Choosing the right roofing system is one of the most important decisions commercial property owners and facility managers can make. In Colorado, where hailstorms, high-altitude UV exposure, heavy snowfall, and dramatic temperature swings put commercial roofs to the test, selecting the right materials can make a significant difference in long-term performance.
Two established options for commercial flat roofing are built-up roofing (BUR) and modified bitumen roofing. Both use asphalt-based materials to protect flat and low-slope buildings, but they differ in construction, flexibility, installation methods, and maintenance requirements.
A built-up roof uses multiple layers of reinforcing materials and bitumen to create a durable roofing membrane, often finished with gravel or another protective surface. Modified bitumen roofing uses asphalt enhanced with polymer modifiers to improve flexibility and performance.
At Integrity Pro Roofing, we work with both systems throughout Denver and Colorado. This guide explains how they compare, which performs better under different conditions, and how to determine whether your existing roof needs repair or replacement.
Commercial Roofing Comparison
What Is Built-Up Roofing (BUR)?
A built-up roof (BUR) is a multilayer roofing system designed for commercial flat and low-slope buildings. It consists of alternating layers of bitumen and reinforcing fabrics, typically finished with gravel, mineral surfacing, or a protective coating. The overlapping layers create a continuous membrane that helps protect the building from water intrusion and weather-related damage.
Built-up roofing has been used on commercial buildings for more than a century. It is commonly called a tar and gravel roof, although modern BUR systems generally use asphalt-based materials rather than traditional coal tar.
The defining characteristic of a built-up roofing system is its multiple reinforcing plies. These layers provide redundancy, meaning damage to one layer does not necessarily compromise the entire roofing assembly.
Understanding 3-Ply, 4-Ply, and 5-Ply Built-Up Roofs
Built-up roofs are often described according to the number of reinforcing plies incorporated into the membrane.
- 3-ply built-up roof: Contains three reinforcing plies embedded in bitumen, creating a multilayer roofing membrane.
- 4-ply built-up roof: Contains four reinforcing plies, offering an additional layer of reinforcement and redundancy.
- 5-ply built-up roof: Contains five reinforcing plies and may be specified for applications requiring a more substantial roofing membrane.
A 4-ply built-up roof is a common example of traditional BUR construction. The term refers to the number of reinforcing fabric layers, not the total number of materials in the roofing assembly.
More plies can provide additional reinforcement, but the number alone does not determine performance. Material quality, installation, substrate conditions, drainage, and maintenance also influence how long the roof lasts.
Components of Built-Up Roofing
A typical built-up bituminous roofing system includes several components:
- Base sheet: Provides a foundation for the roofing membrane and may be mechanically attached, adhered, or otherwise installed according to the system design.
- Reinforcing plies: Fiberglass or other compatible reinforcing sheets provide strength and stability.
- Bitumen layers: Asphalt or another specified bituminous material bonds the reinforcing plies together.
- Surfacing layer: Gravel, mineral-surfaced materials, or another approved protective finish helps protect the membrane from weather exposure.
The exact assembly depends on the manufacturer, building requirements, and installation specifications.
Hot Asphalt vs. Coal Tar vs. Cold-Applied BUR
Not every built-up roof is installed the same way.
Hot asphalt BUR uses heated asphalt to bond reinforcing layers together. This traditional method remains an established option for commercial roofing.
Coal tar BUR uses coal tar pitch rather than asphalt. These systems are associated with certain older commercial roofs and have distinct material characteristics and handling requirements.
Cold-applied BUR uses compatible adhesives or bituminous products that do not require the same hot-asphalt application process. Depending on the system, this may reduce certain installation-related concerns involving heat, fumes, and odor.
Each method has specific installation requirements, and the appropriate choice depends on the building and approved roofing assembly.
Gravel vs. Mineral-Surfaced Built-Up Roofs
Traditional tar and gravel roofs use an aggregate surface to help protect the underlying bitumen from sunlight and physical wear.
Other BUR assemblies may use mineral-surfaced cap sheets or alternative approved surfacing materials.
Gravel provides a protective layer, but loose aggregate can complicate inspections, maintenance, and repairs. Mineral-surfaced systems provide a different finished surface that may be more appropriate for certain building conditions.
Benefits of Built-Up Roofing
One of BUR’s greatest advantages is its multilayer construction. Unlike a single-ply membrane, a built-up roof incorporates several reinforcing layers that work together to protect the building.
This construction offers several potential benefits.
Durability and redundancy: Multiple plies provide a substantial roofing membrane that can withstand normal commercial roof exposure and certain types of physical wear.
Weather resistance: Properly installed BUR systems provide dependable protection against rain, snow, and other environmental conditions.
UV protection: Gravel and mineral surfacing help shield underlying bituminous materials from direct sunlight.
Fire performance: Certain BUR assemblies achieve recognized fire-resistance classifications when installed as tested systems. Ratings depend on the complete assembly rather than the roofing material alone.
Established performance history: BUR has been used for generations, giving commercial property owners a long history of documented applications and maintenance practices.
For large commercial buildings, these characteristics can make BUR an attractive option when long-term durability is a priority.
Drawbacks of Built-Up Roofing
Despite its advantages, built-up roofing has limitations. Installation can be labor-intensive because multiple layers must be applied correctly. This may increase installation time compared with some alternative roofing systems.
BUR assemblies can also be relatively heavy, particularly when multiple plies and gravel surfacing are involved. The building’s structural capacity must be considered before installation.
Hot-applied systems can produce odors and fumes that require careful planning around building occupants, ventilation, and operations.
Another consideration is repair complexity. Identifying the exact source of a leak beneath multiple roofing layers and gravel surfacing can be challenging.
BUR also requires regular maintenance. Although durable, these systems are not immune to flashing failures, blistering, surface deterioration, or drainage problems.
What Is Modified Bitumen Roofing?
Modified bitumen roofing is an asphalt-based commercial roofing system enhanced with polymer modifiers.
These modifiers improve specific characteristics of the asphalt, such as flexibility, elasticity, or resistance to heat-related deformation.
Modified bitumen systems commonly use one or more reinforced membrane sheets and are available in different configurations depending on the manufacturer and installation method.
Two major categories are SBS- and APP-modified bitumen.
SBS vs. APP Modified Bitumen
Understanding the difference between SBS and APP is especially useful when selecting a roofing system for Colorado.
SBS (Styrene-Butadiene-Styrene) modified bitumen incorporates a rubber-like polymer that improves elasticity and flexibility. SBS membranes can accommodate movement due to temperature changes, making them an attractive option for buildings subject to frequent expansion and contraction.
APP (Atactic Polypropylene) modified bitumen uses a plastic-like modifier that improves characteristics such as heat resistance and dimensional stability. APP membranes are commonly associated with torch-applied installation methods, although available systems and application methods vary.
Both materials can perform effectively when properly specified and installed.
For Denver commercial buildings experiencing substantial seasonal and daily temperature fluctuations, SBS may be particularly appealing because of its flexibility at lower temperatures.
However, APP should not automatically be ruled out. Roof configuration, installation requirements, manufacturer specifications, and overall assembly design remain important.
Components of Modified Bitumen Roofing
A typical modified bitumen roofing assembly may include:
- Base sheet: Provides the foundation for the roofing membrane.
- Modified bitumen membrane: Asphalt enhanced with SBS or APP polymers.
- Reinforcement: Polyester, fiberglass, or another reinforcing material incorporated into the membrane.
- Cap sheet: A protective upper membrane, often featuring mineral granules or another approved surface.
- Attachment materials: Compatible adhesives, asphalt, fasteners, or heat-applied bonding methods, depending on the system.
Modified bitumen membranes may be installed using torch application, hot asphalt, cold adhesives, self-adhered products, or other manufacturer-approved methods.
Benefits of Modified Bitumen Roofing
Modified bitumen offers several advantages for commercial flat and low-slope roofs.
Flexibility: SBS-modified systems, in particular, accommodate temperature-related movement and can be useful in Colorado’s variable climate.
Repairability: Localized damage can often be repaired with compatible membrane patches or other manufacturer-approved techniques.
Installation options: Different attachment methods allow contractors to select systems that suit building conditions and operational requirements.
Durability: Reinforced membranes provide protection against normal rooftop exposure, maintenance traffic, and weather-related wear.
System versatility: Modified bitumen can be installed in different configurations, including multilayer assemblies and hybrid roofing systems.
These characteristics make modified bitumen a practical option for many offices, warehouses, retail properties, and industrial buildings.
Drawbacks of Modified Bitumen Roofing
Modified bitumen also has limitations. Because the system uses overlapping membrane sheets, seams require careful installation and ongoing inspection. Improperly installed or deteriorating seams can become potential leak points.
Exposed asphalt-based surfaces also require appropriate UV protection, typically provided by mineral granules, reflective surfacing, or other approved finishes.
Installation methods can introduce additional considerations. Torch-applied systems require strict fire-safety procedures, while adhesives and hot-applied materials may create odors or affect building operations.
Finally, modified bitumen is not necessarily lighter, less expensive, or faster to install in every situation. Those differences depend on the number of layers, attachment method, roof complexity, and existing conditions.
Built-Up Roofing vs. Modified Bitumen: Side-by-Side Comparison
Both systems can provide reliable protection, but their differences become clearer when comparing specific performance factors.
| Factor | Built-Up Roofing (BUR) | Modified Bitumen |
| Typical lifespan | Often 20–30 years or longer | Often 20–30 years |
| Weight | Generally heavier, especially with gravel | Often lighter, depending on assembly |
| Installation time | Typically more labor-intensive | May be faster for some configurations |
| Odor and disruption | Hot asphalt systems can create significant odor | Varies by torch, adhesive, or asphalt application |
| Hail resistance | Multilayer construction can provide durability | Reinforced membranes can perform well; assembly matters |
| Temperature flexibility | Depends on bitumen and assembly | SBS systems offer enhanced flexibility |
| Repairability | Repairable, but multiple layers and gravel can complicate work | Localized membrane repairs can be relatively straightforward |
| Relative cost | Often higher for labor-intensive assemblies | May be lower for some configurations |
These comparisons are general. The complete roof assembly, installation quality, and building conditions ultimately determine performance and project requirements.
How BUR and Modified Bitumen Perform in Colorado
Colorado presents several roofing challenges that should influence system selection.
Hail Resistance
Denver and the Front Range regularly experience hailstorms capable of damaging commercial roofing materials.
BUR’s multiple layers can provide substantial protection against physical wear, while reinforced modified bitumen membranes can also perform well.
Neither system is automatically hail-proof.
The condition of the membrane, underlying insulation, cover board, and overall assembly can influence impact performance. Where hail resistance is a major concern, property owners should consider tested roofing assemblies rather than relying on membrane type alone.
High-Altitude UV Exposure
Colorado’s elevation increases UV exposure, which can accelerate deterioration of exposed roofing materials. BUR systems commonly rely on gravel or mineral surfacing to protect the underlying bitumen.
Modified bitumen systems may use mineral-surfaced cap sheets or other manufacturer-approved UV-resistant finishes. Keeping protective surfacing in good condition is important for both systems.
Freeze-Thaw Cycles and Temperature Swings
Colorado temperatures can change dramatically within a short period. Repeated expansion and contraction place stress on roofing materials, seams, flashing, and penetrations.
SBS-modified bitumen offers enhanced flexibility, which can be beneficial in these conditions. BUR systems can also perform effectively when properly designed and maintained, although aging materials and vulnerable details should be monitored.
Snow, Ice, and Drainage
Flat and low-slope commercial roofs must manage snow accumulation and melting water. Neither BUR nor modified bitumen eliminates the need for proper drainage.
Clogged drains, low areas, and persistent ponding can increase the likelihood of deterioration and water intrusion. For both systems, drainage design and routine inspections are essential to long-term performance.
Can Built-Up Roofing and Modified Bitumen Be Combined?
Yes. Some commercial roofing assemblies combine traditional built-up roofing construction with a modified bitumen cap sheet. These are commonly called hybrid roofing systems.
A hybrid assembly may use multiple BUR-reinforcing plies beneath a compatible SBS- or APP-modified bitumen cap sheet.
The goal is to combine the redundancy of a multilayer built-up membrane with the performance characteristics of a modified bitumen surface.
Potential advantages include enhanced reinforcement, flexibility, and surface protection, depending on the specific assembly.
However, not every BUR and modified bitumen product can be combined. Materials must be compatible, and the system must follow an approved design and manufacturer specifications.
For certain commercial buildings, a hybrid system may provide a useful alternative to a traditional BUR or modified bitumen assembly.
Built-Up Roof Repair vs. Replacement: What Property Owners Should Know
An aging built-up roof does not automatically require replacement. Many BUR systems can remain serviceable for years with appropriate maintenance and targeted repairs.
The important question is whether the damage is localized or indicates broader deterioration.
Common Signs a Built-Up Roof Needs Repair
Several conditions can indicate that a commercial BUR system needs professional attention.
Blistering: Raised areas can develop when air or moisture becomes trapped within the roofing assembly. Some blisters remain stable, while others can split or become vulnerable to damage.
Alligatoring: Cracking patterns resembling reptile skin may indicate deterioration of exposed bituminous materials.
Bare spots in gravel: Missing protective aggregate can expose underlying materials to increased UV exposure and physical wear.
Ponding water: Persistent standing water may indicate drainage problems and can contribute to deterioration around existing weaknesses.
Open flashing: Damaged or separated flashing around parapets, equipment, and penetrations can allow moisture into the roofing system.
Recurring leaks: Repeated water intrusion may indicate unresolved defects or deterioration that extends beyond a single area.
When Built-Up Roof Repair Makes Sense
A targeted repair may be appropriate when damage is confined to a limited area and the surrounding roofing assembly remains in good condition.
Examples include isolated flashing problems, localized membrane damage, or a small section of deteriorated surfacing. Professional built-up roof repair begins with identifying the source and extent of the problem.
Because BUR contains multiple layers, water can travel within the assembly before becoming visible inside the building. An accurate diagnosis helps prevent repeated repairs that address symptoms rather than the underlying cause.
When Commercial Roof Replacement Is the Better Option
Replacement may be more appropriate when:
- Leaks occur across multiple areas.
- Large sections of insulation are saturated.
- The membrane has widespread deterioration.
- Flashing and seams repeatedly fail.
- The roofing assembly has reached the end of its practical service life.
- Continued repairs no longer provide reasonable long-term value.
A professional inspection can help determine whether repairs are still practical or whether commercial roof replacement offers better protection for the building.
Preventative Maintenance for Built-Up Roofs
Routine inspections are particularly important for older BUR systems.
Commercial property owners and facility managers should monitor drainage, flashing, exposed bitumen, gravel coverage, and previously repaired areas.
A professional maintenance plan can help identify developing problems, document roof conditions, and prioritize repairs before damage becomes extensive.
Built-Up Roofing vs. Modified Bitumen: Which Is Right for You?
Now that we’ve explored how these systems are constructed and how they perform in Colorado, which is the better choice for your commercial building?
The answer depends on your property’s structural capacity, roof configuration, maintenance requirements, and long-term priorities.
Choose Built-Up Roofing If:
- You want a multilayer roofing membrane with substantial redundancy.
- Your building can accommodate the weight of the proposed roofing assembly.
- Long-term durability and established performance history are priorities.
- Your property is suitable for the installation methods required.
- You prefer a traditional commercial roofing system with multiple reinforcing plies.
Choose Modified Bitumen Roofing If:
- You want a reinforced asphalt-based membrane with enhanced flexibility.
- Your building experiences significant temperature-related movement.
- You prefer a roofing system with multiple installation options.
- Localized repairability is an important consideration.
- A modified bitumen assembly better fits your roof configuration and operational needs.
Neither system is universally superior. A properly designed and installed BUR roof may outperform a poorly installed modified bitumen system, and the reverse is equally possible.
The best decision comes from evaluating the complete roofing assembly rather than comparing materials in isolation.
Built-Up Roofing FAQs
How Long Does a Built-Up Roof Last in Colorado?
A properly installed and maintained built-up roof often provides approximately 20 to 30 years of service, with some systems lasting longer. Actual lifespan depends on the number of plies, materials, installation quality, drainage, maintenance, and exposure to Colorado’s hail, UV radiation, and temperature fluctuations.
What Is a 4-Ply Built-Up Roof?
A 4-ply built-up roof contains four layers of reinforcing fabric embedded in bitumen. These layers form a multilayer roofing membrane designed to provide durability and redundancy. The four plies do not include every component of the roof assembly, such as insulation, base materials, or protective surfacing.
Is Modified Bitumen Better Than Built-Up Roofing?
Not necessarily. Modified bitumen offers advantages such as enhanced flexibility and several installation options, while BUR provides a multilayer membrane with a long history of commercial performance. The better system depends on the building’s structural requirements, roof design, climate exposure, and maintenance priorities.
Can You Repair a Tar and Gravel Roof?
Yes. Many tar and gravel roofs can be repaired when damage is localized and the surrounding roofing assembly remains serviceable. Repairs may involve addressing damaged membrane sections, flashing, or protective surfacing. A professional inspection helps determine whether repair or replacement is the appropriate solution.
Is a Built-Up Roof More Expensive Than Modified Bitumen?
Built-up roofing can have higher installation costs because of its multilayer construction and labor requirements, particularly when hot asphalt and gravel surfacing are involved. However, costs vary by assembly, roof size, access, installation method, and existing conditions. Some modified bitumen configurations may have comparable or higher costs, so neither system is always the less expensive option.
Trusted Built-Up Roofing Solutions for Colorado Commercial Properties
Choosing between built-up roofing and modified bitumen involves more than comparing materials. The right commercial roofing system should account for Colorado’s climate, your building’s construction, expected maintenance requirements, and long-term operating goals.
At Integrity Pro Roofing, we help commercial property owners and facility managers throughout Denver and the Front Range make informed roofing decisions.
Whether you’re maintaining an existing tar and gravel roof, addressing recurring leaks, or considering a new commercial roofing system, our team provides straightforward assessments and practical recommendations.
We offer professional built-up roofing services, including inspections, repairs, and replacement solutions designed around your building’s needs.
Schedule your complimentary commercial roof assessment and discover the right solution for protecting your commercial property.