Specifying, Installing and Maintaining Built-Up and Modified Bitumen Roofing Systems
- By Joseph W. Hobson
- February 1st, 2000
Built-Up Roofing (BUR) (multi-ply) systems have a proven track record since their origination in the mid-1800s. The introduction of modified bitumen and the hybrid systems in the early 1970s has led to more proven performance with bitumen-based roofing systems. Whether it be a built-up roof, modified bitumen roof or a hybrid system, each offers options and performance advantages from tensile strength to redundancy to flexibility to impact resistance -- all to increase the systems’ longevity and reduce the chances of premature failures.
Defining the Systems
Built-Up Roofing (BUR) System: This is the most common low-slope roofing. Using reinforcement felts and asphalt or coal-tar, a multi-ply system (three to four layers of felt with interply layers of waterproofing bitumens) provides redundancy and strength to resist rooftop stresses and changing environmental conditions. The system benefits are:
- multiple plies offer more protection;
- they have a significantly lower life-cycle cost than other systems;
- they have a time-proven history of success, often significantly exceeding service life;
- a larger critical mass enables the roof to tolerate surface punishment;
- they are easily and economically maintained and, if necessary, repaired with conventional, time-proven materials, thus extending service life; and
- product and material improvements have enhanced an already high-quality system.
Modified Bitumen System: This system uses polymers to modify or change the properties of bitumen so that it can be applied within a wide range of roof-top temperatures. Modified bitumens (modbits) are defined as factory-produced, fabric-reinforced sheets formulated with polymer-modified asphalt (ARMA Modified Bitumen Design Guide for Building Owners, 1997).
Asphalt modified by different combinations of Styrene Butadiene Styrene (SBS) or Atactic (Amorphous) Polypropylene (APP) on a reinforcing carrier such as glass fiber or polyester mat are the most common methods used in manufacturing the modified bitumen membranes. The system benefits are:
- a wide range of colors, styles and specifications to meet specific needs;
- improved thermal shock resistance, puncture resistance and tear resistance over traditional roofing membranes;
- the membrane is prefabricated, and each ply is independently watertight;
- it can be applied with conventional BUR equipment;
- it is easily inspected and corrected during application;
- a ballast is not required for application, therefore lightweight construction needs are met;
- good wind resistance when fully adhered;
- maintenance and repair are easy; and
- excellent tensile strength and elongation.
Hybrid Systems: These are a combination of the BUR and modified bitumen roofing systems.
It has been stated, “a roof is only as good as its installer.” It is also true that a roof is only as good as its scheduled inspections and the maintenance provided by the prudent building owner or manager. Whichever bitumen-based roofing system is used, performance is dependent on:
- proper roof inspections and design for the structure involved;
- acceptable materials and components that are system compatible;
- control over construction site variables;
- proper installation by a professional roofing contractor; and
- inspection and maintenance throughout the life of the roof.
Although roofs represent only five to 10 percent of the building’s capital costs, roofs protect 100 percent of the building and its contents. The importance of the roof sometimes is lost in the concerns of where costs can be reduced. To minimize life-cycle cost and maximize performance, it is important to review and plan out the following criteria.
1. Develop the proper roof specification and design for the structure. An understanding of the bitumen-based roofing systems that are available is essential. Based on the type of roof design being considered, there will be different options available. Everything from the roof geometry, height, slope, square footage, building usage, environmental conditions of building location and expected design service life will influence the selection of a proper roof specification.
2. Use acceptable materials and components that are system compatible. Whether it is a BUR, modified or hybrid system, all materials included in the system must be compatible. The roof deck, vapor retarder(s), insulation, membrane, bitumens, surfacing materials, base ply and cap sheets, and flashings must work together to assure durability, meet performance expectations, meet local building codes and meet compliance with insurance carriers’ requirements.
3. Maintain control over construction site variables. Foremost to controlling site variables is starting and completing the job on schedule. Acceleration of the job schedule or rushing to meet deadlines can result in misapplication, poor performance and cost increase. A slowdown, such as inclement weather, can cause changes in the utility and performance of materials. Therefore, proper storage and protection of materials is a must.
4. Use a professional roofing contractor for proper installation. Regardless of the roofing system type -- and given that the roof design and specifications are proper, the materials and components are compatible, and the site variables are controlled -- the roof will still be only as good as the installer.
5. Inspect and maintain the roof throughout its life. Proper care through regular inspections and maintenance schedules can add years to the roof and reduce costs involved in its design service life.
Inspection and Maintenance
Most roofing specialists suggest spring and fall inspections to help keep potential roof problems under control. Establishing a detailed historical file of the roof is a priority as part of the inspection and maintenance process. Sample checklists of the type of reports, printed materials and records that should be maintained in the historical file are available from most system manufacturers and suppliers, the National Roofing Contractors Association (NRCA) or the Asphalt Roofing Manufacturers Association (ARMA). The same is true for a checklist of what should be included in a thorough roof inspection.
A helpful resource is the ARMA/NRCA Manual for Inspection and Maintenance of Built-Up and Modified Bitumen Systems: A Guide for Building Owners. The inspection and maintenance manual contains general information on the inspection, maintenance and repair of commercial roofing systems and was written specifically for building owners.
Other important considerations in assuring high-quality performance and roof durability include the following.
1. The roof must have proper drainage.
2. Flashing construction and usage are vital. Check with the systems manufacturers, ARMA and NRCA for this information.
3. Make sure roof edges are secured properly.
4. Where rooftop foot traffic is expected, protective walkways are essential. Excessive traffic can cause significant damage to a roof that does not have a walkway system.
5. Inspect, inspect, inspect -- proper maintenance can save much grief and assure expected performance.
Whether it be a built-up roof, modified bitumen roof or a hybrid system, each offers facility managers and specifiers reliable, long-term performance if properly specified, installed and maintained.
Joseph W. Hobson is director of communications for the Asphalt Roofing Manufacturers Association (ARMA) in Calverton, Md.