contractor familiarity and tradecraft, along with genuinely low material costs relative to the coverage and durability it provided — a combination that made it the default, low-risk choice for architects, building owners, and roofing contractors alike throughout most of the 20th century, reaching over 90% of the flat commercial roofing market by the 1960s according to trade industry documentation.

What Actually Drove the Shift Toward Single-Ply Membrane Systems

Single-ply membrane roofing systems, including EPDM (covered in detail in our companion piece on EPDM’s own history), were technically introduced as early as the early 1960s, but genuinely significant market growth away from BUR didn’t happen until after the 1973 oil embargo dramatically spiked asphalt prices and degraded both the availability and consistent quality of the asphalt BUR systems depended on. This price and supply shock created a real economic opening for single-ply alternatives that hadn’t previously existed when asphalt was cheap and reliably available — EPDM specifically gained major market traction through the 1970s specifically because of this oil-embargo-driven pricing disruption, eventually becoming a leading architect and contractor choice based on both cost and meaningfully simpler, faster installation compared to multi-ply BUR’s labor-intensive hot-application process. TPO membrane technology followed a somewhat later timeline, with the first roofing-specific TPO membrane installation documented in Michigan in 1986, and TPO’s market share accelerating particularly sharply from the late 1990s onward, posting double-digit annual growth rates that considerably outpaced BUR’s declining share during that same period.

The Market Share Numbers That Track This Multi-Decade Shift

The scale of this transition is genuinely striking when tracked numerically: single-ply membrane systems already held roughly 35% of the low-slope commercial roofing market by 1984 — notably exceeding industry forecasts of around 25% for that same period — while BUR’s own market share had already fallen to roughly 40% by the 1970s from its earlier dominant position. By 2018, single-ply systems (EPDM, TPO, and PVC combined) had grown to command roughly 67% of the total low-slope commercial roofing market, illustrating just how thoroughly the industry’s default technology choice shifted over the roughly four decades following the 1973 oil embargo’s initial disruption to BUR’s previously unchallenged cost advantage.

Where BUR Remains Specified Today, and Why

Despite its declining overall market share, built-up roofing persists today particularly among institutional buyers like schools and municipal governments, who continue to value BUR’s proven, multi-decade durability track record and its inherent redundant, multi-ply construction — qualities that carry real appeal for public-sector buyers making long-horizon infrastructure investment decisions where a well-understood, century-tested system carries genuine institutional confidence that a comparatively newer single-ply technology, however well-performing, simply hasn’t yet accumulated to the same degree. Some renewed interest in modern BUR variants has also emerged specifically around fiberglass-reinforced felts and their compatibility with LEED certification and broader energy-efficiency considerations, showing that even a century-old roofing technology can continue evolving incrementally alongside newer sustainability-driven building priorities rather than becoming entirely obsolete.

How Modified Bitumen Fits Into This Broader Transition

Modified bitumen roofing represents something of a middle path in this broader single-ply-versus-BUR transition story — it retains BUR’s fundamentally bitumen-based composition and some of its multi-layer redundancy philosophy, while incorporating polymer modifiers that improve flexibility and installation speed compared to traditional hot-applied BUR systems. This page’s research wasn’t able to fully verify the precise decade modified bitumen was first introduced specifically in the U.S. commercial roofing market with full sourcing confidence, though it’s generally understood within the industry as having emerged as a genuine competitive alternative sometime during the same broader 1970s-through-1980s period when EPDM and, later, TPO were also gaining significant market traction against traditional BUR.

How This History Connects to Commercial Roofing Decisions Today

Understanding this century-long competitive history helps explain why a commercial roofer today, evaluating options for a building currently topped with legacy BUR roofing, often frames the conversation specifically around this historical transition — a building owner weighing whether to replace an aging BUR system with a modern EPDM or TPO membrane is, in a real sense, making the same fundamental cost-versus-performance-versus-familiarity decision the broader commercial roofing industry itself worked through gradually over the decades following the 1973 oil embargo, just compressed into a single building-level decision rather than an industry-wide multi-decade shift.

Other Questions People Ask

Is built-up roofing more expensive to install than single-ply membranes today?

Generally, yes — BUR’s labor-intensive, multi-layer hot-application installation process typically costs more in labor than a single-ply membrane’s faster installation, which is part of why single-ply systems have continued gaining market share even though BUR’s material cost itself remains relatively modest.

Does built-up roofing require more maintenance than modern membrane systems?

BUR’s gravel surfacing layer requires periodic inspection and maintenance to ensure even coverage and catch any developing issues, and its overall system, while durable, generally isn’t as simple to inspect and patch as a modern single-ply membrane’s more straightforward, visually inspectable surface.

Can a building with an existing BUR roof be converted to a single-ply system without full tear-off?

In some cases a single-ply membrane can be installed directly over an existing, structurally sound BUR system as a recover roof, though this depends heavily on the existing roof’s condition, local building code requirements, and a qualified commercial roofing contractor’s specific assessment of the existing assembly.

People Also Ask

What is built-up roofing (tar and gravel)?

A multi-layer commercial flat-roof system made from alternating plies of bitumen-saturated felt fused together with hot bitumen and topped with a protective gravel or slag surfacing layer — see the full construction explanation above.

Why was tar and gravel roofing so popular for so long?

Its redundant multi-ply construction eliminated single points of failure, it had few fastener-related leak points, and decades of contractor familiarity plus low material cost made it the default commercial choice, reaching over 90% market share by the 1960s.

When did built-up roofing start being replaced by single-ply membranes?

Single-ply membranes were introduced in the early 1960s but gained major market traction specifically after the 1973 oil embargo spiked asphalt prices, with single-ply systems eventually reaching roughly 67% of the low-slope market by 2018.

Sources

• National Park Service, Technical Preservation Services, historical composition roofing documentation
• WATERPROOF! Magazine, built-up roofing and single-ply membrane historical retrospectives
• EPDM Roofing Association, “The Evolution of EPDM Roofing Technology”
• CRCA/Mediaedge, “The History of TPO”

The precise decade modified bitumen was first introduced specifically in the U.S. commercial market is not fully confirmed in available sources and is presented here at the level of general industry understanding rather than a precisely sourced date. Last updated September 2026.