Green Roofs in Stormwater Design: Function, Modeling, and Engineering Considerations

Green roofs, also called vegetative roofs, are engineered roofing systems that include vegetation and growing media installed over a waterproof membrane.

Instead of behaving like a conventional impervious roof that rapidly sheds rainfall, a green roof captures, stores, and slowly releases stormwater through drainage, evaporation, and plant transpiration.

In dense urban areas where ground-level space is limited but stormwater and heat island concerns are significant, green roofs provide a way to manage rainfall directly at the rooftop.

In this article, we’ll cover:

• What green roofs are

• The types of green roofs

• How they function hydrologically

• Benefits and limitations

• How engineers model them in EPA SWMM 5.2 and WWHM 2025

• Key design considerations

What Is a Green Roof?

A green roof is an engineered system installed above a conventional roof deck and waterproofing membrane. A typical green roof system includes:

• Roof deck

• Waterproofing membrane

• Root barrier

• Drainage layer

• Filter fabric

• Lightweight growing medium

• Vegetation layer

Rainfall is intercepted by vegetation and stored within the growing medium and drainage layers. Over time, stored water leaves the system through evapotranspiration (ET) — evaporation from the soil and transpiration from plants.

Only when storage capacity is exceeded does excess water discharge to roof drains.

The U.S. EPA classifies green roofs as a form of green infrastructure because they manage runoff while providing additional environmental benefits.

Types of Green Roofs

Green roofs are commonly grouped into two primary categories:

Extensive Green Roofs

• Shallow growing media (typically up to ~6 inches)

• Lightweight systems

• Low maintenance

• Drought-tolerant plants such as sedums and grasses

Intensive Green Roofs

• Deeper growing media (generally greater than 6 inches)

• Can support shrubs and small trees

• Higher structural load requirements

• Greater maintenance needs

Both systems may be constructed as continuous built-up layers or modular tray systems. Selection depends on:

• Structural load capacity

• Desired vegetation

• Access requirements

• Maintenance budget

Hydrologic Function of Green Roofs

Hydrologically, a green roof behaves like a shallow soil system.

When rain falls:

1. A portion is intercepted by vegetation.

2. Water infiltrates into the growing medium.

3. Storage occurs in pore spaces and drainage layers.

4. Water leaves over time through evapotranspiration.

5. Excess water drains once saturation is approached.

Studies and EPA guidance indicate that green roofs can significantly reduce both:

• Total runoff volume

• Peak flow rates

compared to conventional impervious roofs.

They also provide limited pollutant filtration as water passes through media.

Environmental and Infrastructure Benefits

Green roofs provide multiple co-benefits beyond runoff reduction:

• Reduced combined sewer overflows in older urban systems

• Lower roof surface temperatures

• Urban heat island mitigation

• Reduced building heating and cooling demands

• Extended roof membrane lifespan

• Habitat creation for birds and pollinators

• Improved visual amenity

These benefits make green roofs attractive in urban redevelopment and district-scale green infrastructure programs.

Constraints and Limitations

Despite their benefits, green roofs require careful planning:

• Structural capacity must support saturated weight (media + water + vegetation).

• Construction costs are typically higher than conventional roofs.

• Maintenance is required for vegetation health and drain function.

• Seasonal variation affects performance (e.g., frozen substrate reduces storage).

• Waterproofing and root barriers must be robust to prevent leaks.

Proper design, inspection, and maintenance are critical.

Typical Green Roof Cross Section

A standard green roof cross-section includes:

1. Roof deck

2. Waterproof membrane

3. Root barrier

4. Drainage layer (may include storage voids)

5. Filter fabric

6. Lightweight engineered growing medium

7. Vegetation layer

Each layer has a specific structural or hydrologic function. Design guides from EPA and state stormwater manuals follow similar configurations.

Modeling Green Roofs in EPA SWMM

In EPA SWMM 5.2, green roofs are modeled using a dedicated Green Roof LID control (earlier versions used bioretention LID).

SWMM simulates:

• Soil layer storage

• Evapotranspiration losses

• Drainage through an underdrain or drainage mat

• Water balance reporting

Users define parameters such as:

• Media depth

• Porosity

• Field capacity

• Hydraulic conductivity

• Drainage configuration

SWMM’s LID reporting tools provide detailed outputs, allowing comparison between conventional and vegetated roof scenarios.

Modeling in WWHM 2025 (Western Washington)

In Western Washington, the WWHM 2025 model is used for flow control compliance under Ecology guidance.

Green roofs are evaluated as one of several Low Impact Development (LID) facilities using continuous simulation with long-term rainfall records.

Designers input:

• Media depth

• Hydraulic properties

• Drainage configuration

• Converted roof area

The model estimates changes in:

• Peak flows

• Flow duration

• Runoff volume

When designed per the Stormwater Management Manual for Western Washington, green roofs may receive flow control credit.

Common Applications

Green roofs are most commonly installed on:

• Commercial buildings

• Multifamily housing

• Industrial facilities

• Schools and public buildings

• Retrofit projects in dense urban areas

They are particularly effective where expanding ground-level stormwater infrastructure is not feasible.

Some cities deploy green roofs at district scale to reduce combined sewer overflows and improve urban livability.

Key Design Considerations

Successful green roof projects begin with:

• Structural verification for saturated loads

• Proper drainage design

• Climate-appropriate plant selection

• Maintenance access planning

• Compliance with local stormwater manuals and building codes

Performance depends on accurate modeling inputs, good engineering practice, and consistent long-term maintenance.

Conclusion

Green roofs are a proven urban stormwater solution that captures rainfall at the rooftop, reduces and delays runoff, and provides multiple environmental co-benefits.

They can be evaluated quantitatively in:

• EPA SWMM 5.2 using the Green Roof LID control

• WWHM 2025 as part of continuous simulation for flow control

As with all LID practices, success depends on proper design, modeling, structural review, and maintenance.

For additional hydrology and stormwater education resources, explore our free materials and instructional videos at Clear Creek Solutions.