The ASCE 7 Standard and Low-slope Roof Drainage
By Stephen Patterson, RRC, PE; and Madan Mehta, PE, PhD
Three important safety issues related to low-slope roofing are: 1) fires on roofs from internal or external sources, 2) roof collapses resulting from the ponding of water (and snow, where present) from extreme storms, and 3) roof blow-offs from extreme windstorms.
Fire safety falls within the realm of fire engineering, and the two important U.S. organizations focusing on its study are the Society of Fire Protection Engineers (SFPE) and the National Fire Protection Association (NFPA). Roof collapses from water ponding and blow-offs are related to the roof’s structural design. The organizations focused on structural safety in buildings are the American Society of Civil Engineers (ASCE) and the Structural Engineering Institute (SEI).
An important document dealing with structural safety in buildings, published jointly by ASCE and SEI, is the ASCE/SEI standard, titled Minimum Design Loads and Associated Criteria for Buildings and Other Structures, commonly known as the ASCE 7 Standard. This standard deals comprehensively with all types of loads on buildings and non-building structures, such as rooftop equipment, decks, billboards, signages, etc. It specifies minimum loads for which buildings and other structures must be designed, such as dead loads, live loads, snow loads, rain loads, earthquake loads, wind loads, and so on.
Although the standard by itself is not legally enforceable, most of its provisions are adopted by building codes of various U.S. jurisdictions, indirectly bestowing legal authority upon it. For example, Chapter 16 of the International Building Code (IBC), titled “Structural Design,” which covers “minimum design requirements so that the structural components of buildings are proportioned to resist the loads that are likely to be encountered,” is based almost entirely on the ASCE 7 standard.
The standard is updated periodically to keep abreast of the developments in this complex but important field because an accurate determination of loads on a building is fundamental to its structural safety. Various editions of the standard are distinguished from each other by a two-digit number at the end of their designation, which refers to the year of publication (or the target year of publication). For example, the ASCE 7-02 standard was published in 2002. On the other hand, its current edition, ASCE 7-16, was planned for 2016 publication, but was released in 2017. Its next edition is planned for release in 2022.
The changes made in ASCE 7 affect almost all aspects of a building’s structural safety. However, the discussion presented here deals with the effect on a building’s structural safety related to the design of its roofing system. More specifically, this paper covers the contribution of the current ASCE 7 standard (ASCE 7-16) to the design of low-slope roof systems to prevent roof collapses from rainwater ponding from extreme rainstorms.
Roof Ponding Fundamentals
Water accumulates on parapeted (raised-edge) roofs, which are typically low-slope roofs. As explained later in this section, water accumulation (commonly referred to as ponding) on a parapeted, low-slope roof is an inescapable design issue. It can be mitigated through good design but cannot be eliminated entirely and must be accounted for in the design of the roof system. Three factors affect ponding on such roofs: 1) roof slope, 2) the roof deck’s structural stiffness, and 3) the roof’s drainage design.
A fundamental strategy to reduce roof ponding is to provide an adequately sloped roof. Increasing the slope helps in several ways. First, it ensures that water will reach the drainage elements (roof drains or scuppers) more rapidly. Theoretically, if the roof is perfectly flat (an ideal dead-level roof), it will drain water toward the drainage elements, but do so slowly.
Additionally, construction tolerances and workmanship lapses can be such that high and low spots are always present on a roof deck. These surface irregularities on an otherwise dead-level roof will produce some pools of ponded water. Therefore, the second advantage of increasing roof slope is that it overcomes the obstructions caused by incidental surface irregularities and reduces the adverse effect of shallow ponds on drainage and a roof membrane’s durability.
Apart from surface irregularities, an additional contributor to ponding is the deflection of roof decks due to the weight of the roof structure and rooftop equipment. The most significant cause of ponding, however, is the deflection of the deck from the buildup of water during rainfall. Stiffness of the deck plays an important role in reducing such ponding. In fact, the provisions of the ASCE 7 standard, as will be described further, clearly highlight the role of roof deck stiffness and roof slope on roof ponding loads.
Read more here: https://iibec.org/asce-7-standard-low-slope-roof/
