Flat Roof Systems: Built-up Roof and Modified Bitumen

By Mark Gregory, General Manager at RSS Roofing Services & Solutions Florida Office

A flat roof is a roof which is almost level in contrast to slopped roofs. Flat roofs or low-slope roofs are commonly found on commercial and industrial buildings. Flat roofs provide a more convenient place to install outdoor HVAC units rather than putting them in high-traffic ground-level areas.

Although there are several different options to take regarding flat roofs used for commercial buildings, built-up roofing (BUR) systems and modified bitumen roofing systems are very common. Both low-slope commercial roofing systems have three principal components: weatherproofing layer or layers, reinforcement and surfacing.

Built-up Roofing Systems

Built-up roofing or BUR systems have been used in the U.S. for more than a century. A traditional BUR system gets its name from the building of a roof on-site using layers of base sheet, interplay sheets and cap sheet or other surfacing such as gravel or liquid coating.

This form of roofing involves alternating layers of bitumen and fabrics that join together to create a membrane. It is then finished with an aggregate layer or coating. The most commonly used bitumen on these types of roofs are asphalt or cold-applied adhesive. The finish of the BUR sometimes looks rocky because the gravel and mineral granules or slag are mixed in with the asphalt, but these roofs can also be smooth.

Modified Bitumen Roofing Systems

A conventional alternative to BUR systems are modified-bitumen roofing systems. Modified bitumen is a roofing membrane consisting of asphalt and plastic or rubber polymers, and is therefore widely considered an evolution of asphalt built-up roofing (BUR). Modified bitumen displays the built-in redundancy of asphalt built up, as well as the UV resistance and flexibility of a modified membrane.

This roof system has two types: atactic polypropylene (APP) and styrene-butadiene-styrene (SBS). The SBS is the most common, boasting highly flexible membranes, giving it stronger expansion and contraction capabilities. APP, on the other hand, has exceptional aging properties, making it a more long-lasting option.

Modified bitumen membranes are hybrids that combine the high-technology formulation and prefabrication benefits of single-ply with traditional roofing installation techniques that are used in built-up roofing. These membranes are factory-fabricated layers of asphalt that are modified using a plastic or rubber ingredient and are combined with a reinforcement.

Advantages of BUR and Modified Bitumen Roofing Systems

A flat roof, including BUR and modified bitumen, are cost-efficient for this roof shape as all room space is used fully, below and above the roof. Having a smaller surface area, flat roofs require less material and are usually stronger than slopped roofs. The flat-style roof also provides ample space for solar panels or outdoor recreational use such as roof gardens.

BUR systems offer several strong qualities that have made it quite popular for commercial buildings. For example, BUR systems provide multi-layer protection. The multiple layers of bitumen and bitumen saturated felts make a watertight barrier capable of providing many years of reliable protection from the elements.

BUR systems are relatively inexpensive when used in standard specifications, and are designed to endure harsh weather conditions all year round. BUR systems exhibit exceptional resistance to the conduction of heat between the exterior and interior of a building, resulting in noticeable reductions in heating and cooling costs.

Compared to other high-performance commercial roofing systems, BUR systems are one of the best investments on the market due to its competitive costs per year of expected service life. The collection of 20, 30, and 40-year old BUR roofs in excellent condition attest to their fact.

BUR systems are ideal for most low-slope commercial roofs. The higher the quality of the BUR materials and asphalt used and the greater the number of layers, the more superior the roofing system will be.

The main advantage modified bitumen roofing has over most other commercial roofing systems is that it is extremely tear-resistant. This is due to the combination of fiberglass and/or polyester reinforcement layers. This feature makes modified bitumen roofing the ideal choice for low-slope commercial roofing with high-foot traffic.

The innate durability of a BUR/modified roof systems are very resistant to overall damage for weather, reducing the need for repairs.

Additionally, modified bitumen roofing systems are energy-efficient. The top layer can be treated to add cool roof features such as solar reflectance and thermal emission, or the ability to reflect sunlight, and release already absorbed heat. This helps reduce the property’s indoor cooling requirements, resulting in lower energy consumption.

Lastly, the roofing system is flexible. Flexible low-slope roofing materials are important not just for impact resistance, but for temperature shifts. Modified bitumen expands and contracts without losing its shape.

Applications and Installation

BUR systems are installed through a build-up of layers. A commercial roofing contractor will have alternating layers of roofing felt and bitumen in the form of asphalt, tar or cold-applied adhesive. Apart from cold-applied built-up roofing systems, tar and gravel installation happens when the bitumen is hot, and then the system can cool and harden. The surfacing is added last and can be either gravel, minerals, elastomeric coating or a cool roofing top sheet. The surfacing is intended to add a layer of UV and thermal resistance, as well as increase the weatherproofing capabilities of the BUR systems.

On occasion, a commercial roofing contractor will install underlayment before laying down any BUR roofing layers. Built-up roofs are, however, considered to be fully adhered when installing directly to the roofing deck, so underlayment isn’t always needed depending on the situation.

Modified bitumen can be installed by heat-welding, hot asphalt, or cold-adhesive processes, meaning that it can be installed on nearly any type of flat-roof building. This is especially useful for buildings with heat-sensitive appliances or structures such as ventilation systems or heating and cooling units already installed on the roof. When the use of torches and other heating elements around these units is not an option, modified bitumen can still be installed by other equally reliable means.

Like with tar and gravel roofing, modified bitumen requires a surfacing material, and the most commonly used are factory applied mineral blends and gravel. A typical lifespan of modified bitumen is 10 to 20 years, depending on climate and weathering. Modified bitumen is generally applied using a heat-application process that melts the seams together to create a seal, yet sometimes a cold-adhesive application system is used.

APP modified bitumen membranes are self-adhering when heated using a roofing torch. SBS modified bitumen membranes can be formulated for torch application, but are more commonly applied using hot asphalt or cold SBS modified-bitumen adhesive. The result is a BUR system with significantly enhanced performance capabilities. When an SBS or APP cap sheet is combined with traditional BUR-base sheets and interplay sheets, the result is a hybrid roof offering the best of both systems.

Maintenance

Both roofing systems last longer if they are properly maintained. Some assessors use 10 years as an average life cycle, although this is dependent on the type of roof system in place. Some old tar and gravel roofers acknowledge that unless a roof has been neglected for too long and there are many problems in several areas. BUR systems have been utilized dating back to the 1900s and can last up to 20 to 30 years.

General commercial roof maintenance for these systems include removing ponding water, typically within 48 hours. This is accomplished by adding slope, roof drains or scuppers. The best time to address the issue of ponding water is during the design phase of a new roofing project. The quicker the water is removed, the less chance there is for a roof leak to occur.

All roofs should be inspected by a professional commercial roofing contractor at least semi-annually and after major storms. Attention should be paid to the flashings around all the rooftop penetrations. The sharp bends at such places can come open and need to be sealed. Also, it is important to keep the roof drains free of debris. A clogged roof drain will cause water to pond, leading to increased load weight on a building that may not be engineered to accommodate the extra weight. Also, ponding water has the chance to freeze which can weaken the roof seams.

About the Author

Mark Gregory is the general manager of RSS Roofing Services & Solutions in Florida. RSS, headquartered in St. Louis, Mo. and with multiple locations in the Central and Southeastern region of the United States, is a leader in commercial, industrial and institutional roofing services. The company has multiple locations including Florida; St. Louis, MO; Evansville, IN; Nashville, TN; Columbia, MO and a special projects division. RSS is a subsidiary of MHS Legacy Group, a diversified national holding corporation also based in St. Louis with roots back to 1895. For more information, please email Mark at mgregory@roofingsands.com or visit www.roofingsands.com.

For more information and photos on this project, visit the Stone Hall Mansion Project Page

This was originally published in Florida Roofing magazine’s November issue.

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