Prior to the mid-to-late 1970s, almost all low-slope roofs were asphalt or coal tar built-up roofs. However, during the last two decades of the 20th century, a variety of other types of low-slope roof systems began to compete with traditional built-up roofs (BUR). These newer systems included modified bitumens, single-plies, sprayed polyurethane foam, and metal panels. While the modified bitumen systems are related to BUR, the other low-slope alternatives are radically different. Along with new choices of membrane materials, plastic foam roof insulations also emerged in the 1970s. The abundance of materials from which to choose has greatly complicated roof system design.
To select, detail and specify the most appropriate roof system for a project, the architect should ideally have at least a general understanding of the available material options.
Note: Low-sloped roofs are defined as those roofs with a slope less than or equal to 3:12 (25 percent). However, with the exception of metal roofs, most low-slope roofs have a slope of about ¼:12 (2 percent). Steep-slope roofs are defined as those roofs with a slope greater than 3:12 (25 percent). Some materials can be used on both low- and steep-slopes, while others are limited to either low- or steep-slope.
Ref.: TLSmith Consulting Inc.