The Roofing Resource Blog

Why Is there Ponding water on my Flat Roof ?

Wed, Feb 25, 2015 @ 08:00 AM | Perry

ponding water_hail_damaged_roof

Ponding water on a flat roof can not only cause damaging leaks, it can drastically reduce the performance and life of the flat roofing membrane. For this reason most flat roofing material manufacture’s warranties are void in the presence of or exclude ponding water. Ponding water is defined by the roofing industry as water that remains on the roof for longer the 48 hours after rain.

Understanding why your flat roof is holding water is the first step to finding a solution. 

1. Inadequate or Obstructed Roof Drainage 

flat_roof_drainThe most common reason that flat roofs hold or pond water is because of inadequate  or obstructed drainage. The good news is that this cause of ponding water is usually the easiest to correct with scheduled roof maintenance. Flat roofs drain in one of three ways: 1) through roof drains, 2) sloping towards a gutter, or 3) through a scupper - collector head that leads to a downspout.  When these drains / gutters are clean and water is able to flow ponding water issues  are greatly diminished or non existent. 

Everything from soda pop cans , tennis balls, basket balls, leaves, paper, plastic bags and other flying debris can partially or total block your flat roof drainage. Scheduled flat roof maintenance will ensure that you avoid this totally avoidable type of ponding water and the damage it can cause. 

2. Structure, Design, Deflection. 

The second cause of ponding water on a flat roof is more complicated and cannot be resolved by simple cleaning the drains. This type of standing water is usually caused by either poor design or from deflection or sag in the buildings roof structure. Improperly sized , not enough, or poorly placed flat roof drainage can cause mild to sever ponding water issues. However this standing or ponding water is usually never as bad a when the roof structure has deflected or sagged over time. 


When the structure that the flat roof is installed upon has settled or sagged the amount to ponding water will be equal to the distance that the structure has deflected below the roof gutters or scuppers. Flat roofs with internal drains are rare affected by a sagging deck as the water will still drain toward the low spot. This is a common problem on buildings where the flat roof is installed over a concrete deck that has deflected or  sagged over time. The sag or low spot in the roof causes water to stand instead of running towards the scupper or gutter. Water can be removed from the roof only to return the very next rain or snow event. 

3. Compressed or Saturated Roof Insulation


The third type of ponding water condition is caused by roofing insulation that has either been compressed or smashed down with heavy roofing materials or HVAC equipment during a construction project or by roof insulation that has become saturated from a leak and has degraded. 

Many types of flat roofing insulation are sensitive to water and will quickly degrade or dissolve when wet. As the degraded insulation begins to dissolve and loose its shape, water is able to pond in the indentation. This problem compounds itself because the more water stands on the flat roof the more it leaks and the more it leaks the more degraded the roof insulation becomes causing even more water to stand. When this happens the screws and plates that originally held the roof insulation down will now appear to be sticking up and will be visible under the flat roofing membrane. This type of ponding water can be corrected by replacing the saturated and degraded roofing insulation the the next the the flat roof is replaced.  

Also see Three Ways to Eliminate Ponding Water on Your Flat Roof