I’ve had this Q & A page at JLC online on my to do list for a while now to post up here for you all…while the discussion is about drainage problems on low slope roofs, the photo they use is a deck over living space with tile on it and ponding water. And of course, decks over living space with tile or pedestrian traffic coatings on them are roofs as well, just that we walk on them too.

Frank discusses how a roof deck can pond water even when it’s “built to code” from loads placed on the deck. Recommended reading for designers, architects, builders, waterproofers and anyone interested in increasing their knowledge and skills.

Drainage Problems on Low-Slope Roofs

By Frank Woeste

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: 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 below).

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