How to Prevent Disastrous Drainage Problems on Low-Slope RoofsHow to Prevent Disastrous Drainage Problems on Low-Slope Roofs

Q: Can deflection of a low-slope roof cause ponding? How can this be avoided?

A: Frank Woeste, P.E., professor emeritus at Virginia Tech, responds to the Journal of Light Construction: Historically, structural designers and builders have assumed that a design slope of 1/4 inch per foot (1:48) is sufficient to prevent ponding action, thinking that the installed roofing system will maintain at least a 1:48 slope in-service as required by some roof covering systems.

However, in many cases and for different reasons, ponding on limited areas of low-slope roofs is common. That’s due to roof deflection, which over time can cause water to collect in some areas of a roof where the design slope for drainage is not adequate, and in fact changes from a “positive” drainage slope to a “negative” slope (see photo above).

A Study Case

As shown on the vertical axis of the graph below, the 1/4-inch-per-foot design slope results in the left bearing being 5 inches above the right bearing. The span of 240 inches is depicted along the horizontal axis of the graph. The straight black line represents the slope (1:48) of the 2x10s prior to any type of loading. This slope matches the “design slope,” which is normally communicated to the joist or truss designer by the construction drawings or a specification.

To read the rest of Woeste's article for JLC, including steps to address ponding potential, click here.