What Is a Built-Up Roof?

A built-up roof is a multi-layered roofing system designed for flat or low-slope roofs. Unlike pitched roofs, where shingles or tiles allow water to slide off, flat roofs require a watertight barrier to prevent leaks. The BUR system achieves this with multiple layers of bitumen (asphalt or coal tar) and reinforcing fabrics that work together to create a tough, weatherproof seal.

The construction of a built-up roof typically involves three to five layers, each made of bitumen (often applied hot) and roofing felt. The final layer is a coating of aggregate, like gravel or stone, which shields the roof from sun, weather, and debris. This layering system creates a thick, impenetrable roofing membrane that protects against water infiltration, UV rays, and extreme weather.

Built-up roofs (BURs) are most commonly found on commercial buildings, especially those with flat or low-slope roofs, like warehouses, retail spaces, and office buildings. Flat roofs are typical in commercial construction because they provide easier access for maintenance, HVAC installations, and other rooftop equipment. The durability and long lifespan of BURs make them a cost-effective choice for these types of structures.

However, BURs can also be used on residential properties with flat or low-slope roofs, though this is less common. Homeowners may choose built-up roofing for flat-roofed home additions, porches, or garage roofs where durability and weather resistance are priorities. Residential use tends to be less frequent due to the installation complexity and weight of BUR systems, which aren’t typically needed for traditional sloped residential roofs.

Built-Up Roof Terminology

There a few alternative terms and abbreviations commonly used for built-up roofs (BURs):

1. Tar and Gravel Roof: Named for the combination of bitumen (tar) and a gravel top layer.
2. BUR Roof: An abbreviation for built-up roofing, commonly used in the industry.
3. Hot Mop Roof: Refers to the hot application of asphalt or tar during installation.
4. Asphalt and Gravel Roof: Highlights use asphalt as the bitumen component and a gravel finish.
5. Layered Roofing System: Describes the multi-layer approach typical of built-up roofs.
6. Membrane Roof: While broader, this term sometimes includes BUR systems as a membrane installation on flat or low-slope roofs.

Each of these terms emphasizes a particular aspect of BUR, whether it’s the roofing system’s materials, method, or structure.

How Built-Up Roofs Work

Built-up roofs are typically constructed with alternating layers of bitumen and reinforcing fabric, such as fiberglass or organic roofing felt. Depending on the roofing system and the installer’s preference, these layers are applied in a hot or cold process. Here’s a breakdown of the layers:

1. Base Layer: The first layer is often a felt or fabric that adheres directly to the roof deck, forming the base for additional layers.
2. Bitumen Layers: Each layer of fabric or felt is followed by a layer of hot or cold bitumen, usually asphalt or coal tar. These layers are bonded to create a watertight seal.
3. Reinforcing Fabric: Between each layer of bitumen, reinforcing fabrics add strength, flexibility, and resilience to the roofing system.
4. Top Layer: The final layer is usually a coating of gravel or stone. This aggregate protects the underlying layers from UV rays, weather exposure, and foot traffic, enhancing the roof’s lifespan.

Benefits of Built-Up Roofing

Built-up roofs offer several advantages, making them a popular choice for residential and commercial properties with flat or low-slope roofs.

1. Durability: Built-up roofs are known for their exceptional durability. The multiple layers make it resistant to water, extreme temperatures, and wear from foot traffic. A well-maintained BUR system can last 15-30 years, depending on the quality of materials and installation.
2. Weather Resistance: BUR systems offer excellent protection against weather. The layers create a thick barrier that prevents water infiltration, making it highly effective against leaks, especially in areas with heavy rainfall or snow.
3. Fire Resistance: With materials like gravel and stone on the top layer, built-up roofs are fire-resistant, adding a layer of protection to your building, which is especially valuable in areas prone to wildfires or high temperatures.
4. Energy Efficiency: Built-up roofs can improve energy efficiency. The gravel layer helps reflect sunlight, reducing heat absorption and keeping the building cooler in summer. Additionally, BUR systems can be designed with additional insulation to enhance energy savings further.
5. Low Maintenance: BUR systems are low-maintenance compared to other flat roofing options. The tough outer layer protects against damage, and when minor repairs are needed, they’re usually simple and inexpensive.

Drawbacks of Built-Up Roofing

While built-up roofs have many advantages, they’re not without potential downsides. Here are a few considerations to keep in mind:

• Heavyweight: BUR systems are quite heavy due to the layers of bitumen and gravel. Some buildings may require structural reinforcements to support the additional weight.
• Installation Complexity: Installing a built-up roof is labor-intensive and requires specialized skills. This can increase installation costs compared to other roofing options.
• Odor During Installation: The hot bitumen used in BUR systems can create a strong odor during installation, which may be a concern for buildings in populated areas or for residents who are sensitive to smells.

Installation Process of a Built-Up Roof

Installing a built-up roof involves several steps and requires specialized tools and expertise. Here’s a general outline of the process:

1. Preparation: The roof deck is thoroughly cleaned and prepared to ensure the BUR system adheres properly. Any old roofing material is removed, and any structural repairs are made.
2. Applying Base Layers: The first layer of felt or fabric is installed, creating a solid foundation for additional layers. This layer is bonded to the roof deck using bitumen or a cold adhesive.
3. Adding Layers: Additional layers of bitumen and reinforcing fabric are applied, with each layer adding thickness and durability to the roofing system. Typically, three to five layers are used, depending on the desired durability and the property’s climate conditions.
4. Top Layer Application: The final layer consists of gravel or stone aggregate applied over the top bitumen layer. This layer protects against UV damage, reduces heat absorption, and adds fire resistance.
5. Quality Check and Clean-Up: After installation, the roofing team inspects the roof to ensure all layers are properly applied and the edges are sealed to prevent water infiltration.