When it comes to protecting the low-slope commercial structures, only a few solutions come in handy. In this matter, built up roof (BUR) stands the test of time. Also called “tar and gravel roof”, it is the most reliable roofing system for flat and low-slope structures.

BUR consists of multiple layers of roofing, which are alternated with the layers of bitumen, including coal tar, asphalt, and cold-applied adhesives. A built up roofing system is often finished with a top layer of gravel that acts as a reflective coating and offers extra protection. These layers are known for offering durability, weather resistance, and environmental thickness.

For over a century, these roofs have been considered reliable for commercial and industrial buildings, delivering consistent results where other materials fall short of performance. The multilayer structure not only offers waterproofing but also offers superior protection against UV rays, mechanical wear, and temperature fluctuations.

The blog will delve deep into the details of the built-up roof and its components, advantages, and applications. Let’s get started!

Key Takeaways

What Is a Built-Up Roof?

A Built-Up Roof (BUR) is a multi-layer roofing system that is made up of various alternating layers of bitumen, which could be coal tar or asphalt, and reinforcing fabrics. On top of that, this roofing system is covered with a protective surface, such as mineral granules, gravel, or a reflective coating. All these layers are built up on-site to develop a smooth and watertight barrier that offers superior protection against harsh weather conditions.

The roofing is ideal for exceptional durability as it offers protection against leaks, punctures, and other structural damage. Built-up roofing systems are also designed to withstand harsh environmental conditions. Whether your commercial low-slop structure is located in an area prone to heavy landfall, extreme heat, or high winds, a BUR provides excellent protection. Energy efficiency is another key feature of roofing.

Components of a Built-Up Roof System

Base Sheet:

The base sheet acts as the foundation of the system that is applied directly to the roof deck. The base ensures adhesion and acts as a barrier against moisture. Roofers attach this sheet directly to the building by using the fasteners. Creating a seamless and flat surface protects against humidity. The base sheet is important as, without it, the rest of the layers cannot adhere properly.

Bitumen Layers:

Bitumen layers consist of hot asphalt or cold-applied bitumen that is used to adhere the layers together. The adherence results in a flexible and waterproof membrane. It also acts as the waterproofing agent in a built up roof system. Roofers heat bitumen and spread it between the ply sheets in order to create tightness among the layers. This substance not only fills gaps between layers but also creates a watertight seal across the entire roof due to its water-resistant nature.

Reinforcing Ply Sheets

These are typically made from fiberglass, organic felt, or polyester, adding strength and tear resistance to the roof. These are also known as reinforcing fabrics, which provide flexibility. The roofers soak these ply sheets in the bitumen before laying them down.

These ply sheets consist of 3 to 5 layers. The addition of more layers makes the roof more challenging. These sheets act as a buffer as they absorb stress from weather changes.

Surfacing Layer:

The surfacing or the top-most layer comprises mineral granules, gravel, and reflective coating. This layer is known for shielding against UV rays, mechanical tears, and weathering. Gravel is the most conventional option. It is spread over the top while the bitumen is still hot, so it sticks. It blocks UV rays and prevents damage from flying debris. Mineral cap sheets are a choice. These are pre-coated sheets that are applied on top of the final bitumen layer, providing a clean finish.

Reflective coatings are also standard, especially in warm climates. These coatings help lower cooling costs by bouncing back the sunlight and reducing wear caused by heat.

Types of Built Up Roof Systems

After understanding what is built up roof system is, you must be wondering about its types of installation. Installing a BUR is not an easy task, as it demands care and expertise. Roofers must follow safety protocols to create a strong and waterproof system for high durability. Here are the two types of Built Up Roof Systems! Hot built-up roof and Cold built-up roof.

Features	Hot built-up roof	Cold built-up roof
Definition	A traditional built-up roof system that uses hot asphalt or bitumen as the adhesive between layers of roofing felt or fiberglass.	A modern version of BUR that uses cold-applied adhesives instead of hot asphalt or bitumen.
The installation method	Asphalt is heated in a kettle and mopped onto the roof to bond layers of felt or ply sheets.	Adhesive is applied directly from containers without heating; layers are pressed together.
Adhesive type	Hot asphalt, coal tar, or modified bitumen heated to 400–450°F (204–232°C)	Cold-applied liquid adhesives
Curing time	Immediate cooling and hardening upon application.	Requires curing or drying time, depending on adhesive type and weather.
Layers	3–5 plies of bitumen and reinforcing felt, topped with gravel or reflective coating.	3–5 plies of felt or fabric embedded in cold adhesive, topped with mineral or reflective surfacing.
Equipment used	Kettle, mopping tools, torches for modified bitumen	Hand tools, rollers, or spray equipment; no heating equipment.
Weather limitations	Best installed in warm, dry conditions; weather-sensitive due to hot asphalt handling.	They can be applied in cooler or more variable weather, less affected by temperature extremes
Maintenance	Low maintenance; easily repairable with asphalt patches.	Easier to repair and recoat with cold adhesives; safer for occupied sites.
Durability	Extremely durable; proven performance over decades.	Durable but slightly less proven over the long term than hot systems.
Safety concerns	Fire hazards from hot kettles and burns from hot asphalt.	Safer installation; no open flames or hot materials.
Cost	Slightly lower material cost but higher labor and safety costs.	Slightly higher material cost but reduced labor and safety costs. Ideal Use
Ideal use	Large commercial or industrial roofs where durability and longevity are priorities.	Occupied or sensitive buildings (schools, hospitals, offices) where odor, noise, or fire risks must be minimized.

Installation Process of Built-Up Roofs

The installation process of built-up roofs consists of various steps. Here is the detail of these steps!

Surface Preparation

In the surface preparation, the first step involves the roof deck inspection for structural integrity, moisture, and cracks. The deck must be dry, clean, and free of debris to ensure proper adhesion. In the case of improving adhesion, you can apply primer to the deck.

Installing a Vapor barrier

If necessary, one must install a vapor barrier. A vapor barrier prevents moisture from penetrating and damaging the roof layers. The barrier is usually made up of bitumen material or polyethylene sheets. For the application, one must lay over the prepared deck and seal all the joints.

Installing roof insulation

Common insulation boards include perlite or mineral wool. The roof insulation is mechanically fastened and adhered with hot asphalt or cold-applied adhesive, depending on the structure.

Installing base sheets

Installation of the base sheets forms the foundation of the BUR system. These sheets are either mechanically fastened or adhered directly to the vapor barrier or insulation. The base sheets are known for providing waterproofing. You can apply it either through the hot-mopped method or the cold method.

Applying bitumen & roofing plies

This is the core of the BUR system. The process involves alternating layers of hot bitumen (asphalt or coal tar pitch) and reinforcing felt sheets (usually consisting of fiberglass or polyester). Each ply is embedded into the bitumen while it’s still hot, forming a seamless, bonded structure. You can add three, four, and five-ply systems for better protection.

Surfacing layer application

After completing the number of plies, the roof is finished with a surfacing layer. This layer protects bitumen from UV radiation, mechanical damage, and weather. You can add gravel surfacing, mineral surfaced cap sheet, and reflective coating.

Detailing and Finishing

The last step involves detailing and finishing. All edges, penetrations, and seams are sealed using flashing materials and bitumen, ensuring waterproofing.

Benefits of Built-Up Roofing Systems

There are multiple advantages of built up roofs. Here is the detail of these benefits!

Durability and Longer Life Span

BUR systems are ideal for delivering durability and a longer life span. The multiple layers of bitumen offer excellent protection against leaks, punctures, and other forms of damage. This multi-ply construction offers redundancy, meaning that even if one layer is compromised, the others will continue to protect the building. Moreover, the reflective coating will provide additional durability.

Waterproofing

The multiple layers of bitumen and fabric form a protective and watertight barrier, making it ideal for waterproofing properties.

Resistance against weather conditions

Built-up roofings are designed in such a way that they can resist harsh environmental conditions. Whether your construction project or building is located near an area that is prone to heavy rainfall, heatwaves, or gusty winds, the BUR system offers excellent protection due to its thick and multi-layered design. The gravel coating will also help in deflecting UV rays and reducing the risk of heat damage. Moreover, the gravel ballast helps to stabilize the roof against high winds, providing additional security in areas that experience frequent storms or hurricanes.

Fire and UV Resistance

The BUR systems are built with Class A fire ratings. As a result, the built-up roofs are more suitable for protecting against fire. The mineral and gravel top layers will help reflect UV rays and prevent degradation.

Energy Efficiency

Energy efficiency provided by the built-up roofing system is another key advantage. The layers of bitumen act as an excellent insulator. These layers will help in regulating the internal temperature of buildings and commercial spaces. In terms of winter, BUR systems help retain heat, thereby reducing the need for more energy and increasing energy efficiency. During the scorching summer, the reflective coating will deflect sunlight and lower cooling costs. By reducing energy consumption, businesses can lower their environmental footprint and save on utility bills.

Customization Options

Built-up roofing systems are versatile when it comes to personalization. These systems can be tailored according to the specific needs of a low-slope commercial building. For instance, you can choose the different types of roofing systems based on building requirements. This flexibility ensures that the roofing system is optimized for its environment, providing the highest level of performance possible.

Drawbacks of BUR

Besides advantages, there are some drawbacks to the BUR system.

• Multiple layers plus gravel can be heavy for your low-slop commercial spaces. Not all structures can support it, so it could cause problems for you.
• Bitumen is a petroleum-based product that can emit fumes during installation. Additionally, BUR materials are challenging to recycle, presenting difficulties at the end of the roof’s life.
• Heating materials and applying multiple layers takes time and skill.
• While durable, it requires regular inspections to identify problems beforehand, such as cracks or pooling water. Ignoring them can lead to damaged structural integrity and costly repairs.

Built-Up Roofing vs. Other Roofing Systems

Built-Up Roofing (BUR) systems differ from other roofing systems in terms of composition, performance, installation, and longevity.

The BUR system consists of multiple layers of bitumen (asphalt or coal tar), which are alternated with reinforcing fabrics, forming a durable and watertight membrane. It is primarily used on flat or low-slope roofs. Compared to single-ply roofing systems like EPDM, TPO, or PVC, BUR offers enhanced redundancy. The multiple layers provide extra protection against leaks and mechanical damage.

However, Single-ply membranes are lighter, easier, and quicker to install, making them more cost-effective for commercial projects. BUR systems generally require more labor and time but are highly preferable for their robustness, UV resistance, and longer lifespan of up to 30 years or more with proper maintenance. On the contrary, metal roofing systems provide high energy efficiency, longevity, and aesthetic appeal, especially for sloped structures. Modified bitumen roofing combines features of BUR and single-ply systems, offering flexibility, ease of installation, and durability. Ultimately, BUR stands the test of time for its reliability and multilayer defense against harsh weather.

Cost of Built-Up Roofing

Category	Details	Average cost
Material costs	Includes asphalt, bitumen, reinforcing felts, and surfacing materials (gravel, mineral cap sheet, etc.)	$2.50 – $4.00 per sq. ft.
Labor cost	Skilled labor for heating asphalt, layering, and sealing	$2.00 – $4.00 per sq. ft.
Tear-off and disposal	Removing old layers before installation	$0.50 – $1.50 per sq. ft.
Insulation cost	Adding roof insulation for energy efficiency	$1.00 – $2.50 per sq. ft.
Surfacing finish	Protective gravel or mineral surfacing	$0.50 – $1.00 per sq. ft.
Maintenance and repairs	Periodic inspections, patching leaks, and resealing	$0.10 – $0.25 per sq. ft. per year
Cost compared to other roofing systems	BUR remains one of the most cost-effective flat roof systems	Single-Ply (TPO, EPDM): $5–$9/sq. Ft. Modified Bitumen: $4–$8/sq. Ft. Metal Roof: $7–$14/sq. ft.

Warranties and Lifespan Expectations

A Built-up roof system is one of the most reliable roofing systems for flat or low-slope commercial spaces. It offers better protection due to its multi-layered features, giving remarkable durability against the weather. But when investing in a new BUR system, building owners naturally want to understand how long it will last and what kind of warranties they can expect.

The 3-ply Bur system has an expected lifespan of 15-20 years with the standard commercial option with moderate durability.

The 4-ply BUR system has a lifespan of 20-25 years. It features added layers to improve the resistance.

The 5-ply BUR system has a lifespan of 25-30 years with maximum protection against harsh climates. The BUR with reflective coating has an expected lifespan and warranty of 30-35 years.

Conclusion

No one can deny the importance of a built-up roofing system for better protection and resistance. BUR consists of multiple layers of roofing, which are alternated with the layers of bitumen, including coal tar, asphalt, and cold-applied adhesives. These layers are known for offering durability, weather resistance, and environmental thickness.

If you are looking for a professional company that can help you with a built in roofing system, look no further than Cost Bidding. For more details, visit our website.

Faqs

What is the average cost of a built-up roof per square foot?

A typical built-up roof installation per square foot ranges between $2.50 and $6.00 in the US markets.

How much does built-up roofing cost?

The cost of built-up roofing depends on various factors, including the material chosen, labor rates, equipment charges, old roof removal, permitting, flashing, surfacing, and site-related issues.

What is the typical lifespan of a modified bitumen roof, and what maintenance does it require?

The typical lifespan of a modified bitumen roof is 15-15 years. For maintenance, one must regularly inspect, repair minor damage, clear debris, and coat the surfaces for better protection.

Is built-up roofing suitable for residential buildings?

A built-up roofing system is well-suited for flat and low-slope roofs. But sometimes, they can be used for residential buildings.