When it comes to protecting large commercial buildings from the elements, few systems have stood the test of time like the built-up roof, or BUR. This roofing method has been around for over a century, and good reason. It’s rugged, reliable, and especially well-suited for flat or low-slope rooftops. But what exactly is a built-up roof, and why do so many professionals still choose it today?
This guide is handy for commercial building owners, facility managers, and roofing contractors who want to make informed decisions about roofing investments. Whether you’re overseeing maintenance for a shopping center, managing a school campus, or handling contracts for government buildings, understanding the value of built-up roofing could help you save money and extend the life of your roof.
Let’s take a closer look at what makes BUR systems such a dependable choice and why they continue to earn trust after more than a century in use.
KEY Takeaways
What is a Built-Up Roof?
A built-up roof (BUR) is one of the oldest types of roofing systems used for commercial buildings, especially those with flat or slightly sloped roofs. Its name says a lot about how it works. This roof is literally “built up” by stacking multiple layers of materials. These layers typically consist of bitumen (a thick, sticky form of asphalt or tar) and reinforcing fabrics, such as felt or fiberglass. Once everything is in place, the top is often finished with gravel or a special coating to protect it from sunlight and weather damage.
The history of built-up roofing dates back to the 1800s, when people began using hot tar and paper to create waterproof barriers for flat roofs. Over time, the materials and methods improved, but the basic idea stayed the same. Even today, BUR is considered one of the most reliable and time-tested roofing systems for commercial buildings.
So how does it work? Each layer in a BUR system has a purpose. The bitumen acts like glue and waterproofing, while the fabric adds strength. By applying these layers repeatedly, usually three to five times you achieve a thick, rigid surface that can withstand considerable wear and tear. The final layer, whether it’s gravel or a reflective coating, helps protect the roof from sun damage, which can break down materials over time.
Built-up roofs are best suited for low-slope or flat roofs. These types of roofs don’t drain water as quickly as steep ones, so they need a system that can seal tightly and stay waterproof even during heavy rain. That’s precisely what BUR is designed to do.
Components of a Built-Up Roof System
The main components work together to create a roof that’s strong, waterproof, and built to lasteven under harsh weather conditions.
Base Sheet
The base sheet is the very first layer laid down on the roof deck. It acts like a foundation for the rest of the roof system. Roofers usually attach this sheet directly to the building using adhesive or fasteners. It creates a clean, flat surface for the subsequent layers and provides additional protection against moisture. While it may seem simple, the base sheet is important because it helps the rest of the layers adhere more effectively and perform more reliably over time.
Ply Sheets (Felts or Fabrics)
Next come the ply sheets, also called felts or reinforcing fabrics. These sheets provide the roof with strength and flexibility. They are usually made from materials like fiberglass or organic felt. Roofers soak them in bitumen (a sticky, waterproof substance) before laying them down.
Most built-up roofs consist of multiple ply sheets, typically three to five layers. The more layers you have, the more challenging the roof becomes. These sheets also help absorb stress from movement or weather changes, which makes the roof less likely to crack or split.
Bitumen Layers (Asphalt or Coal Tar)
Bitumen is the glue that holds everything together. It also acts as the waterproofing agent in a BUR system. Bitumen is typically either hot asphalt or coal tar, although some systems utilize cold-applied alternatives.
Roofers heat it and spread it between the ply sheets so everything bonds tightly. This sticky substance fills in gaps and creates a continuous, watertight seal across the entire roof. Because bitumen is naturally water-resistant, it provides an additional layer of protection against leaks.
Surfacing
The final layer of a built-up roof is the surfacing, which protects the system from the sun, wind, and physical damage. There are a few different types of surfacing:
Gravel is the most traditional option. It’s spread over the top while the bitumen is still hot, so it sticks. It blocks UV rays, reduce fire risk, and prevent damage from foot traffic or flying debris.
Mineral cap sheets are another choice. These are pre-coated sheets that are applied on top of the final bitumen layer, providing a clean finish and built-in protection.
Reflective coatings are also standard, especially in warm climates. These coatings bounce sunlight away from the roof, which helps lower cooling costs and reduce wear caused by heat. They provide dimensional stability and tear resistance. Or Polyester fabric offering superior elongation and puncture resistance.
How is a Built-Up Roof Installed?
Installing a built-up roof (BUR) is a careful process that takes time and skill. Roofers need to work in layers, making sure each one is placed and sealed correctly. The purpose is to create a robust, waterproof surface that can withstand years of sun, rain, and temperature fluctuations.
There are two primary methods for applying a built-up roof: hot-applied and cold-applied.
Hot Built Up
In the hot application, roofers heat the bitumen until it becomes a thick, liquid-like substance. They pour it between each layer, using it as glue to hold everything together. This method has been in use for a long time and works very well; however, it also requires specialized tools and safety gear.
Cold Built Up
The cold application skips the heating step. Instead, it uses cold-applied adhesives or bitumen products that work at room temperature. This method is safer and easier in some situations, especially when using heat would be risky or complex to control. Both methods can produce strong, reliable roofs—the choice depends on the building, the climate, and the contractor’s preference.
While there are different ways to apply a BUR system, the basic steps are mostly the same.
- First, the roofers start by preparing the surface. They clean the roof deck and make sure it’s dry and smooth. Then, they install the base sheet, which acts like a starting layer. This sheet gets attached directly to the roof, creating a solid base for everything else.
- Next, they begin adding ply sheets, which are made of felt or fiberglass. After each ply sheet is laid down, it is coated with bitumen—a sticky, waterproof substance similar to hot asphalt or coal tar. This layering continues until there are usually three to five plies in place. The heat from the bitumen helps the sheets bond together tightly, creating a thick, sealed surface.
- After the final ply, roofers apply the top surfacing layer. This could be gravel (pressed into the hot bitumen), a mineral cap sheet, or a reflective coating. The surfacing protects the roof from UV rays, weather, and physical damage, helping it last longer and stay strong.
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Benefits of Built-Up Roofing Systems
- Waterproofing: Multiple layers of bitumen and fabric form a sealed barrier that blocks water, even if one layer gets damaged. This makes BUR ideal for areas with heavy rain or snow.
- Durability: With three to five layers, BUR roofs can withstand foot traffic, debris, and temperature fluctuations without cracking. The bonded layers flex with the building, reducing the risk of splits or leaks.
- Fire Resistance: A gravel or mineral-coated top layer helps slow fire spread, making them a safe choice for commercial buildings with strict safety standards.
- UV Protection. BUR’s thick top layer shields against sunlight, wind, and extreme temperatures, helping the roof last longer under harsh conditions.
Drawbacks of BUR
- Weight: Multiple layers plus gravel can be heavy. Not all structures can support it, so a structural check is essential before installation.
- Complex Installation: Heating materials and applying multiple layers takes time and skill.
- Maintenance: While durable, it requires regular inspections to identify problems early, such as cracks or pooling water. Ignoring them can lead to costly repairs.
- Environmental Issues. 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.
Comparison Between BUR With Other Roofing Systems
Choosing the right roofing system depends on your building’s needs. Built-up roofing (BUR) is a layered system that offers strength and waterproofing; however, other systems, such as single-ply membranes and modified bitumen, provide different advantages.
Single-ply membranes, such as TPO and EPDM, consist of a single layer of material. They’re lighter, quicker to install, and often more affordable. TPO reflects sunlight well, enhancing energy efficiency, while EPDM handles temperature fluctuations and movement. However, with only one layer, they’re more prone to punctures and tears.
Modified bitumen is similar to BUR, but it uses sheets of asphalt combined with rubber or plastic for added flexibility. It’s easier to install and offers solid waterproofing but usually doesn’t last as long or handle foot traffic as well as BUR.
Lifespan and Maintenance Tips
With proper care, a built-up roof can last 20 to 30 years. Its lifespan depends on installation quality, weather exposure, and maintenance.
Inspect the roof twice a year and after major storms. Look for cracks, blisters, or standing water. Clear debris and check that drains are functioning properly.
A BUR may need repairs or replacement if leaks appear often, the surface wears down, or sections start to sag. Minor issues can frequently be fixed, but extensive damage usually means it’s time for a new roof.
Regular upkeep helps protect your investment and keeps the roof performing well for decades.
See also: How to estimate for your roofing system
FAQs
The cost to install a built-up roof generally ranges from $4 to $10 per square foot. Prices depend on factors such as material type (asphalt or coal tar), the number of layers, the roof size, and local labor rates.
A well-installed built-up roof (BUR) typically lasts 20 to 30 years. Its lifespan depends on several factors.
