Designing and Verifying Water Control Strategies for Steep-Slope Gutters

23 Dec 2025

How Analysis and Testing Improved Roof Drainage Performance at the Mann Center for the Performing Arts

Effective roof drainage design supports the long-term performance of a building enclosure. In the Mid-Atlantic, where rainfall intensity and seasonal extremes continue to increase, steep-slope assemblies demand more than code-based gutter sizing to manage high runoff velocity and complex roof geometry.

At the Mann Center for the Performing Arts in Philadelphia, Pennsylvania, recurring gutter overflows during heavy rain revealed those limitations. After a capital improvement project introduced expansion joints and a new polymethyl methacrylate (PMMA) liner, overflow began even during moderate storms. The owner engaged Intertek to analyze the problem and verify a solution for Philadelphia’s increasingly intense rainfall.

Understanding the Existing Conditions

What Roof and Site Conditions in Philadelphia Led to Gutter Overflow at the Mann Center?

The Mann Center’s steep, standing-seam metal roofs drain into built-in gutters that wrap the perimeter of the pavilion. Each section discharges through multiple inlets connected to rainwater conductors. Expansion joints were added during the refurbishment to accommodate thermal movement and prevent liner fatigue.

While structurally beneficial, the joints created localized dams that restricted water flow. The PMMA liner further reduced inlet capacity by nearly 40 percent. Without sufficient relief between gutter segments, water traveling downslope at high velocity overflowed the edges, spilling onto pedestrian areas and damaging assemblies below.

Field assessment confirmed that overflow points aligned with the new expansion joints. Debris buildup, limited maintenance access, and the steep geometry compounded the failures. A combined analytical and physical testing program was required to understand flow behavior and validate proposed modifications.

Computational Analysis and Design Iteration

How Did CFD Modeling Help Evaluate Steep-Slope Drainage Performance in a Mid-Atlantic Climate?

To analyze system performance, Intertek engineers developed a computational fluid dynamics (CFD) model replicating the Mann Center’s steep-slope geometry and rainfall conditions typical of Philadelphia’s climate zone. The analysis simulated rainfall above code minimums to capture extreme weather behavior.

Boundary conditions based on observed field data allowed accurate prediction of pressure, velocity, and dissipation across the gutter system. Results showed about 10 percent flow loss from bypass and splash. Multiple design iterations evaluated sump geometry, flange transitions, and expansion joint placement to increase inlet efficiency and reduce turbulence.

The optimized design featured curved sidewalls and tapered geometry to slow velocity and improve drainage pressure at each inlet. The final configuration encouraged debris wash-through, essential for the Mann Center’s tree-lined Fairmount Park site.

Performance Mock-Up Testing

How Was the Mann Center’s Roof Drainage Design Verified Through Full-Scale Testing in Philadelphia?

Intertek built a full-scale mock-up replicating the Mann Center’s roof and gutter layout. Testing simulated rainfall up to 9.5 inches per hour, representing a 100-year design storm with a regional safety factor for Philadelphia’s rainfall patterns.

Calibrated flow instruments measured drainage efficiency while high-speed videography documented flow and debris movement. Testing required at least 85 percent capture with minimal splash or bypass.

The mock-up exceeded expectations. The optimized sump captured approximately 86 percent of total flow with only 0.5 percent dissipation. Debris washed through the system unobstructed, confirming effective self-clearing.

Results and Lessons Learned

What Did Field Data and Testing Reveal About Improving Roof Drainage Systems in the Northeast?

Since the redesigned gutter assemblies were installed, the Mann Center has experienced several storm events exceeding modeled rainfall intensity with no reported overflow. The project showed that performance-based design delivers more reliable results than prescriptive methods.

Key Findings

• A 15 percent safety factor above code rainfall produced reliable results for Philadelphia.
• CFD modeling allowed precise iteration without the time and cost of multiple prototypes.
• Combining digital analysis and testing validated assumptions
• Long-term maintenance considerations such as debris wash-through and cleanout access should be integrated early in the design process.

Advancing Drainage System Performance

How Can Performance-Based Analysis Improve Future Roof Drainage Design Across the Mid-Atlantic Region?

As storm intensity increases across the Northeast and Mid-Atlantic, gutter systems must evolve from prescriptive sizing to performance-based verification. CFD modeling and physical testing provide the data-driven insight needed to predict real-world behavior, reduce failure risk, and extend building service life.

Intertek helps design teams evaluate and optimize water management systems for real-world performance.