When it comes to roofing systems designed for durability and protection against harsh weather, the built up hot mopped roof remains one of the most time-tested options. This traditional roofing method has been widely used for commercial buildings and flat or low-slope roofs for decades. Known for its layered construction and tar-based sealing process, a built up hot mopped roof offers strong waterproofing and long-lasting performance. To understand why it continues to be a popular choice today, it’s important to explore how it’s made, its benefits, and how it compares to modern alternatives.
What Is a Built Up Hot Mopped Roof?
A built up hot mopped roof, often called a BUR system, is a multi-layer roofing design that combines bitumen (asphalt or coal tar) with layers of roofing felt or fabric. The hot mopped term refers to the method of applying heated asphalt between each layer, creating a watertight seal. Typically, the process involves alternating layers of hot asphalt and roofing felt until several plies are built up, finishing with a protective top layer such as gravel or a reflective coating.
This type of roof is most commonly found on flat or low-slope structures such as office buildings, warehouses, schools, and apartment complexes. Because of its proven ability to resist water penetration and withstand heavy use, the built up hot mopped roof has earned a reputation for reliability in demanding environments.
How a Built Up Hot Mopped Roof Is Installed
The installation of a built up hot mopped roof involves several detailed steps. This process requires skilled labor and specialized equipment to ensure a proper and durable finish. The key stages include
- 1. Surface PreparationThe existing roof surface or decking must be cleaned and made smooth. Any debris, moisture, or loose materials are removed to ensure proper adhesion.
- 2. Base Sheet ApplicationA base sheet, usually made of fiberglass or organic felt, is applied to serve as the foundation for the roofing system.
- 3. Hot MoppingHot asphalt is heated to around 400°F and spread across the surface using a mop or mechanical sprayer. This forms a sticky layer that bonds the felt to the surface.
- 4. LayeringAdditional layers of felt and hot asphalt are alternated to create multiple plies, typically three to five, depending on the roof design and desired thickness.
- 5. SurfacingThe top layer may be finished with gravel, mineral surfacing, or reflective coatings to protect against UV rays and improve durability.
This meticulous layering process gives the built up hot mopped roof its strength and weather resistance. Each layer adds redundancy, meaning that even if one layer is compromised, the others continue to protect the building from water intrusion.
Advantages of Built Up Hot Mopped Roofs
One of the main reasons built up roofs continue to be used today is their exceptional durability and performance. Below are some of the most significant benefits
- Proven longevityWith proper maintenance, a built up hot mopped roof can last 20 to 30 years or longer.
- Superior waterproofingThe multiple layers of asphalt and felt create a seamless, watertight barrier against rain and snow.
- Resistance to heavy foot trafficBuilt up roofs are strong enough to handle maintenance workers and equipment without damage.
- Fire resistanceAsphalt and gravel surfaces are naturally resistant to fire, adding a layer of safety to commercial properties.
- UV and thermal protectionWhen finished with a reflective coating or gravel, these roofs reduce heat absorption, helping lower indoor temperatures and energy costs.
- Cost-effectivenessAlthough installation requires more labor, the long lifespan and low maintenance costs make it an economical choice over time.
These features make built up hot mopped roofing particularly suited for large, flat commercial buildings that need dependable, long-term protection.
Disadvantages to Consider
Despite its strengths, the built up hot mopped roof also has certain drawbacks. Understanding these can help property owners make informed decisions
- Heavy weightBecause it consists of multiple layers, the system adds significant weight to the structure, which may not be ideal for some buildings.
- Complex installationThe use of hot asphalt requires experienced roofers and safety precautions due to high temperatures.
- Odor during installationThe heating and application of asphalt produce strong fumes that can be unpleasant or hazardous if not properly ventilated.
- Maintenance of surface materialsOver time, gravel can shift or be displaced, requiring periodic re-leveling to maintain full coverage.
- Slower installation timeCompared to newer roofing materials, BUR installation can take longer and may be weather dependent.
These challenges are manageable with professional installation and regular inspections, which help extend the life and reliability of the roofing system.
Built Up Roof vs. Modern Roofing Alternatives
In recent years, alternative flat roofing materials have gained popularity, such as modified bitumen, TPO, and EPDM membranes. However, built up hot mopped roofs still hold several advantages over these newer systems, especially in durability and multi-layer protection.
- BUR vs. Modified BitumenModified bitumen uses asphalt combined with polymers for flexibility and is often applied with a torch. While easier to install, it typically has fewer layers than BUR, offering less redundancy.
- BUR vs. TPOTPO (Thermoplastic Olefin) roofing is lightweight and reflective but can be more prone to punctures and seam failures over time.
- BUR vs. EPDMEPDM (rubber roofing) offers excellent resistance to UV rays but may shrink and crack in certain conditions, requiring maintenance to prevent leaks.
While each system has its strengths, the built up hot mopped roof stands out for its time-tested durability and ability to perform under demanding environmental conditions.
Maintenance and Longevity
Maintaining a built up hot mopped roof is essential to preserving its performance and extending its lifespan. Regular inspections can catch small issues before they develop into costly repairs. Recommended maintenance practices include
- Inspecting for cracks, blisters, or loose gravel every six months.
- Clearing debris, leaves, and standing water to prevent drainage issues.
- Checking flashing and edges for signs of wear or separation.
- Applying a reflective coating periodically to reduce UV damage and heat absorption.
- Scheduling professional inspections after severe storms or temperature changes.
With consistent upkeep, the roof can easily surpass its expected service life and maintain its watertight integrity for decades.
Environmental and Energy Considerations
Modern advancements have made built up hot mopped roofs more eco-friendly than ever. Many systems now use asphalt blends with recycled materials and reflective top layers that reduce heat gain. This not only benefits the environment but also lowers building energy costs by keeping interiors cooler during hot months.
Additionally, the layered design provides natural insulation, helping to stabilize indoor temperatures. For businesses focused on sustainability, pairing a BUR system with a cool roof reflective coating can significantly enhance energy efficiency.
Why Built Up Hot Mopped Roofs Remain Popular
Despite the rise of newer roofing technologies, the built up hot mopped roof continues to hold a strong position in the construction industry. Its combination of proven performance, strength, and water resistance makes it ideal for buildings that demand long-term durability. Many contractors still prefer BUR systems for commercial and industrial properties because they provide consistent results and have a long record of success in all types of climates.
While newer materials may promise easier installation, few can match the multi-layer protection and redundancy offered by the traditional built up system. When installed and maintained properly, it remains one of the most dependable roofing choices available.
The built up hot mopped roof is more than just a roofing systemit’s a legacy of craftsmanship and engineering that has stood the test of time. With its multiple layers of protection, excellent waterproofing, and adaptability to large commercial roofs, it remains a reliable solution for those who value longevity and performance. Though it requires skilled installation and regular maintenance, the investment pays off in decades of dependable protection. Whether used on new buildings or as a replacement for older systems, the built up hot mopped roof continues to be one of the most trusted roofing methods in the world of construction.