The Comprehensive Built Up Roof Guide You Need

Built up roofs, also known as BUR systems, are a popular type of low-slope roofing solution. Constructed from multiple layered materials, BUR systems create a continuous, watertight covering ideal for flat or gently-pitched roofs.

You’ll learn what a BUR system is, the different types available, key benefits, considerations, installation process, and answers to common questions.

What is a Built Up Roof?

A built up roof refers to a roofing system commonly installed on flat or low-pitched roofs. It involves alternating layers of bitumen (asphalt or coal tar), reinforcing fabrics, and surfacing materials to form a solid, continuous waterproof membrane.

The key characteristics of a built up roof are:

what is a built up roof
  • Designed for low-slope or flat roofs
  • Employs multiple layers fused into a monolithic covering
  • Waterproof and weatherproof membrane

The layers work together to create seamless protection across the entire roof. The bitumen acts as a bonding agent and waterproofer while the reinforcing fabrics provide strength. The top layer shields the roof from ultraviolet light, impacts, and other damage.

How Does a Built Up Roof Work?

During installation, layers are assembled from bottom up directly onto the roof deck. The key components include:

  • Primer: Coats the roof deck to improve adhesion
  • Bitumen: Hot asphalt or cold adhesives used as the waterproofing agent
  • Reinforcing fabric: Fibrous mats like roofing felt placed between bitumen layers
  • Surfacing material: Top layer such as aggregate for UV protection

These materials fuse and bond to form a durable, monolithic membrane. The multiple layers provide redundancy if a breach occurs in any single layer. Built up roofs can be customized by adding more bitumen and fabric layers to achieve desired thickness.

Types of Built Up Roofs

There are three main types of built up roof assemblies:

1. Hot Built Up Roofs

Hot built up roofs use hot asphalt or coal tar pitch as the waterproofing agent between layers. The asphalt is heated to about 400degF before application. The heat causes the asphalt to partially penetrate layers and creates a strong bond.


  • Hot asphalt provides a durable, seamless membrane
  • Ability to customize thickness as needed


  • Asphalt odors during installation
  • Fire safety risks require special equipment
  • Temperature dependent application

2. Cold Built Up Roofs

Cold built up roofs utilize adhesives applied at ambient temperatures as the waterproofing bonding agent. These can be liquid solvent-based or water-based emulsified asphalt coatings.


  • Lower odor and fewer safety risks
  • Can install in wider temperature range


  • Joints and seams more critical
  • Adhesives can lack strength of hot asphalt

3. Ballasted Asphalt Built Up Roofs

These systems use hot asphalt waterproofing but the top protective layer is an aggregate ballast, such as gravel or stone pavers. The weight of the ballast provides fire, wind, and UV resistance.


  • Economical protective layer
  • Ballast weight enhances wind performance


  • Ballast can damage roof if not properly installed
  • Difficult to detect leaks; ballast must be removed

Benefits of Built Up Roofs

Built up roofs offer numerous advantages that make them a smart choice:


With proper maintenance, built up roofs can last 15-40 years. Some offer warranties up to 50 years thanks to material advances.


The thick, continuous membrane provides superior water resistance. Dual reinforcement between bitumen layers enhances redundancy.

Fire Resistance:

Multiple layers of non-combustible materials give built up roofs excellent fire ratings.

Energy Efficiency:

The layers have insulating properties. Coatings like reflective paint increase energy savings further.


Built up roofs can be adapted by adding more bitumen and fabric layers to achieve desired thickness and durability.

Modern cool roof technologies, recycled content, and vegetative cover further expand the sustainability and performance benefits of built up roofing.

Considerations for Built Up Roofs

While built up roofs have many advantages, there are also important factors to keep in mind:

Higher Installation Costs:

Due to intensive labor and multiple materials, built up roofs have greater installed costs than single ply or asphalt shingle roofs.

Professional Installation:

These systems require experienced contractors following proper procedures to ensure successful performance.

Regular Maintenance:

Inspections, repairs, and preventative maintenance are necessary to achieve the full lifespan potential.

Climate Considerations:

In extreme climates, the lifespan may decrease. Proper design for rain and snow loads is crucial.

Slope and Drainage:

Adequate roof slope and drainage are needed to prevent ponding water which can compromise integrity.

Advanced inspections using drones and infrared scanning can now identify potential issues early before they require extensive repairs.

Built Up Roof Installation Process

Proper installation by qualified roofers is key to achieving lasting performance. It involves meticulous attention to detail and strict adherence to procedures. The main steps include:

  1. Tear off old roofing if replacing existing system
  2. Inspect deck condition and make any necessary repairs
  3. Prime the roof deck
  4. Install base bitumen layer
  5. Embed reinforcing fabric into bitumen
  6. Apply 2-4 more alternating bitumen and fabric layers
  7. Install surfacing layer such as aggregate

The number of layers and exact materials depend on the specific type of built up roof specified. Installation is also highly weather dependent due to the use of hot asphalt or liquid coatings. The cost can range from $4-$12 per square foot depending on the system used and local labor rates.

Frequently Asked Questions

How long do built up roofs last?

With proper maintenance and barring any extreme weather damage, built up roofs typically last 15-40 years. Enhanced materials and professional installation can result in warranties up to 50 years.

Do built up roofs require a lot of maintenance?

Built up roofs are relatively low maintenance. However, regular inspections and prompt repairs are still essential to maximize durability. Annual inspections and preventative maintenance are recommended.

Are built up roofs energy efficient?

Traditional built up roofs absorb heat but modern options like reflective white coatings can reduce air conditioning costs by up to 30%. Multiple layers also provide good insulation.

Can you install a built up roof over an existing roof?

Yes, built up roofs can be installed over existing roof systems, but the roof deck must be evaluated. Only up to two additional layers are typically recommended before completely removing old roofing.

What causes built up roofs to fail?

Common factors leading to failure include inadequate slope/drainage, puncture damage, underlying deck issues, poor maintenance, and installation defects like improper layer bonding.

Built up roofs offer time-tested waterproofing, durability, and customization ideal for low-slope commercial buildings. With professional installation and routine maintenance, these systems provide decades of reliable performance. We hope this guide helps you gain a better understanding of the composition, types, benefits, considerations, and installation process for built up roofs.

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