What Is A Built Up Roof? Comprehensive Guide For Homeowners And Building Professionals

Built up roofs have long been a preferred solution for flat and low-slope roofing, popular among commercial buildings and larger residential structures. This article explores what a built up roof is, its essential components, installation process, benefits, drawbacks, and key maintenance aspects. Whether you’re a property owner making roofing decisions or a contractor advising clients, this in-depth guide provides the knowledge needed to evaluate built up roofing systems effectively.

Summary Table: Built Up Roof Key Facts

Aspect	Description
Definition	Multi-layered roofing system for flat/low-slope roofs using bitumen and reinforcing plies
Common Use	Commercial & industrial buildings, some residential applications
Average Lifespan	20-30 years (with proper maintenance)
Main Materials	Bitumen (asphalt or coal tar), roofing felts, aggregate surfacing
Installation Time	Medium to High (multi-stage process)
Key Benefits	Excellent waterproofing, durability, UV resistance
Drawbacks	Weight, odor during installation, slower installation, cost
Maintenance	Regular inspections, minor repairs as needed

What Defines A Built Up Roof?

A built up roof, or BUR, is a multi-layered roofing system designed for flat or low-pitch surfaces, most commonly used in commercial construction but also suitable for certain homes. BURs utilize alternating layers of bitumen (asphalt or coal tar) and reinforcing fabrics, generally covered with a final protective layer such as gravel or a mineral cap sheet.

These layers are “built up” on the roof deck, one after another, which is how the system earns its name. The construction process delivers enhanced waterproofing, redundancy, and overall resilience.

Core Components Of Built Up Roofing Systems

1. Bitumen (Asphalt Or Coal Tar)

The bitumen layer serves as both adhesive and waterproofing agent. Traditionally, hot asphalt is used, but cold-applied and self-adhering variants are also available. Coal tar pitch is less common today due to environmental concerns but offers flexibility and chemical resistance in certain situations.

2. Roofing Felts Or Reinforcement Sheets

Each built up roof system requires several layers of reinforcing sheets, known as roofing felts. Made from organic materials, fiberglass, or polyester, these felts provide structural strength, stability, and puncture resistance for the roof membrane.

3. Surfacing Materials

The final surface layer is crucial for UV protection and weather resistance. It often consists of gravel set in bitumen (“tar and gravel roof”), mineral cap sheets, or reflective coatings. Some installations use stone or mineral granules to further enhance durability and appearance.

Types Of Built Up Roofing Systems

• Hot-Mopped BUR: Traditional method utilizing hot liquid asphalt applied to the roof deck and between plies.
• Cold-Applied BUR: Uses liquid adhesives at ambient temperatures, emitting less odor and VOCs.
• Ballasted (Gravel) BUR: Features river-washed or crushed stone as the final surfacing for added weight and wind resistance.
• Modified Bitumen BUR: Incorporates polymer-modified bitumen sheets for increased flexibility and temperature resistance.
• Hybrid Systems: Combine BUR with single-ply membranes for customized performance.

Table: Built Up Roof Variants Comparison

Type	Key Features	Ideal Application
Hot-Mopped	Long-standing tradition, strong adhesion	Large commercial flat roofs
Cold-Applied	Less odor, flexible application	Occupied buildings, sensitive environments
Ballasted	Uses heavy stone/gravel for protection	Areas with high winds, puncture-prone roofs
Modified Bitumen	Added polymers, temperature and impact resistant	Extreme weather or temperature regions

How Is A Built Up Roof Installed?

Installation of a built up roof is a complex process that requires skill and precision. The steps involved are:

1. Substrate Preparation: Roof deck is cleaned and prepped for adherence.
2. Vapor Barrier Installation: Optional layer to prevent condensation and moisture infiltration.
3. Insulation Placement: Insulating boards are mechanically fastened or adhered to meet energy requirements.
4. Base Sheet Application: First layer of roofing felt is set in hot or cold-applied bitumen.
5. Successive Ply Application: Multiple plies of felt and bitumen are alternately layered (typically 3 to 5 plies). More plies equal increased protection and redundancy.
6. Surfacing: Top layer of aggregate, mineral cap, or coating is installed for UV/impact protection.
7. Flashing and Edging: Critical detailing around penetrations, edges, and joints to prevent leaks.

Advantages Of Built Up Roofs (BUR)

• Excellent Waterproofing: Multiple layers reduce chances of leaks, making BURs highly watertight when installed correctly.
• Durability: BUR systems are renowned for their long lifespan, typically lasting 20 to 30 years or more with proper care.
• UV and Fire Resistance: Aggregates or mineral granules provide substantial protection against sunlight and fire hazards.
• Low Maintenance: With routine inspections and minor repairs, BURs demand less frequent major work compared to some other systems.
• Redundancy: If a single layer is breached, the remaining plies maintain system integrity and keep interiors dry.
• Good Insulation Compatibility: Works well with various rigid foam or composite insulation boards.
• Proven Track Record: Used for over a century in the US, BURs have a well-established record of performance in virtually all climates.

Disadvantages And Limitations Of Built Up Roofs

• Installation Complexity: The multilayer system is labor-intensive, requiring professional expertise for best results.
• Weight: BURs are heavier than most single-ply systems, sometimes necessitating structural reinforcement of the building frame.
• Odor And Fumes: Hot-applied bitumen emits strong odors and VOCs during installation, requiring precautions with air intakes and occupancy.
• Cost: Initial installation is often more expensive versus some alternatives, though the cost may be offset by longevity.
• Slow Installation: Multi-stage process generally takes more time compared to membrane or spray systems.
• Flexibility Limitations: Built up roofs are less suited to roofs with high movement or excessive foot traffic unless specifically reinforced.

Where Are Built Up Roofs Commonly Used?

Built up roofs are most commonly found on commercial buildings, schools, warehouses, hospitals, and governmental facilities, especially in the United States. Their low-slope design makes them well-suited for larger structures, but they can also be found on some modern, flat-roofed homes and multifamily housing complexes.

Material Choices For Built Up Roof Systems

Bitumen Varieties

• Asphalt: The most widely used option, offering balanced cost, performance, and UV stability. Typically used hot, though cold-applied versions exist.
• Coal Tar Pitch: Provides unique chemical properties and flexibility, but is less common due to health and environmental concerns.
• Modified Bitumen: Blends asphalt with synthetic rubbers or plastics for enhanced durability and elastic response.

Reinforcement Layers

• Organic Felts: Traditional paper or cellulose felts. Less durable, but still in use for budget applications.
• Fiberglass Felts: Higher strength, moisture and fire resistance, and dimensional stability.
• Polyester Felts: Excellent elasticity and puncture resistance for demanding environments or higher foot traffic.

Surfacing Options

• Gravel/Ballast: Stones embedded in final layer of bitumen offer UV protection and reflectivity.
• Mineral Cap Sheets: Pre-coated, mineral-surfaced membranes provide a cleaner finish and color options.
• Reflective Coatings: White or silver coatings enhance solar reflectivity for energy-efficient roofs.

Built Up Roofs Vs. Single Ply Roofs

Aspect	Built Up Roof (BUR)	Single Ply Roof
Layers	Multiple plies with bitumen	Single membrane layer
Waterproofing	Redundant, robust	Relies on one membrane; less redundancy
Installation	Complex, time-consuming	Faster, less labor-intensive
Weight	Heavier, may require reinforcement	Lightweight
UV/Weather Resistance	High (with surfacing)	Varies by membrane type (TPO, PVC, EPDM)
Repairability	Easy to patch; many layers	Repairs require careful sealing; single layer punctures are critical
Cost	Generally higher at install; may prove cost-effective over time	Lower to mid-range upfront costs

Key Steps In Maintaining A Built Up Roof

To ensure a maximum lifespan for a built up roof, consistent maintenance is required. The most critical steps include:

• Routine Inspections: Schedule inspections twice a year and after major storms. Look for blisters, cracks, ponding water, and gravel displacement.
• Prompt Repairs: Address cracks, punctures, and flashing separations as soon as they are detected to prevent water intrusion.
• Debris Removal: Remove leaves, branches, and debris to prevent clogged drains and water pooling.
• Drainage Maintenance: Keep scuppers, gutters, and downspouts clear to avoid standing water, which can accelerate membrane breakdown.
• Resurfacing: Reapply aggregate or protective coatings as needed when the existing layer thins or wears away.
• Professional Evaluations: Engage a qualified roofing contractor for annual or biennial professional roof evaluations and more extensive repairs.

Expected Lifespan And Warranties

With proper installation and regular maintenance, a built up roof typically lasts 20 to 30 years. Some high-quality systems, especially those with four or five plies and excellent surfacing, can surpass the 30 year mark.

Warranty terms for BUR systems often range from 10 to 20 years, depending on materials, number of plies, and installer certification. Some manufacturers may offer longer warranties for systems with premium components and thorough maintenance programs.

Cost Considerations: Installation And Long-Term Value

Initial installation costs for built up roofs usually range from $4.00 to $10.00 per square foot, depending on the number of plies, material selection, and regional labor rates. Additional expenses may arise with insulation upgrades, roof deck reinforcement, or custom surfacing.

While higher than some single-ply options, the reduced frequency of major repairs and excellent service life can make BURs cost-effective in the long run, especially for structures with large, open roof areas.

Energy Efficiency And Sustainability

Modern built up roofs can be engineered for energy efficiency with reflective surfacing and high-quality insulation. Light-colored aggregate or coatings minimize solar heat gain, helping reduce building cooling loads and energy costs during summer.

Some manufacturers offer BUR components with recycled content, and cold-applied adhesives further reduce environmental impact. End-of-life recycling options are expanding, with higher rates for aggregate and some felts, though overall system recyclability still trails the best single-ply alternatives.

Best Practices For Selecting A Built Up Roof Contractor

• Licensing and Insurance: Select fully licensed, bonded, and insured professionals.
• Experience: Choose contractors with extensive BUR installation and repair history.
• Manufacturer Certification: Ask for certification or approved installer status from major material suppliers.
• Reference Projects: Review completed BUR projects similar in size and complexity to your own.
• Comprehensive Quotes: Compare detailed estimates covering materials, labor, warranties, and optional services.
• Safety Record: Safe roofing practices are critical. Ask about OSHA training and protocols for hot-applied bitumen or working in occupied buildings.

Signs It’s Time To Replace Or Upgrade Your Built Up Roof

Over time, even the best maintained BURs will require replacement. Watch for these indicators:

• Widespread Blistering Or Bubbling: Indicates trapped moisture and possible membrane separation.
• Multiple Layer Failures: Numerous leaks, splits, or plies coming loose signal end of service life.
• Gravel Or Aggregate Loss: Accelerated wear of the top surface leads to UV and thermal damage of underlayers.
• Insulation Damage: Prolonged water intrusion may compromise underlying insulation, impacting energy efficiency.
• Persistent Ponding Water: Poor drainage, sagging, or deck deterioration can result in standing water, increasing risk of leaks and shortening lifespan.

Most experts recommend replacing a built up roof around the 25-30 year mark, even if no severe damage has occurred, to prevent catastrophic failure and costly interior repairs.

Frequently Asked Questions About Built Up Roofs

Is A Built Up Roof Suitable For Residential Homes?

While originally developed for commercial and industrial buildings, built up roofs can be a strong choice for modern homes with flat or low-slope roof areas, particularly where long-term durability is a priority. Adequate structural support and professional installation are crucial.

How Does A Built Up Roof Compare To Modified Bitumen?

Modified bitumen is a type of hybrid system that incorporates polymer additives for greater flexibility and temperature resistance. Both offer robust waterproofing, but modified bitumen may be easier to install in rolls and accommodates minor roof movement better than traditional BUR systems.

Is Built Up Roofing Fire Resistant?

Yes, built up roofs typically achieve strong fire resistance thanks to multiple plies and noncombustible aggregate surfacing. Fire ratings depend on specific material selection and overall roof assembly. Consult manufacturer data for UL Class ratings.

What Kind Of Maintenance Does A Built Up Roof Require?

Regular inspections, minor patching, and resurfacing as needed are key. Keep drains clear and address minor splits or punctures early. With this routine, a BUR system will maximize its service life with fewer emergency repairs.

Final Thoughts On Built Up Roofs

Built up roofs remain an industry standard for flat and low-slope structures due to their proven performance, robust waterproofing, and long-term value. With skilled installation and regular maintenance, BUR systems deliver decades of reliable protection, justifying their place atop commercial and select residential projects throughout America.