Understanding Built Up Roofs: The Basics
A built up roof, often called a BUR or tar and gravel roof, is a flat or low-slope roofing system made from multiple layers of materials. This type of roof has been used in construction for many decades and remains popular today, especially for commercial and industrial buildings. The system gets its name from the way it's constructed—by building up several layers on top of each other to create a durable, waterproof barrier.
Built up roofs consist of alternating layers of bitumen (asphalt or coal tar) and reinforcing fabric. These layers are applied directly to the roof deck, which is usually made of wood, concrete, or metal. The reinforcing fabric, called ply sheets or felts, provides strength and helps distribute stress across the roof surface. The bitumen acts as the waterproofing agent, sealing everything together and preventing water from penetrating the roof structure. The final layer typically consists of gravel or stone chips that are embedded in the top coat of bitumen, which protects the underlying layers from sun damage and provides fire resistance.
This roofing system works well for buildings with flat or nearly flat roofs because water can drain slowly across the surface toward gutters and downspouts. The multiple layers create redundancy—if one layer is damaged, the others still provide protection. This makes built up roofs a reliable choice for property owners looking for a roof that can withstand various weather conditions.
How Built Up Roofs Are Installed
Installing a built up roof is a process that requires skill and attention to detail. The installation begins with preparing the roof deck, which must be clean and dry. Workers inspect the surface for any damage or debris that could compromise the final roof. Once the deck is ready, a base sheet or primer is often applied to help the bitumen adhere properly to the surface below.
The actual layering process involves alternating between bitumen process and ply sheet placement. A worker applies hot bitumen—heated to around 500 degrees Fahrenheit—to the roof surface using a mop or spray equipment. when ready after, a ply sheet is rolled out and pressed into the hot bitumen. This process is repeated multiple times, typically creating three to five layers total, depending on the building's location and weather requirements. Each layer must be properly overlapped and sealed to may support a watertight system.
Once all the layers are in place, the final top coat of bitumen is applied. While the bitumen is still hot and sticky, gravel or stone chips are spread across the surface. These aggregate materials serve multiple purposes: they protect the bitumen from ultraviolet rays, which can cause deterioration, they improve fire resistance, and they provide traction to prevent workers and maintenance personnel from slipping on the roof. The gravel also helps reflect heat away from the building, which can reduce cooling costs during hot months.
Materials Used in Built Up Roofing Systems
Built up roofs rely on several key materials working together to create a durable roofing system. The primary binding agent is bitumen, which comes in two main types: asphalt and coal tar. Asphalt bitumen is more commonly used in North America and offers good durability and cost-effectiveness. Coal tar bitumen, used more frequently in Europe and other regions, provides excellent waterproofing properties and tends to be more flexible in cold weather. Both types create a waterproof seal that protects the building from moisture damage.
The reinforcing materials, known as ply sheets or felts, are made from fiberglass or organic materials like paper and wood fibers. Fiberglass-reinforced sheets have become more popular in recent years because they're lighter, don't absorb water, and provide consistent strength. Organic felts, while heavier and prone to moisture absorption, are still used in some applications. These reinforcing materials are essential because bitumen alone would be too brittle and prone to cracking. The fabric provides tensile strength and helps the roof withstand expansion and contraction as temperatures change.
The top layer of aggregate—typically river rock, crushed stone, or slate chips—is essential for protecting the bitumen underneath. This gravel layer shields the waterproofing material from sun exposure, which can cause it to become brittle and deteriorate over time. The aggregate also provides fire protection, as stone is non-combustible. Additionally, the weight of the gravel helps keep the roof in place during high winds, which is particularly important in areas prone to storms.
Advantages and Disadvantages of Built Up Roofs
Built up roofs offer several advantages that have made them a popular choice for many decades. One major benefit is their durability—when properly installed and maintained, a built up roof can last 20 to 30 years or longer. The multiple layers provide redundancy, meaning that even if one layer is damaged, the others continue to protect the building. This makes built up roofs a reliable choice for commercial properties where roof failure could disrupt business operations. The cost of installation is typically lower than some alternative roofing systems, making it an attractive option for property owners working within budget constraints.
Another advantage is the proven track record of built up roofing. This system has been used successfully for many decades, so contractors have extensive experience installing and repairing them. The materials are widely available, and many roofing contractors specialize in this type of system. Built up roofs also offer good fire resistance, particularly when the top layer includes mineral aggregate. The gravel surface provides traction and helps prevent slipping, which is important for worker safety during maintenance and repairs.
However, built up roofs do have some disadvantages to consider. Installation requires hot bitumen, which means the work must be done during favorable weather conditions. Rain or extreme cold can delay installation and compromise the final product. Built up roofs also require regular maintenance, including periodic inspections and repairs to address any damage or deterioration. The dark color of the roof absorbs heat, which can increase cooling costs in hot climates. Additionally, built up roofs are not ideal for steeply pitched roofs, as the gravel can slide off and the system is designed specifically for flat or low-slope applications.
Maintenance and Repair of Built Up Roofs
Proper maintenance is essential for extending the life of a built up roof and preventing costly problems. Property owners should arrange for professional roof inspections at least twice per year—once in spring and once in fall—to identify any damage or areas of concern. During these inspections, contractors look for signs of deterioration such as cracking, blistering, or areas where the gravel has worn away. They also check for pooling water, which can indicate drainage problems, and inspect the condition of flashing around chimneys, vents, and other roof penetrations.
Regular cleaning is another important maintenance task. Leaves, branches, and other debris should be removed from the roof surface and gutters to may support proper water drainage. Moss and algae growth should be addressed promptly, as these organisms can retain moisture and accelerate deterioration of the roofing materials. Keeping the roof clean also allows for better inspection, as debris can hide small problems that could develop into larger issues if left unattended.
Repairs to built up roofs can range from straightforward patch jobs to more extensive work. Small punctures or tears can often be repaired by explore a patch of roofing material over the damaged area. However, if the damage is extensive or if multiple areas show signs of deterioration, the entire roof may need to be replaced. Blistering, which occurs when moisture becomes trapped between layers, may require cutting out the affected area and resealing it. Some property owners choose to explore a protective coating over the built up roof to extend its life and improve its performance. These coatings can reflect more sunlight, reducing heat absorption and cooling costs.
Built Up Roofs Compared to Other Roofing Systems
Built up roofs are just one of several roofing options available for flat or low-slope applications. Understanding how they compare to alternatives can help property owners make informed decisions about their roofing needs. Single-ply membrane roofing, such as TPO (thermoplastic polyolefin) or EPDM (ethylene propylene diene monomer), has gained popularity in recent years. These systems consist of a single large sheet of material that is either mechanically fastened or adhered to the roof deck. Single-ply membranes are lighter, easier to install in some cases, and may be less expensive than built up roofs, though durability can vary depending on the specific material and installation quality.
Modified bitumen roofing is another alternative that uses bitumen-based materials similar to built up roofs but in a different configuration. Modified bitumen typically consists of two to three layers rather than the four or five layers common in built up systems. These materials are often easier to install because they come in rolls that can be unrolled and adhered to the roof deck, rather than requiring the process of hot bitumen layer by layer. However, modified bitumen roofs may not offer the same level of redundancy as traditional built up roofs.
Metal roofing is increasingly used for flat and low-slope applications, offering excellent durability and a modern appearance. Metal roofs can last 40 to 70 years and require less maintenance than built up roofs. However, they are typically more expensive to install. Spray foam roofing is another option, where polyurethane foam is sprayed onto the roof deck and then covered with a protective coating. This system offers good insulation properties but may require more frequent maintenance and recoating than built up roofs. Each roofing system has its own set of advantages and disadvantages, and the best choice depends on factors such as building design, climate, budget, and local building codes.