Ponding Water on Flat Roof
Not an emergency, but track for changes
Water pooling on your flat roof? Standing water after 48 hours causes membrane damage. Learn fixes ($500–$3,000) and drainage improvement options. Estimated repair cost: $500–$3,000. For a precise estimate based on your actual roof, get a free satellite quote at RoofVista.
Likely Causes
Inadequate roof slope — flat roofs should have minimum 1/4" per foot slope to drains
Clogged roof drains or scuppers preventing water from exiting
Sagging roof structure creating low spots where water collects
HVAC equipment or other rooftop loads compressing insulation and creating depressions
Previous patch repairs creating uneven surface that traps water
How to Diagnose This Problem
- 1
Check the roof 48 hours after rain — any remaining water indicates a ponding problem
- 2
Measure water depth in ponding areas — over 1 inch deep is a structural concern
- 3
Check all drains and scuppers for debris blockage — clean and test drainage
- 4
Look at the membrane under ponding areas — accelerated wear, bubbling, or soft spots indicate damage
- 5
Check for evidence of algae growth in ponding areas — this signals chronic standing water
Estimated Repair Cost
$500 – $3,000
Typical repair range (varies by location and severity)
When to Replace Instead of Repair
If ponding is caused by structural sagging rather than drainage issues, tapered insulation systems ($3-$6/sqft) installed during a membrane replacement solve the problem permanently. Membrane replacement is needed if ponding has degraded the existing membrane.
Materials Affected
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