The roof design decision is the most important decision in the roofing process. It encompasses a number of important issues that can only be determined at this stage of a roof’s progression. Ensuring proper roof performance is the main issue with roof design. Consideration must be taken in determining proper design, material and application techniques.
It is at the design stage that all aspects of the roof system are devised. The roof assembly generally falls into two component groups: substrate and roofing.
The substrate consists of all components that are protected from the environment. This group would include the roof deck, any vapor retarders and the insulation. Roofing is considered to be all of the components that provide protection for the substrate. This group includes the membrane, flashings, exposed trim, drains and all other items required for a complete roof installation.
Low-slope roofing systems are defined by code as roof systems applied on slopes of 2:12 or less. It is the architect or designer’s responsibility to specify the system that is best suited for the facility. There are three types of conventional roofing systems used in commercial construction. The type of material used in the system determines the application methods. The types of systems include:
• Built-up roof systems.
• Modified bitumen roof systems.
• Single-ply roof systems.
Built-Up Roof Systems
Built-up roof systems have been on the U.S. commercial roofing market since the late 1800s. They are considered conventional roof systems. The documented use of bituminous products such as coal tar and asphalt as waterproofing products dates back thousands of years.
BUR systems are comprised of multiple plies (three or more) of felt set in an application of heated bitumen (coal tar or asphalt). The membrane system is covered with a surfacing of roof aggregate that is set in an application of a heated bitumen (coal tar or asphalt), smooth surfaced or covered with a mineral cap sheet.
Coal Tar in BUR Systems
There are two types of coal tar used in commercial roof applications. The most common formulation is classified as Type I Coal Tar, which has been used since the late 1800s. In the 1980s the coal tar manufacturers responded to concerns over fumes with a low-fume Type III Coal Tar. The basic difference between these types is that there is less cryosyle in Type III Coal Tar. There have been reported problems with Type III Coal tar and Type I is preferable.
Asphalt Used in BUR Systems
There are four classifications of asphalt used in commercial roof systems.
• Type I (Dead Level) asphalt has a low softening point and is recommended for aggregate surfaced roof systems that have a slope of ½ inch in 12 or less.
• Type II (Flat) asphalt is moderately susceptible to flow and is recommended for aggregate surfaced roof systems that have a slope between ½ inch and 1½ inches in 12 or less.
• Type III (Steep) asphalt is relatively non-susceptible to flow and is recommended for roof systems that have a slope between 1 inch and 3 inches in 12 or less.
• Type IV (Special Steep) asphalt is moderately non-susceptible to flow and is recommended for roof systems that have a slope between 2 inches and 6 inches in 12 or less.