What Is a Built Up Roof: A Comprehensive Guide for Home and Commercial Uses

A built up roof, commonly known as a BUR, is a traditional waterproofing system that layers multiple asphalt or coal tar felts with reinforcing fabrics to form a durable, weather-resistant surface. This article explains what a built up roof is, how it is constructed, its pros and cons, common applications, and maintenance considerations to help readers determine if BUR is the right choice for their building.

Understanding The Built Up Roof System

A built up roof is a multi-ply roofing system that combines bitumen (asphalt, coal tar, or cold-applied liquids) with reinforcing sheets and a top surface protection. The typical BUR assembly includes three parts: a primer or base sheet, multiple plies (felt sheets) embedded in bitumen, and a surface cap sheet or coating. These layers work together to provide waterproofing, durability, and UV protection. The staggered ply arrangement reduces the risk of leaks and distributes foot traffic, ponding, and temperature fluctuations more evenly than single-layer roofs.

Key Components And Their Roles

• Substrate or Deck: The roof surface that provides a solid base for BUR layers. It must be clean, dry, and structurally sound.
• Base Sheet: The first layer applied over the deck, helping with adhesion and moisture resistance.
• Plies (Felt Sheets): Typically asphalt- or coal tar-impregnated felts that are embedded in hot bitumen to create redundancy and waterproofing.
• Surfacing: The final layer, which can be granulated cap sheets, reflective coatings, or clean gravel to protect against UV radiation and weathering.
• Insulation (optional): Insulation boards or blankets may be added above or below BUR to improve energy efficiency.

Common Variations And Modern Adaptations

Historically, coal tar was common, but modern BUR systems typically use asphalt or modified bitumen to improve performance and environmental profile. A popular variation is the fiberglass reinforced BUR, which uses fiberglass sheets for added strength and fire resistance. Other modern adaptations include:

• Polymer-Modified Bitumen BUR: Enhances flexibility and durability in temperature extremes.
• Reflective BUR: Cap sheets with reflective granules or white coatings to reduce heat absorption, improving energy efficiency.
• Hybrid Assemblies: Combine BUR with advanced membranes or silicone coatings for extended life and easier maintenance.

Advantages Of Built Up Roofs

• Durability: Multiple layers provide redundancy; damage to one ply does not immediately compromise the entire roof.
• Weatherproofing: Excellent waterproofing, particularly when properly installed and maintained.
• Proven Longevity: When properly maintained, BUR roofs can last decades, making them cost-effective over time.
• Versatility: Compatible with various insulation types and roof shapes, including flat and low-slope designs.
• Cost Stability: Material costs are typically predictable, and BUR has a long track record in both commercial and residential markets.

Disadvantages And Considerations

• Weight: BUR systems are heavier than single-ply membranes, which may require structural verification.
• Labor Intensity: Installation is technique-sensitive and often requires skilled labor, potentially increasing labor costs.
• Maintenance: Requires periodic maintenance to address bitumen cracks, blisters, or exposed reinforcing fabric.
• Aesthetic Limitations: Generally less visually appealing than some modern single-ply membranes, though reflective coatings can improve appearance and energy use.

Installation Process And Best Practices

Bur installation begins with surface preparation, including cleaning, repairing defects, and ensuring proper drainage. The base sheet is applied and secured, followed by the successive plies that are embedded in hot bitumen or adhesive. The final cap sheet or surface coating provides UV protection. Key best practices include ensuring proper ventilation, controlling temperature during application, and performing a detailed inspection after installation to verify seam integrity and drainage paths. Hiring experienced contractors with BUR credentials is crucial for lasting performance.

Insulation, Energy Efficiency, And Reflectivity

BUR roofs can incorporate various insulation strategies. If installed with above-deck insulation, the system improves thermal performance and can reduce heating and cooling costs. Reflective or white cap sheets and coatings minimize heat absorption, lowering cooling loads in hot climates. In areas with severe weather, reflective BUR helps curb the urban heat island effect and may qualify for energy efficiency incentives in some jurisdictions. Proper integration with attic or roof-top mechanical systems is essential to prevent condensation and ensure airflow.

Maintenance, Repairs, And Common Issues

Regular inspections are essential for maintaining a built up roof. Common issues include:

• Ponding Water: Standing water over extended periods can accelerate deterioration and lead to leaks if not properly drained.
• Blistering And Cracking: Result from moisture, heat, or poor installation; requires repair to prevent water intrusion.
• Cap Sheet Wear: The surface layer can erode due to UV exposure or foot traffic; timely resealing or resurfacing extends life.
• Membrane Delamination: Layers separate due to moisture or improper adhesion; professional assessment is needed for repair or replacement.

Cost Considerations And Lifespan

The upfront cost of a built up roof varies by thickness, insulation, flat or low-slope surface, and labor rates. BUR systems typically have moderate to long lifespans, often 15 to 40 years depending on climate, maintenance, and installation quality. Ongoing costs include periodic inspections, minor repairs, and potential recoating or resealing to preserve reflectivity and waterproofing. When evaluating options, compare total cost of ownership rather than only initial price to account for durability and maintenance intervals.

Applications And Suitability

BUR roofs are widely used in commercial buildings, warehouses, schools, and multifamily housing, as well as some residential flat-roof homes. They perform well on roofs with low slope, challenging weather exposure, and need for long-term waterproofing. The system’s robustness makes BUR a favored choice in regions with heavy rainfall, wind, or hail. However, newer alternatives like single-ply membranes may be preferred in projects prioritizing lighter weight or quicker installation. A professional assessment can determine compatibility with existing structures and climate goals.

Choosing A Contractor And Ensuring Quality

Due diligence in selecting a contractor is essential for BUR success. Consider: verified licenses and insurance, references and portfolio, detailed written proposals outlining materials, ply count, and warranty terms, and a clear maintenance plan. Ask about ventilation requirements, drainage design, and how UV protection will be maintained. A thorough pre-installation inspection and a post-installation warranty provide additional protection against premature failure. Clear communication on inspection schedules and recommended maintenance helps maximize the roof’s life cycle.

Environmental And Safety Considerations

Modern BUR practices emphasize safer, more environmentally friendly materials. Polymer-modified bitumen and ventilated cap sheets reduce odor and emissions during installation. Safety protocols, including fall protection and heat handling procedures for hot asphalt, are essential on every job site. For buildings pursuing green certifications, reflective BUR and appropriate insulation can contribute to energy performance goals while maintaining reliable weather protection.

Frequently Asked Questions

1. How long does a built up roof typically last? Depending on climate and maintenance, BUR roofs commonly last 15–40 years.
2. Can BUR be applied over existing roofs? In many cases, yes, but a structural and moisture assessment is needed to prevent trapped moisture and ensure proper adhesion.
3. Is BUR suitable for metal buildings? Yes, BUR can be applied over metal decks with the right substrate and insulation strategy.

Conclusion: Is A Built Up Roof Right For Your Project?

Built up roofs offer proven waterproofing, durability, and versatility for a range of commercial and residential applications. By understanding the components, installation methods, maintenance needs, and life-cycle costs, builders and property owners can determine if BUR aligns with performance goals, climate conditions, and budget. When properly installed and maintained, a built up roof stands as a resilient solution that protects structures against the elements for many years.