Choosing the right roofing systems for commercial buildings can be confusing. There are a lot of materials to choose from, and they all serve the same general purpose. When it comes to comparing a built-up roofing system to a modified bitumen system, both options have their applications. We’ll look at these two commercial roof systems to see how they compare.

What are Built-Up Roof Systems and Modified Bitumen Roofs?

When considering flat and low-slope roofs, built-up roof systems (also known as BUR roof systems) and modified bitumen (commonly called mod bit or mod bitumen roofing) are two popular options. They create durable, long-lasting waterproofing layers over a commercial building.

Built-Up Roof Systems: A Traditional Flat Roofing Option

Built-up roof systems, or BURs, are a traditional type of flat roof system, and they’ve been in use for over 100 years. They consist of alternating layers of bitumen and reinforcing fabrics, creating a durable, waterproof membrane. The base sheet is applied to the roof, bitumen is typically mopped onto the surface, and then fabrics (or roofing felts) are sandwiched on top. The final layer is typically a cap sheet, gravel, or protective coating. This layered design is why the system gets its name, as it’s built up from the roofing deck a layer at a time.

BUR roof membranes are often chosen for their ability to withstand harsh weather conditions. Each layer applied adds strength and durability to the system. But, these systems are also known for being very heavy, so structural strength is always a priority.

Modified Bitumen Roofs: A Modern Alternative

Modified bitumen roofing systems are similar to built-up roofs in that they’re both a form of asphalt-based roofing system and consist of layers of modified bitumen sheets. However, rather than mopping the bitumen onto the surface, the bitumen sheets can be applied with torches or the sheet might be self-adhering. In either case, the layered sheets are strong and puncture-resistant.

A modified bitumen roofing system will be durable, weather-resistant, and fire-resistant. It can also be coated with a protective coating to reflect UV rays and improve the building’s energy efficiency.

A built-up roofing system with a layer of protective gravel on top.

Comparison of a Built-Up and a Modified Bitumen Roof

Now that we understand the main construction of these two commercial roof systems, it’s time to compare their pros and cons. It’s important to note that both systems can last 20 to 30 years with proper maintenance, so the lifespan is generally similar.

BUR Advantages and Disadvantages

Built-up roofing systems might be over a century old, but they’re still in use today. For that reason, it’s important to understand these systems’ pros and cons.

Pros:

  • The layers of roofing felt and sticky, flexible bitumen makes these roofs highly durable.
  • Bitumen is extremely water-resistant, ensuring these roofing systems create a waterproof membrane.
  • There are lower maintenance needs over time than some other roofing materials.

Cons:

  • The built-up roof installation process is time and labor-intensive.
  • BUR roof membranes are very heavy compared to the other roofing systems.
  • Built-up roof systems offer excellent waterproofing capabilities and a dense structure but can be heavy and require structural reinforcement.

Modified Bitumen Advantages and Disadvantages

Modified bitumen is a modern approach to solving the same issue as a BUR system. However, these systems have their own pros and cons to consider.

Pros:

  • Mod-bit systems are highly resistant to tears and punctures due to their layered design.
  • They’re generally more flexible than built-up systems, preventing tears and or seam splitting.
  • They’re easier and faster to install than built-up roofs since there are fewer layers and less adhesive to apply.
  • Mod-bit roofs have fewer layers, and those layers are lighter, making these systems lighter than BURs

Cons:

  • Mod-bit systems aren’t quite as durable as BUR systems, though they can outperform other materials
  • Requires more frequent maintenance than single-ply roofing systems like TPO, EPDM, or PVC
  • Requires UV-resistant surfacing materials to protect it from sun damage

Installation and Maintenance

While the systems themselves are similar in nature, there are some distinct differences in the installation and even the maintenance of both roofing systems.

A worker installed a new modified bitumen roof by heating the seams with a torch.

Built-Up Roofing BUR Installation

There are basically three methods for installing BUR roofing systems. They include hot-applied, cold-applied, and torch-applied. Hot applied is the most common, explained below:

  1. Inspect the deck to ensure that it’s in good condition and suitable for the weight of a BUR system.
  2. Any insulation that will be installed is applied to the roof deck, and a vapor barrier may be installed on top.
  3. A base sheet is applied to the roof deck or the vapor barrier as the first layer, fastened either mechanically or with adhesives.
  4. Heated bitumen is applied to the base sheet with a mop or sprayer.
  5. Roofing fabric or felts are applied to the bitumen and embedded in the surface while the bitumen is still hot.
  6. Alternating layers of bitumen and felts are applied to the roof following these steps.
  7. A final layer of bitumen is applied to the felt and gravel or crushed stone is embedded. In some cases, a reflective coating or cap sheet might be applied instead.
  8. Flashing is installed around the roof edges and any penetrations, with the edges sealed by bitumen.

Modified Bitumen Installation

Mod-bit roofing systems are easier and faster to install than BUR simply because the steps take less time. There are a few methods of installation as well, including heating the membrane and then applying it, cold adhesive application, and a peel-and-stick application. Heat applied is the most common method, detailed below:

  1. Inspect the deck to ensure that it’s structurally sound and in good condition. Weight is less of an issue, but still something to consider.
  2. Install any insulation and vapor barriers desired, but again, they’re not required.
  3. A base sheet is installed on top of the roof deck or insulation, and it can be applied with mechanical fasteners or adhesives.
  4. Additional sheets (two to three are common) of mod-bit are applied on top following the same methods.
  5. A cap sheet is applied on top of the mod-bit system, with either granules embedded in the sheet or a reflective coating applied on top.
  6. Flashing is installed around roof edges, penetration, and other areas, and edges are heat welded and sealed.

Maintenance

Maintenance is similar for both systems, with BUR systems requiring slightly less attention over time:

  • Regular inspections (semi-annually) to check for signs of damage or issues with seams
  • Debris should be removed during the inspections
  • Areas where ponding water is occurring should be noted, as this can degrade the surface over time. Ponding water may be the result of clogged drains but it might also require the attention of a professional roofing contractor.
  • Inspect seams and overlaps to ensure they’re properly sealed, and repair as needed.
  • Check for dents, bending, or cracking in flashing around the edge of the roof and around penetrations.
  • Reapply any coating applied to the roof every 5 to 10 years.
  • Modified bitumen roofs can be installed using the torch-application method, hot-mopped, or applied with “cold-process” adhesives.

If any issues are located, it’s best to contact a professional roofing contractor to ensure they’re handled and repaired correctly.

Built-Up Roof vs Modified Bitumen: Let Us Help You Decide for Your Commercial Property

If you’re hunting for your commercial building’s next roofing system, let us help you decide if a built-up roof or a modified bitumen roof is the right choice for you. At AAA Roofing, we have over 35 years of experience in the roofing industry, tackling almost any roofing project you can think of. We believe in providing excellent customer service, a proper installation, and decades of protection. We can even handle your flat roof maintenance needs, as well. Contact AAA Roofing for your commercial roofing needs today.