Built Up Smooth Roof: A Comprehensive Guide to BUR Roof Systems

The built up smooth roof, commonly referred to as BUR (Built-Up Roofing), combines multiple layers of bitumen and reinforcing fabrics to create a durable, water resistant surface. The smooth finish is achieved by applying a final coating or flood coat, producing a seamless, aesthetically clean roof edge for commercial and some residential projects. This article explains what a built up smooth roof is, how it’s installed, its benefits and drawbacks, and best practices for maintenance and longevity.

What Is Built-Up Smooth Roof

A built up smooth roof is a traditional roofing system that stacks alternating layers of asphalt or coal tar pitch with fleece or fiberglass reinforcement. The top layer is a smooth, reflective coating designed to seal the surface and resist weathering. BUR systems are known for their long track record, excellent waterproofing, and versatility across flat and low-slope roofs. The smooth finish reduces debris buildup and simplifies cleaning, while protecting the underlying insulation and structure.

Materials And Layers Of A BUR System

A typical BUR assembly includes several key components:

• Insulation and deck: Plywood, gypsum, or concrete decks, plus insulation to minimize heat transfer and improve energy efficiency.
• Bitumen or tar: Asphalt, coal tar pitch, or modified bitumen as the waterproofing matrix.
• Reinforcement: A reinforcing fabric such as fiberglass or organic mats embedded between bitumen layers for strength and dimensional stability.
• Surfacing: The final smooth coating, which may be a coal tar or asphalt-based cap sheet, or a silicone or acrylic topcoat for enhanced weather resistance.
• Optional ballast: Gravel or crushed stone can be added to protect the top layer and provide UV protection, depending on design requirements.

The result is a continuous, multi-ply membrane with a perimeter and flashing detailing designed to handle expansion, contraction, and ponding water on flat to gently sloped roofs.

Advantages Of A Built Up Smooth Roof

• Durability: Multiple layers offer redundancy against leaks and punctures.
• Waterproofing: Excellent seam and edge protection, making BUR highly resistant to water intrusion.
• Longevity: A well designed BUR system can last 20 to 40 years with proper maintenance.
• Versatility: Suitable for large commercial roofs, plazas, and roofs with parapets and unique details.
• Reflectivity options: Smooth cap coats can be selected for improved solar reflectance to reduce cooling loads.

Disadvantages And Considerations

• Weight: BUR systems are heavier than single-ply membranes, requiring structural adequacy.
• Installation disruption: The application process can be noisy and produce odors from hot bitumen.
• Maintenance: Regular inspections are needed to identify blisters, cracks, or membrane shrinkage.
• Repair complexity: Repairs can be more involved than with some newer single-pply systems.

Installation Process For A Smooth BUR Roof

Professional installation follows a sequence to create a durable, smooth surface:

• Preparation: Roof deck repair, cleaning, and flashing installation around penetrations.
• Insulation placement: Install and secure insulation boards if required to meet U-value targets.
• First ply and reinforcement: Apply a hot or cold bitumen layer and embed reinforcement fabric.
• Additional plies: Add one or more additional layers of bitumen with reinforcement in between, ensuring proper adhesion and curing between plies.
• Surface coat: Apply a smooth cap sheet or coating to create a uniform, flat finish. In some designs, reflective or elastomeric coatings are used.
• Curing and inspection: Allow proper curing and conduct a warranty-quality inspection of seams, penetrations, and edge details.

Maintenance And Longevity

Proper maintenance extends BUR life significantly. Seasonal inspections should focus on:

• Seams and flashings: Look for lift, splitting, or opened mastic around curbs, vents, and penetrations.
• Surface coatings: Re-coat or refresh reflective top coats to maintain UV protection and appearance.
• Drainage: Ensure gutters and downspouts are clear and that ponding is minimized.
• Patching repairs: Address blisters or gouges promptly with approved repair methods to prevent leaks.

Maintenance intervals depend on climate, roof use, and building exposure. Regular professional inspections are recommended every 3 to 5 years, with more frequent checks for extreme weather regions.

Weather Performance And Energy Considerations

BUR systems perform well in diverse climates due to multiple protection layers. In hot climates, reflective cap coats help reduce cooling loads, while in cold, snowy regions, the waterproof nature prevails against moisture infiltration. Energy efficiency can be improved by pairing BUR with proper insulation and reflective coatings to minimize heat gain and heat loss, contributing to lower operating costs over the roof’s life.

Cost Considerations And Value

Initial installed cost for BUR can be higher than some modern single-ply membranes, but lifetime cost often stays competitive due to durability and repair ease. Factors influencing price include:

• Deck condition and insulation: The base state affects preparation time and material needs.
• Number of plies: More plies provide greater durability but raise cost.
• Topcoat material: Reflective or elastomeric coatings add upfront cost but save energy over time.
• Local labor: Regional differences in labor rates influence total cost.

Owners should balance upfront expenditure with long-term performance, maintenance needs, and potential energy savings when evaluating BUR options.

Environmental And Safety Considerations

BUR systems have environmental implications primarily related to material production and disposal. Modern installations favor low-VOC bitumen and recyclable components where feasible. Safety during installation requires certified crews, proper ventilation for asphalt fumes, heat management for hot asphalt processes, and adherence to fall protection and fire safety protocols.

Selecting A Contractor And Design Details

Choosing a qualified contractor is essential for reliable BUR performance. Key qualifications include:

• Experience with BUR systems: Demonstrated track record on similar roof configurations.
• References and warranties: Clear warranties covering materials and workmanship.
• Code compliance: Adherence to local building codes and fire ratings.
• Inspection readiness: Availability for post-installation inspections and maintenance guidance.

Design decisions should address climate, drainage, insulation levels, and roof height, ensuring a smooth, durable finish that meets both performance and aesthetic goals.