To select, detail, and specify the most appropriate roof system for a project; past experience with several of the available material options and an understanding of roof assembly materials and system options, and an understanding of roof design considerations is recommended. The purpose of this section is to provide design guidance for designing high-performance low- and steep-slope roof assemblies. This document relies on many other industry standards, which should also be consulted. It is the intent to provide recommendations beyond the content of those standards, especially as they relate to integrating the roofing assembly into a total building enclosure and mechanical system design. It is intended to provide a “Best Practice” and shall not be construed in any manner to establish the legal standard of care required from licensed professionals.
Prior to the mid-to-late 1970s, almost all low-slope roofs were asphalt or coal tar built-up roofs. In fact, in the earlier part of the century, coal tar roofs were often used to cool buildings by allowing the intentional ponding of water on the coal tar surface of the roof for evaporation and cooling effect. Coal tar pitch is not composed of solvents like asphalt, and so will not dissolve and evaporate the solvent oils out of the roofing compound like asphalt in a pond situation. This is why coal tar pitch can be applied on a dead level roof surface; ponding water has no negative effect on it. Type I Coal Tar Pitch has good adhesive and self-healing properties and is used with aggregate surfacing on roof slopes up to 1/8″:12. Designers should specify coal tar for use in coal tar built-up roof membranes comply with ASTM Standard D450, Type I. Also, coal tar built-up membrane systems should be detailed in accordance with manufacturer requirements; especially at drains, scuppers, and roof edges.
Asphalt continues to be the more common built-up roofing material compared to coal tar pitch. One must be aware of the critical difference in the oil solvent composition of asphalt, in that these solvents can leach out of the asphalt in ponding conditions, evaporate off, and leave the asphalt membrane dried and cracked just where the ponding is most prevalent. Be aware that, for this reason, asphalt roofing manufacturers require a minimum of 1/4″ slope per foot to prevent any possibility of ponding. Manufacturers typically consider moisture that remains on the roof surface OK as long as it evaporates within 48 hours under conditions conducive to drying. Asphalt system warranties are typically void for slopes less than 1/4″ per foot, and certainly where any ponding occurs, which unfortunately, is where leaks will occur.
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, metal panels, and reinforced liquid-applied roof membranes. Liquid-applied roofing was added to the International Building Code, 2012 Edition (IBC 2012) in Section 1507.15. The NRCA has a section and details for this type of roofing in The NRCA Roofing Manual: Membrane Roof Systems.
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 and applications from which to choose has created a complex and challenging subject matter.
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 1/4:12 (2 percent) slope. It is recommended that low-slope roofs have a slope of 1/2:12 (4 percent) where possible. Steep-slope roofs are defined as those roofs with a slope greater than 3:12 (25 percent). As discussed in the Description section, some materials can be used on both low- and steep-slopes, while others are limited to either low- or steep-slope. Steep slope materials may require additional enhancements when installed on slopes less than 4:12 (33 percent).