What Causes Ponding Water on Flat Roofs?
Ponding water is one of the most common - and most misunderstood - problems on commercial flat roofs. While a small amount of water after rain is normal, water that remains for extended periods signals a deeper issue.
For commercial buildings in Greenville and the Upstate, understanding why ponding occurs helps prevent leaks, structural damage, and premature roof failure.
What Counts as Ponding Water?
Ponding water is defined as water that remains on a roof more than 48 hours after rainfall, assuming normal weather conditions.
When water doesn’t drain properly, it increases stress on the roofing system and accelerates wear.
Improper Roof Slope
Flat roofs aren’t perfectly flat - they rely on subtle slopes to move water toward drains or scuppers.
If the roof wasn’t designed or modified properly, low areas can form where water collects instead of draining.
Clogged or Restricted Drains
One of the most common causes of ponding is blocked drainage.
Debris such as:
Leaves
Dirt
Trash
Roofing granules
can prevent water from exiting the roof, even if the slope is adequate.
Settling of the Building Structure
Over time, buildings naturally settle.
When this happens:
Decking can dip
Structural supports shift
New low spots develop
These changes often create ponding areas years after installation.
Saturated or Compressed Insulation
Water that penetrates the roofing membrane can soak into insulation.
Once insulation becomes wet, it:
Loses rigidity
Compresses
Creates depressions that trap more water
This leads to a cycle of worsening ponding.
Improper Repairs or Roof Additions
Adding rooftop units, walk pads, or patch repairs can unintentionally disrupt drainage patterns.
If water is redirected without proper planning, new ponding areas can form.
Why Ponding Water Is a Serious Problem
Standing water:
Breaks down roofing materials
Increases leak risk
Adds weight to the structure
Accelerates membrane failure
Voids many manufacturer warranties
Ignoring ponding water often leads to costly repairs.
When Ponding Water Needs Professional Attention
Ponding water should be evaluated when:
Water remains after 48 hours
The same areas collect water repeatedly
Leaks or interior moisture appear
The roof feels soft underfoot
A professional inspection can determine whether drainage improvements, repairs, or restoration options are needed.
Addressing the Cause - Not Just the Water
Removing water without fixing the underlying issue doesn’t solve the problem.
For Greenville commercial properties, identifying why ponding occurs is the key to protecting the roof and extending its lifespan.
