Commercial flat roofs often use either built-up roofing (BUR) or modified bitumen systems, each offering unique advantages in durability, installation, and cost. This guide compares built-up roofs with modified bitumen, detailing their materials, longevity, cost factors, installation processes, and suitability across climates and building types. Discover which system best meets your property’s needs and budget.
| Feature | Built-Up Roof (BUR) | Modified Bitumen |
|---|---|---|
| Materials | Alternating layers of asphalt & ply sheets topped with gravel or cap sheet | Polymer-modified asphalt sheets (APP or SBS) |
| Installation | Hot-mopped, labor-intensive | Heat-applied, cold-adhered, or self-adhesive |
| Longevity | 20-30 years | 15-25 years |
| Water Resistance | Excellent | Very Good to Excellent |
| Cost | Moderate to high | Moderate |
| Best Use | Large flat commercial roofs | Small/medium or complex shapes |
What Is Built-Up Roofing (BUR)?
Built-up roofing (BUR) has proven itself over a century on American commercial buildings. It consists of alternating layers of roofing felt (ply sheets) and hot-applied asphalt or bitumen, creating a waterproof membrane. The top layer typically features gravel, mineral cap sheets, or coatings for additional protection.
How BUR Is Constructed
- Ply Sheets: Several layers (usually 3-5) of reinforced felt or fiberglass mats serve as the core.
- Bitumen: Hot-applied asphalt or coal tar binds and waterproofs each ply.
- Surfacing: Embedded gravel, mineral granules, or reflective coatings shield the roof from UV rays and physical damage.
The result is a heavy, durable roof ideal for large, low-slope structures like warehouses, factories, and schools.
Key Benefits Of Built-Up Roofing
- Exceptional Waterproofing: Multiple layers make BUR extremely leak-resistant.
- Long Lifespan: With proper upkeep, BUR lasts 20-30 years or more.
- Fire And Impact Resistance: The gravel or cap sheet helps protect against fire and hail.
- Thermal Stability: Thick mass resists temperature fluctuations.
What Is Modified Bitumen Roofing?
Modified bitumen emerged in the 1970s as a technologically advanced alternative to traditional BUR. It uses asphalt sheets blended with modifiers like APP (atactic polypropylene) or SBS (styrene-butadiene-styrene), enhancing flexibility, strength, and weather resistance.
How Modified Bitumen Roofs Are Installed
- Sheet Rolls: Delivered in rolls, modified bitumen sheets can be applied with heat (torch-down), cold adhesives, or peel-and-stick methods.
- Single Or Multi-Ply: Typically installed as a single or double ply, sometimes on top of existing BUR systems for added protection.
- Surfacing Options: Finished with ceramic granules, reflective materials, or aluminum coatings for UV resistance and aesthetics.
This system is especially suitable for small-to-mid-size roofs, roofs with more complex shapes, or buildings that need flexible, robust membranes.
Advantages Of Modified Bitumen Roofing
- Speed And Versatility: Quicker, less messy installation using various methods (torch-down, self-adhered, cold-applied).
- Flexibility: Polymer modification allows sheets to expand or contract, resisting cracks.
- Strong Seams: Overlapping joints can be fused for a watertight seal.
- Good Weather Resistance: Performs well against moderate hail, wind, and foot traffic.
Material And Layer Comparisons
BUR Material Composition
- Ply sheets/felt (organic/fiberglass base)
- Hot-applied asphalt or coal tar pitch
- Gravel/mineral cap for UV & impact
Modified Bitumen Composition
- APP or SBS modified asphalt sheets (2-4 mm thick)
- Fiberglass or polyester reinforcement
- Granulated, foil, or reflective surfacing
Beneath both types lies insulation and a vapor barrier, chosen based on climate and energy needs.
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Installation Methods Explained
BUR Installation
- Deck cleaning & preparation
- Applying adhesive and laying base ply
- Alternating layers of bitumen and ply sheets (3-5 times)
- Topping with bitumen and embedded surfacing aggregate or cap sheet
BUR installation is labor-intensive, requires hot equipment onsite, and is best performed by experienced crews.
Modified Bitumen Installation
- Roof deck preparation
- Applying base layers/primers (if required)
- Rolling out sheets using one of three techniques:
- Torch-down: Sheet heated with torch during application for secure adhesion
- Cold-applied: Liquid adhesive spread before rolling the sheet
- Self-adhered: Peel-and-stick backing simplifies installation
- Sealing seams with a torch or adhesive
Modified bitumen’s flexibility makes it easier to install on smaller or irregularly shaped roofs.
Lifespan And Durability In American Climates
| System | Average Lifespan | UV/Heat Resistance | Storm Resistance |
|---|---|---|---|
| Built-Up Roof | 20-30 years | Very High (with gravel/cap) | Excellent (with proper surfacing) |
| Modified Bitumen | 15-25 years | Moderate-High (with reflective cap) | Good-Excellent (depends on ply & surfacing) |
BUR roofs tend to outlast modified bitumen in many harsh climates, given their multi-layer construction. However, advancements in modified bitumen allow for great performance in regions with extreme temperature swings—such as the Northeast and Midwest—when installed correctly.
Cost Analysis: Which Is More Affordable?
The cost of either system depends on factors like location, labor rates, material thickness, size of the roof, and existing conditions. Here’s a comparative breakdown:
| Cost Factor | BUR | Modified Bitumen |
|---|---|---|
| Installation (per sq. ft.) | $4.50 – $8.00 | $4.00 – $7.00 |
| Material Cost | Moderately high (multiple plys, surface aggregate) | Moderate (single/double ply, polymer sheets) |
| Maintenance | Low to moderate | Low to moderate |
| Replacement/Repair Complexity | Challenging/labor-intensive | Simpler (patch & reseal methods) |
While BUR may cost slightly more upfront, its longevity sometimes balances out the initial investment. Modified bitumen, with lower labor and faster installation, is often more affordable for smaller or unique roof designs.
Maintenance Expectations For Each System
BUR Maintenance Requirements
- Regular inspection for surface damage or ponding water
- Re-applying coatings or replacing gravel as needed
- Timely repair of cracks or split seams
- Maintaining drainage and cleaning debris
Proper BUR maintenance can significantly extend the system’s lifespan.
Modified Bitumen Maintenance
- Flashing inspection and re-sealing seams
- Checking for punctures, blistering, or membrane shrinkage
- Cleaning debris and ensuring regular water run-off
- Applying reflectivity coatings if surface begins to fade
Modified bitumen is easier to repair—small damages usually require only patching and re-sealing.
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Energy Efficiency And Reflectivity
BUR Cool Roof Options
Traditional BUR systems absorb heat, but modern versions include reflective cap sheets or solar coatings that lower rooftop temperatures, reducing cooling costs in southern states.
Modified Bitumen Energy Savings
Many new modified bitumen membranes meet Energy Star or Cool Roof Rating Council (CRRC) standards with white or light-colored granule surfaces. These effectively reflect sunlight and heat, making them popular in warm, sunny regions.
Fire And Impact Resistance
- BUR: Inherently fire-resistant thanks to gravel or mineral surfacing. Excellent for buildings requiring high fire ratings.
- Modified Bitumen: Typically meets Class A fire ratings with fire-resistant cap sheets. Handles hail and moderate impacts well, especially SBS-modified types.
Manufacturers may recommend specific surface layers or reinforcements depending on local fire codes and hail exposure risk.
Environmental Impact And Sustainability Factors
Both roofing systems raise concerns about asphalt use, petroleum content, and end-of-life disposal. However, roofing industry innovations increasingly support energy savings and recycling options.
- BUR: Long lifespan and high mass reduce replacement frequency, but disposal typically involves landfill.
- Modified Bitumen: Thinner profile means less material. Some manufacturers recycle old membranes into new products.
- Reflective coatings available for both can earn LEED credits and lower urban heat impact.
Ideal Applications For Each System
| Roof Situation | Suggested System |
|---|---|
| Large, simple commercial roofs | BUR |
| Small or complex-shaped roofs | Modified Bitumen |
| Areas with high fire risk or hail | BUR (with gravel/cap), SBS-modified bitumen |
| Buildings requiring quick installation | Modified Bitumen (self-adhered or cold-applied) |
| Owners concerned about initial cost | Modified Bitumen |
| Roofs expecting heavy foot traffic | BUR or multi-ply SBS-modified |
Choosing the best roof depends on building design, location, traffic, and owner priorities. Always consult a commercial roofing expert for site-specific recommendations.
Installation Safety And Disruption
- BUR: Hot-applied bitumen means greater safety risks and odors. Project duration is typically longer, causing potential disruption.
- Modified Bitumen: Cold-applied and self-adhered versions reduce fire risk, odors, and building occupancy disruptions. Torch-down method requires trained installers.
Modern modified bitumen is often preferred where occupant comfort and minimal disruption are priorities.
Warranty Coverage And Manufacturer Support
Both systems are eligible for standard or enhanced manufacturer warranties if professionally installed and maintained.
- BUR warranties can range from 10 to 30 years, typically covering material defects and leaks.
- Modified bitumen warranties are similar, with options for up to 25 years.
- Warranties may exclude poor maintenance, unqualified repairs, or extreme weather events.
Trends In Commercial Flat Roofing
- Green and cool roofing: Energy codes, rebates, and sustainability goals drive demand for reflective membranes and vegetated BUR assemblies.
- New application methods: Self-adhered and cold-applied modified bitumen are increasingly popular for their ease, reduced risk, and eco-friendliness.
- Roof asset management: Owners invest in routine inspections and preventive maintenance to maximize roof life and warranty value.
How To Decide: Key Questions To Ask A Roofer
To choose the best commercial flat roofing system, consider these questions:
- What is the typical lifespan for each system in your region?
- How will roof design and usage influence material choice?
- Are building codes or insurance requirements a factor?
- What is the total lifecycle cost (installation, maintenance, repairs)?
- Can either system be overlaid on your current roof for savings?
For most large, straightforward flat commercial roofs, built-up roofing still reigns for proven longevity. In contrast, modified bitumen offers exceptional flexibility, faster installation, and higher performance on complex or smaller structures.
Recommended Resources And Next Steps
- National Roofing Contractors Association (NRCA)—standards, best practices, and contractor verification
- U.S. Department of Energy on Cool Roofs
- Consulting local contractors with experience in both systems for site-specific pricing and benefit analysis
Whether you opt for a built-up roof or modified bitumen, both systems can deliver robust, waterproof protection with proper material selection, installation, and upkeep.
How to Get the Best Roofing Quotes
- Prioritize Workmanship
A roof is one of your home’s most important investments. Always choose a contractor based on experience and reputation — not just price. Poor installation can lead to expensive problems down the road. - Compare Multiple Estimates
Don’t settle for the first quote you receive. It’s always a smart move to compare at least three bids from local roofing professionals. You can 877-801-4315 to get local quotes from roofing contractors in your area, available across the United States. - Use Negotiation Tactics
After selecting a trusted roofer, be sure to use our proven tips — How to Negotiate with Roofing Contractors — to secure the best possible final price without cutting corners.