Commercial Roofing

Common Flat Roof Problems and How to Fix Them

Updated October 2026 · 4 min read · By the Knoxville Roofing Quotes team

Flat and low-slope roofs are everywhere around Knoxville: commercial buildings, modern homes, porches, additions and garages. They're practical, but they come with problems that sloped roofs don't have, and with about 52 inches of precipitation in a normal year here, according to the National Weather Service, drainage gets tested often.

If you've got a flat roof, here's what to watch for.

1. Ponding Water

This is the big one. The National Roofing Contractors Association defines ponding as water that stays in low spots more than 48 hours after rain, in weather that would normally dry it. On a sloped roof, water runs off. On a flat roof, drainage has to be designed in, and when it fails, water collects.

Why It Matters

• Standing water accelerates membrane deterioration
• Added weight stresses the structure
• Creates a perfect environment for algae and vegetation
• Ice expansion in winter can damage membranes
• Attracts mosquitoes and pests

Solutions

• Improve drainage: Add or unclog drains, install crickets to direct water
• Add tapered insulation: Creates slope toward drains
• Install additional drains: More drainage points means less ponding
• Pond coating: Specialized coatings can protect areas that pond (temporary solution)

2. Membrane Damage

Flat roofs are typically covered with a membrane – EPDM, TPO, PVC, or modified bitumen. These membranes can get damaged by:

• Foot traffic (especially near HVAC units)
• Dropped tools or equipment
• Tree branches and debris
• UV degradation over time
• Thermal expansion/contraction stress

Signs of Damage

• Visible tears, punctures, or cracks
• Bubbling or blistering
• Seams pulling apart
• Worn or thin spots

Solutions

Small punctures can often be patched with membrane-compatible patches and adhesive. Larger damage or widespread deterioration may require section replacement or full re-roofing. Prevent foot traffic damage by installing walk pads in high-traffic areas.

3. Flashing Failures

Where the flat roof meets walls, curbs, drains, and penetrations requires flashing. It's the most common leak source on flat roofs.

Problem Areas

• Base flashings at walls and curbs
• Pipe boots and vent flashings
• HVAC curb flashings
• Drain flashings
• Expansion joints

What to Look For

• Sealant cracking or separating
• Flashing pulling away from surfaces
• Rust on metal flashings
• Gaps or openings

Solutions

Flashing repairs often just need new sealant and possibly new membrane termination bars. Badly deteriorated flashings need complete replacement. Regular inspection catches these before they leak.

4. Blistering and Bubbling

Blisters are raised areas where the membrane has separated from the substrate, usually with air or moisture trapped underneath.

Causes

• Moisture trapped during installation
• Inadequate adhesion
• Moisture migration from inside the building
• Volatile gases escaping from insulation

Why It Matters

Small blisters aren't immediate emergencies. But they can grow, eventually cracking and allowing water intrusion. They also indicate underlying problems that may be getting worse.

Solutions

Small stable blisters can be monitored. Growing or cracked blisters need to be cut open, dried, and patched. Widespread blistering might indicate bigger moisture problems requiring investigation.

5. Shrinkage and Cracking

Over time, some membranes shrink, a problem often seen on older EPDM roofs. Shrinkage pulls the membrane away from edges and flashings, creates stress at seams, and can expose underlying substrate.

Signs

• Membrane pulling away from edges
• Exposed substrate around perimeter
• Stretched or thinned areas
• Cracking patterns

Solutions

Minor shrinkage can be addressed with additional flashing at edges. Severe shrinkage usually means it's time to replace the roof. There's no way to "unshrink" a membrane.

6. Clogged Drains

Flat roofs depend entirely on drains for water removal. When drains clog, water backs up, and that leads to ponding (see problem #1).

What Clogs Drains

• Leaves and debris
• Roofing granules and particles
• Dirt and sediment buildup
• Bird nests (yes, really)
• Ice in winter

Prevention

• Install drain covers/strainers
• Regular cleaning schedule
• Clear debris after storms
• Consider secondary (overflow) drains

Maintenance Is Everything

Here's the truth about flat roofs: they need more attention than sloped roofs. Problems don't self-correct. Water doesn't just run off. Small issues become big issues faster.

A solid maintenance program includes:

• Twice-yearly professional inspections (spring and fall)
• Clearing drains regularly, and more often if trees hang over the roof
• Checking after major storms
• Addressing small issues immediately
• Keeping foot traffic to a minimum

How long a flat roof lasts depends heavily on the system and the upkeep. InterNACHI's life-expectancy chart gives broad ranges: 7 to 20 years for TPO, 15 to 25 for EPDM, about 20 for modified bitumen and about 30 for built-up roofing. Maintenance is what pushes a roof toward the long end.

If you have an active leak, start with our roof leak repair page. For business properties, our commercial roofing guide covers system choices and costs, and our roof inspection page explains what a professional inspection should include.

Sources

• National Roofing Contractors Association: Glossary definition of ponding
• InterNACHI: Life expectancy of home components
• National Weather Service Morristown: Knoxville climate