Built Up Roof System: Comprehensive Guide To BUR Roofing For Commercial And Residential Buildings

Built up roof systems (BUR) have been a trusted solution for flat and low-slope roofs across America for over a century. This thorough guide explores what BUR roofing is, its construction, types, pros and cons, cost, application process, modern innovations, and essential maintenance. Whether you’re a property owner, facility manager, roofer, or architect, this article provides valuable insights to help you evaluate, install, or maintain a built up roofing system effectively.

Key Aspect	Details
Definition	Multi-layered roofing system using bitumen and reinforcing fabrics, usually on flat or low-slope roofs.
Common Components	Base sheet, ply sheets (felts), bitumen (asphalt/tar), surfacing (gravel, cap sheet, coatings).
Typical Lifespan	20–40 years (with proper maintenance and quality materials).
Main Benefits	Waterproof, durable, UV-resistant, low maintenance, time-tested performance.
Common Applications	Commercial, industrial, and some residential buildings with flat or low-slope roofs.

What Is A Built Up Roof System (BUR)?

A built up roof system (BUR) is a tried-and-true method for roofing flat and low-slope structures. It consists of alternating layers of bitumen (asphalt or coal tar) and reinforcing felts that build up a protective, waterproof membrane. The result is a thick, durable covering that has withstood harsh weather and heavy use for generations.

The layered construction creates redundancy, meaning if one membrane layer is compromised, others still provide protection. BUR roofs are often surfaced with gravel or mineral cap sheets to enhance durability, UV resistance, and fire protection.

Key Components Of A Built Up Roof System

The strength and performance of any BUR system come from combining several important elements, each with its unique function:

• Base Sheet: The first layer, often mechanically fastened or adhered to the roof deck, provides a smooth surface and initial barrier.
• Bitumen (Asphalt Or Coal Tar Pitch): A waterproofing agent, usually hot-applied, that binds all the layers and creates a seamless membrane.
• Reinforcing Ply Sheets (Felts): Usually made of organic, glass fiber, or polyester mats, these are embedded in multiple layers of bitumen to build up the roof’s thickness and strength.
• Surfacing Layer: Protects BUR from weather, sunlight, and impact damage. Options include gravel (ballast), slag, mineral cap sheets, or reflective coatings.

This multi-layer approach ensures a tough, long-lasting roof that adapts to the demands of commercial and select residential projects.

How Is A BUR Roofing System Constructed?

The application of a built up roofing system is a meticulous, skilled process:

1. Preparation: The roof deck (often steel, concrete, or wood) must be cleaned, dried, and free from debris or protrusions. Existing roofing (if present) may be removed.
2. Base Sheet Installation: The first layer, usually adhered or fastened mechanically.
3. Hot Mopping/Bitumen Application: Heated bitumen (asphalt or coal tar) is spread or mopped on, providing a sticky base.
4. Ply Sheet Placement: Reinforcing felts are rolled into the hot bitumen, creating the first ply. This process is repeated, alternating bitumen and felts, typically three to five times for optimal durability.
5. Surfacing: The finished membrane is covered with gravel or a mineral cap sheet while bitumen is still hot, or coated with an energy-saving reflective product.

Modern cold-applied BUR uses liquid bitumen adhesives instead of hot tar for safer, fume-free installation in certain settings.

Types Of Built Up Roofing Systems

There are several variations of BUR, tailored to different needs:

Traditional Hot-Applied Asphalt BUR

Heated liquid asphalt is mopped between layers of reinforcing felts to create a seamless and watertight membrane, finished with gravel or mineral cap.

Coal Tar Pitch BUR

This utilizes coal tar instead of asphalt as the bituminous binder, which provides enhanced resistance to standing water and some chemicals. However, coal tar’s use is limited today due to safety and environmental regulations.

Cold-Applied BUR

In this modern approach, solvent-based adhesives or modified bitumens are used instead of hot mopping. This method is popular in areas where open flame or hot kettles are restricted.

Hybrid And Modified BUR Systems

Some manufacturers incorporate polymer-modified bitumen sheets into standard BUR assemblies, combining the redundancy of traditional systems with advanced flexibility and performance of modern modified bitumen products.

Benefits Of A Built Up Roof System

BUR systems offer several distinctive advantages, making them a preferred option for many commercial and industrial properties:

• Waterproofing Redundancy: Multiple layers mean that the system remains watertight even if one layer is breached.
• Proven Longevity: With proper installation and maintenance, BUR roofs last 20–40 years, some even longer.
• Fire Resistance: Surfacing with gravel or mineral caps provides an additional layer of fire protection.
• Low Maintenance: Fewer moving parts and self-sealing characteristics reduce long-term rooftop issues.
• UV And Weather Resistance: Aggregate layers or coatings protect from sun, wind, hail, and heavy precipitation.
• Excellent For Heavy Foot Traffic: Perfect for roofs with frequent maintenance or HVAC equipment.

These attributes make BUR a solid investment for many U.S. building owners, especially in urban or industrial settings.

Potential Drawbacks And Challenges Of BUR Roofing

No roofing system is without its limitations. Considering these factors is essential before choosing BUR:

• Installation Complexity: Hot-applied BUR requires experienced professionals and strict safety protocols, increasing labor costs.
• Weight: BUR systems are heavy; structural reinforcement may be needed for some retrofits.
• Fumes And Odors: Hot mopped bitumen produces strong odors and, in some areas, volatile organic compounds (VOCs).
• Mess And Clean-Up: The application process can create spills, splatter, and debris if not managed carefully.
• Limited Slope Use: BUR is best for roofs with slopes below 3:12; steep slopes are generally unsuitable.

Evaluating your building’s structure, location, and use helps determine if a built up roof is your best option.

Typical Lifespan And Performance Of BUR Roofs

Most built up roof systems are expected to provide reliable service between 20 and 40 years when high-quality materials and skilled installation are used. With regular maintenance—sealant touch-ups, prompt repair of blisters, and proper drainage management—some BUR roofs have lasted over 50 years.

Factors influencing lifespan include:

• Quality of Bitumen and Felts: Higher quality materials increase resilience to weather and traffic.
• Number of Plies: More reinforcing layers generally mean better performance and longer service life.
• Climate and UV Exposure: Extreme weather or persistently standing water can lower lifespan if not properly managed.
• Maintenance Regimen: Regular inspection and prompt attention to minor issues are crucial for longevity.

Well-maintained BUR systems often outperform many single-ply or lightweight roofing options in demanding environments.

Cost Of Built Up Roof Installation

The cost of a built up roof system depends on several variables, from material quality to region and project complexity. On average in the United States (as of 2025):

• Bare-Bones Asphalt BUR: $4–$7 per square foot installed
• High-End Multi-Ply BUR: $6–$12 per square foot installed

Project Type	Typical Cost/SF	Key Features
Commercial Flat Roof	$6–$10	3–5 plies, gravel surfacing, up to 30-year lifespan
Residential Low-Slope	$6–$12	Specialty cap sheet, tailored for aesthetics
Re-Roof/Overlay	$4–$8	Over existing BUR with minor repairs

Labor and safety requirements, preparation work, insulation upgrades, and local code compliance play significant roles in determining final project cost.

Comparison: Built Up Roof Vs. Modified Bitumen And Single-Ply

Feature	BUR	Modified Bitumen	Single-Ply (EPDM/TPO/PVC)
Installation	Labor intensive, hot-applied	Heat, cold, or adhesive applied; easier than BUR	Fast, less labor—usually fully adhered or mechanically attached
Layers	3–5+ plies	1–2 plies (thicker membrane)	Single membrane
Waterproofing	Excellent redundancy	Very good	Good; relies on membrane integrity
Cost	Mid to high	Mid	Low to mid
Lifespan	20–40 years	15–25 years	10–25 years
Best For	Heavy use, long-term low-slope	Retrofitting, moderate slopes	Budget, quick installation, light loads

Single-ply roofs are typically faster and cheaper to install, but BUR offers greater resistance to damage and weather when correctly maintained.

Where Are Built Up Roof Systems Most Commonly Used?

BUR systems are predominantly found on flat and low-slope roofs (slopes less than 3:12) for commercial, institutional, industrial, and some multi-family residential buildings.

• Office buildings and warehouses
• Schools and universities
• Hospitals and healthcare facilities
• Retail and shopping centers
• Apartment complexes

Modern Innovations In BUR Technology

Although BUR is a traditional system, innovations have increased its efficiency and sustainability:

• Cold-Process BUR Systems: Safer application without hot kettles, using solvent- or water-based adhesives.
• Polymer-Modified Bitumen: Modified asphalts offer improved flexibility, UV resistance, and longer lifespan.
• Reflective/White Top Coatings: Reduce heat absorption, improve energy efficiency, and meet “cool roof” standards.
• Recycled Materials: Use of recycled content in felts, bitumens, and surfacing aggregates for sustainability.

Eco-friendly options and improved safety standards make modern BURs relevant and competitive with newer roofing technologies.

Key Factors In BUR Roof Design And Specification

Designing an effective BUR system requires careful consideration of these factors:

• Roof Slope: Slope between 1/4:12 and 3:12 is optimal for drainage and durability.
• Local Climate: UV exposure, temperature ranges, precipitation, and potential for standing water all influence system design.
• Insulation: High-quality insulation beneath BUR can enhance energy efficiency and reduce condensation issues.
• Surface Protection: Select gravel, cap sheet, or coating based on traffic, appearance, and climate needs.
• Flashing Details: Well-installed flashings are crucial for waterproofing at penetrations and edges.

Consult with a roofing professional or manufacturer’s technical rep to tailor your BUR design for maximum performance in your region.

Maintenance Guidelines For Built Up Roofs

Like any roof, BUR requires routine maintenance to maximize its life and performance:

• Biannual Inspections: In spring and fall, inspect for ponding water, surface cracks, blisters, open seams, and punctures.
• Debris Removal: Keep the roof and drains clear of debris to prevent water backup and damage.
• Prompt Repairs: Address blisters, splits, and flashing failures as soon as they are found, using compatible materials.
• Check Surfacing: Reapply gravel or coatings as needed to preserve UV resistance and fire rating.
• Professional Assessment: Annual inspection by a qualified roofer can catch small problems before they escalate.

Routine upkeep ensures that a built up roof delivers decades of trouble-free protection for your property.

Signs Your BUR Roof Needs Repair Or Replacement

Recognizing early warning signs helps prevent expensive damage:

• Persistent Standing Water (Ponding for more than 48 hours)
• Large Blisters Or Bubbles in the membrane
• Visible Cracks, Splits, Or Open Seams
• Widespread Loss Of Gravel/Surfacing
• Leaks Or Stains Inside The Building, particularly after heavy rain
• Expanding Damage Near Flashings Or Penetrations

Timely professional repair or replacement by a certified contractor secures your investment and protects your property from further damage.

How To Choose The Right Contractor For BUR Systems

Quality installation is vital. Look for roofing contractors with:

• Specialized Experience in built up roofing and low-slope applications
• Manufacturer Certifications and access to warranted systems
• Strong References on similar projects
• Proven Safety Standards (especially for hot-applied work)
• Comprehensive Proposals detailing all materials, application steps, and cleanup

Investing in a qualified professional ensures your new BUR system meets all performance and warranty requirements.

Frequently Asked Questions About Built Up Roof Systems

• Q: How many layers does a typical BUR have?
  A: Most systems have 3–5 ply sheets, but custom solutions may use more for demanding environments.
• Q: Can BUR be installed over an existing roof?
  A: Yes, overlays are possible if the existing roof is sound and building codes permit. Weight and compatibility must be considered.
• Q: Is BUR roofing eco-friendly?
  A: Many systems use recycled materials, and white coatings can reduce urban heat islands; however, asphalt is a petroleum product and has an environmental impact.
• Q: How do you fix a leak in a built up roof?
  A: Isolate the source, remove surface gravel, dry the area, and apply compatible patching felts and bitumen. Always consult a professional for permanent repairs.
• Q: What kind of warranties are available?
  A: Manufacturer warranties range from 10–30 years. Ask your contractor for details and limitations based on system choices.

Summary Table: Key Features Of Built Up Roof Systems

Attribute	BUR System
Ideal Slope	Flat and low-slope (up to 3:12)
Typical Layers	3–5+ (bitumen and felts)
Service Life	20–40+ years
Best For	Commercial, industrial, high-traffic rooftops
Energy Efficiency	Medium, improved with reflective coatings
Cost Range	$4–$12/SF (installed, US 2025)
Maintenance Level	Low to moderate (routine inspections)
Environmental Factors	Gravel/CAP protects from fire, UV, hail
Modern Upgrades	Cold-applied, modified bitumen, cool roof coatings

Choosing The Right Built Up Roof: Next Steps

With its time-tested durability and redundancy, built up roofing continues to serve as a staple choice for American flat and low-slope structures. Work with a knowledgeable roofing professional to assess your project’s needs, evaluate cost versus life cycle value, and select the best BUR system to protect your investment for decades to come.