The majority of the roofs in which metal plate connected wood trusses are used are designed with adequate slope and drainage. Rain and the meltwater from snow quickly drain away. For these roofs, designers don’t need to account for rain load as one of the environmental loads the roof will support (unless conditions require a rain-on-snow surcharge).
However, as multi-story multi-family buildings and other urban in-fill structures increasingly feature low-slope (and sometimes even green) roofs, water management is more and more frequently a significant design consideration. The key question for every designer: is the roof system you’re working on susceptible to ponding, the condition in which water is retained on the roof?
A small amount of anticipated ponding might not significantly affect the building, but if the structural roof members, including the sheathing, purlins, trusses, joists, beams and girders, are not strong enough or stiff enough to support the additional weight of the water, ponding instability can occur and has the potential to result in the failure of all or part of the roof system.
Truss technicians who recognize the conditions that can lead to ponding can implement appropriate design practices to ensure the trusses are designed to perform as intended by the building designer.
1611.1 Design rain loads. Each portion of a roof shall be designed to sustain the load of rainwater that will accumulate on it if the primary drainage system for that portion is blocked plus the uniform load caused by water that rises above the inlet of the secondary drainage system at its design flow. […]
Ponding is most often associated with flexible flat roofs—that is, flat roofs with large deflection limits. With these roofs, the initial dead load deflection is potentially large enough to collect and hold water from rain or snow melt. The weight of this water will cause additional deflection, which can lead to additional accumulation. Over time, the deflection and accumulation cycle can lead to structural failure.
Ponding can also occur when a roof drainage system is blocked or impeded. For example, even sloped roofs might feature parapets or other obstructions that prevent water from freely draining over the edge. Roofs without free drainage typically include a primary drainage system—such as roof drains at or near the lowest elevations of the roof plane—and one or more secondary drainage systems—such as drains or scuppers at slightly higher elevations than the primary drains. When the primary drains are clogged with debris or ice, water accumulates until it reaches the inlet of the secondary drainage system. The difference in elevation between the primary and secondary drainage systems can be several inches, and the weight of the water that accumulates in this space must be considered in the design of the supporting roof members.
R = Rain load on an undeflected roof in pounds per square foot (kN/m2). Deflection from loads isn’t considered when determining the amount of rain on a roof.
ds = Depth of water on an undeflected roof up to the inlet of the secondary drainage system in inches (mm).
dh = Depth of water on an undeflected roof above the inlet of the secondary drainage system in inches (mm).
Ponding instability can be prevented. Adequate drainage, a large enough roof slope to prevent water accumulation, or a design that ensures all structural members of the roof system have enough strength and stiffness to prevent progressive deflection can mitigate potential issues. ASCE/SEI 7-10, for instance, notes that a roof slope of a quarter of an inch per foot or greater is adequate for preventing ponding instability (assuming adequate drainage is also provided), and the ANSI/TPI 1-2014 commentary to section 7.6.2 indicates that a camber of one and a half times the dead load deflection is typically sufficient to prevent ponding.
In any scenario, section 188.8.131.52 of ANSI/TPI 1-2014 requires building designers to include in construction documents the location, direction and magnitude of any rain and ponding loads to be supported by trusses, as well as any dead load, live load and in-service creep deflection criteria for roofs subject to ponding loads. A wise truss designer should take note of this information and, if unclear, make a request for verification to avoid the potential problems of an under-designed roof system.
7.11 Ponding Instability. Roofs shall be designed to preclude ponding instability. For roofs with a slope less than ¼ in./ft (1.19˚) and roofs where water can be impounded, roof deflections caused by full snow loads shall be evaluated when determining the likelihood of ponding instability […].