What causes ponding (standing water) on a flat roof?
Tuesday, July 20, 2021
A few puddles on a flat roof after a heavy rain are normal. But when they are still there two days later, you have a problem. Standing water deteriorates the area of roofing under it, causing premature leaks, and tends to collect unsightly and acidic leaf debris around its outline. Here's our list of five possible causes:
1) Sagging roof - Flat roofs are never built truly flat, or at least they are not supposed to be. The building code requires a minimum 1/4” drop over each lineal 12" of roof, referred to as a quarter-inch per foot slope. A lesser slope would be possible if the roof surface was as perfectly even as a tabletop, but roof rafters and trusses have a natural sag under load (called deflection by engineers) and it usually occurs at the middle of the span.
Part of the mandated minimum slope is intended to allow for a little sagging and still have a positive slope to the roof edge. When the sag exceeds the built-in slope, you get standing water. One cause of sag is undersized roof rafters with excessive deflection, and deterioration of the rafters in a humid environment over time, along with just plain sloppy construction creating built-in dips, are two more.
2) Adding a heavy item on roof - Installing an air conditioner on a flat roof that was not designed for the additional weight is one example. Another is a wood roof deck bearing directly on a roof beneath it that has not been beefed-up to handle it. Deflection and ponding will occur over time as the roof reacts to the new load.
3) Leaking roof - When the surface of a roof deteriorates and starts to crack, especialy when it’s a built-up roof, puddles form in the cracks and the degradation of the surface and enlargement of the puddles speeds up. Little craters form in an aging SPF (Sprayed Polyuethane Foam) roof and allow leaks to reach the wood below, damaging and weakening the roof structure. Sagging roof sheathing between rafters or at the roof edge can be caused by leaks too.
4) Structural settlement - Uneven settlement of the walls or columns supporting a roof, particularly where a flat roof intersects a wall that rises above it, can cause large areas of ponding. See photo at top of page. Signs of structural distress below the area can confirm this diagnosis.
5) Clogged roof drains - This problem only occurs when a flat roof has is wrapped with a parapet wall above it around the perimeter, or it has drains at the center of the roof that are piped to the ground. It can be dangerous because there is sometimes the potential for enough water to accumulate to collapse the roof. But very few residential roofs have this design.
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