Built-Up Roofing (BUR) for Commercial Buildings: Pros and Cons
A breakdown of built-up roofing (BUR) systems for flat and low-slope commercial buildings — how they’re built, where they perform well, and where they fall short.
Built-up roofing, often shortened to BUR and sometimes called a tar-and-gravel roof, has been used on flat and low-slope commercial buildings in the United States for well over a century. It is one of the oldest roofing systems still in active use today, and for good reason — but it is not the right fit for every building. Here is a full breakdown of how BUR works and where it holds up, and where it does not.
What is a built-up roofing system?
A BUR system is made of alternating layers of roofing felt (sometimes called ply sheets) and bitumen — either asphalt or, less commonly today, coal tar pitch. A base sheet is installed over insulation board on the roof deck, and additional plies are added on top, each one mopped down with hot or cold bitumen. Most commercial BUR systems use three or more plies, which is where the system gets its “redundant layer” reputation.
The finished assembly is typically topped with a surfacing layer — most commonly gravel embedded in a final bitumen coating, though mineral cap sheets and reflective coatings are also used. That surfacing layer protects the underlying plies from UV exposure and physical wear.
The case for BUR: where it performs well
Proven, redundant waterproofing
Because BUR is built from multiple overlapping layers of felt and bitumen, a small amount of localized damage does not automatically compromise the whole roof the way a single-ply membrane puncture can. This layered redundancy is BUR’s signature advantage and the reason it has remained a viable choice for so long.
Strong fire resistance
A gravel-surfaced BUR roof has notably good fire resistance, since gravel does not burn and can help stop the spread of embers or flying debris landing on the roof — a meaningful consideration for buildings near wildfire-prone areas or with certain fire code requirements.
Durability under foot traffic
BUR systems generally tolerate regular foot traffic — from maintenance crews accessing rooftop HVAC units, for example — better than some thinner single-ply membranes, since the gravel surfacing and multiple plies distribute wear more effectively.
Long service life when properly maintained
A properly installed and well-maintained BUR roof can last up to 30 years, which is competitive with or better than several modern low-slope alternatives, provided the roof receives regular inspection and upkeep.
The case against BUR: where it falls short
Weight
The gravel surfacing that gives BUR its fire resistance also makes it significantly heavier than modified bitumen or single-ply systems like TPO or EPDM. Not every building’s structure is designed to carry that additional load without an engineering review, particularly on older buildings or retrofits.
Harder to diagnose leaks
Gravel surfacing tends to accumulate dirt and debris over time, and it can make it genuinely difficult to locate the exact point of water entry when a BUR roof does develop a leak. Loose gravel can also work its way into drains and gutters, contributing to ponding water if maintenance is deferred.
Installation conditions and odor
Hot-applied BUR installation involves heating bitumen on-site, which produces strong odors and requires specific safety precautions during application — a real consideration for occupied buildings, tenants, or nearby businesses during a reroofing project.
Slower installation than single-ply systems
Because BUR requires multiple layers to be applied and allowed to bond in sequence, installation generally takes longer than single-ply membrane systems, which can extend project timelines and labor costs on larger commercial roofs.
BUR vs. modern single-ply alternatives
Facility managers evaluating BUR are often also considering TPO or EPDM, two of the most common single-ply membrane alternatives for flat commercial roofs. TPO in particular offers high reflectivity that can reduce cooling costs in Arizona’s climate, lighter weight, and generally faster installation, though it has a shorter track record than BUR’s century of field performance and depends heavily on installation quality since it relies on heat-welded seams rather than BUR’s layered redundancy.
Neither system is universally “better.” The right choice depends on your building’s structural load capacity, budget, timeline, and how the roof will be used day to day.
Is BUR the right choice for your building?
BUR tends to be a strong fit for buildings that can support the additional weight, where fire resistance is a priority, where the roof will see regular foot traffic for equipment access, and where the ownership plans to invest in the ongoing maintenance a gravel-surfaced roof requires. It is a weaker fit for lightweight structures, buildings prioritizing the fastest possible reroofing timeline, or owners who want a lower-maintenance system.
A structural assessment and a clear conversation about your building’s specific use case are worth having with a commercial roofing contractor before deciding between BUR and a single-ply alternative.
Questions to ask your commercial roofing contractor
- Can my building’s structure support a gravel-surfaced BUR system, or would it need reinforcement?
- How many plies are being specified, and what does that mean for expected lifespan?
- What is the maintenance plan for keeping drains and gutters clear of gravel and debris?
- How does the installation timeline and process account for an occupied building?
- How does BUR compare to TPO or EPDM specifically for my roof’s use case and budget?
FAQs
How long does a built-up roof last?
A properly installed and maintained BUR system can last up to 30 years, though actual lifespan depends on ply count, materials used, climate exposure, and how consistently the roof is maintained.
Is built-up roofing heavier than other commercial roofing systems?
Yes, generally. The gravel or mineral surfacing that gives BUR its fire resistance and durability also adds substantial weight compared to single-ply membranes like TPO or EPDM.
Can a leak be repaired on a BUR roof?
Yes, but locating the source can be more time-consuming than on a single-ply membrane, since gravel surfacing can obscure the exact point of water entry. Regular inspections make repairs easier to catch early.
Is BUR a good option in Arizona’s climate?
BUR can perform well in Arizona given its durability and fire resistance, though a reflective surfacing option is worth discussing to help manage heat absorption, and the weight and maintenance considerations should be evaluated against your building’s specific structure and budget.
Have more questions? Check out our FAQ page.
