FAQ Purple Roof tools

The Purple Roof system is a type of blue/green roof that functions as both an evapotranspiration system and a detention system on the roof. It combines the benefits of a green roof (vegetation to absorb and transpire rainwater) with a blue roof (storage and slow release of stormwater). That dual behavior helps manage runoff in dense urban environments.

Key functions

Evapotranspiration. Vegetation and soil capture and store rainwater; water returns to the atmosphere through evaporation from the soil and transpiration from plants, reducing runoff volume.

Detention. During heavy rain, when upper layers saturate, excess water enters the honeycomb detention reservoir and is released to the storm system at a controlled rate, easing peak loads on sewers and reducing flood risk.

Layers of the system (top to bottom)

• Sedum mat: Pre-vegetated mat of hardy, drought-tolerant plants.
• Engineered soil media: Lightweight growing medium that supports plants and retains water.
• Needled mineral wool (retention fleece): Highly absorbent layer that holds water for evapotranspiration.
• Honeycomb (detention reservoir): Structural void space for detaining stormwater.
• Detention drainage layer: Low-transmissivity fabric that controls discharge so detention storage is used through the profile.

Curve numbers (CN) for green roofs in these tools are grounded in ASTM-style testing and variable profile depths, not a single generic CN for all assemblies.

For a full technical discussion and applications specific to Purple Roof systems, read the memo “Defining a Curve Number in Purple Roof Systems.”

Open the memo (Google Drive)

Hydraulics of the outlet control drain box

The drain box around each roof drain acts as a hydraulic outlet control. The box typically has four openings (one per side). Opening width is adjustable in design to target the desired outflow rate. The lowest layer of the system—the detention drainage layer—passes through each opening and is modeled as an orifice. Multiple drains are represented by multiplying the effective drainage opening width.

Video: Watch the drain box in action.

Because the Purple Roof orifice is not a standard free-flowing opening and the assembly includes the low-transmissivity drainage layer, a typical C_d ≈ 0.6 is not appropriate. Validation used direct stage–discharge measurements in the lab: a 20′×4′ cassette with depth sensors in 2″ honeycomb storage above the detention drainage layer, drainage at one end, scales on the cassette and collection basins, and equilibrium stages to build a stage–discharge curve.

Compared to the orifice equation, the data fit well with C_d = 0.07, most accurate at higher storage depths. Because ~2″ storage is common and that depth drives peak conditions, C_d = 0.07 is the recommended value for design calculations in this context.

Video: Direct stage–discharge measurement test.

Purple Roof installation

Step 1 – Pre-construction planning

• Confirm waterproofing and flood testing are complete.
• Verify roof penetrations, curbs, and drain locations.
• Store materials dry; DL5 can stretch and NMW can tear when wet.
• Centralize loading; install from the farthest point back toward access.
• Hold a pre-installation meeting; verify fall protection, logistics, and safety.

Step 2 – Drain enclosures and edging

• Place prefabricated drain enclosures per shop drawings.
• Confirm drain domes are clean and functional.
• Secure enclosures (temporary tape or ballast if needed).
• Install edging per hydraulic layout (solid or perforated); confirm alignment before DL5.

Step 3 – Detention layer (DL5)

• Install over waterproofing or root barrier; start at drains and work outward.
• Butt seams tightly (no overlap); purple line faces up.
• Tape or ballast seams; keep DL5 dry until placed.
• Inspect for gaps or misalignment during placement.

Step 4 – Honeycomb reservoir

• Install above DL5 in running bond; 4′×8′ panels flat and tight.
• Secure in wind; fill gaps >1″ with cutoffs.
• Cover promptly with mineral wool or fabric to keep debris out of honeycomb.
• If metal edging sits above honeycomb, install before proceeding.

Step 5 – Needled mineral wool (NMW)

• Keep wrapped until install; do not install wet.
• Start farthest from access; single layer: 1″ seam overlap; multi-layer: alternate 90°, no overlap.
• Extend up verticals to 1″ below final grade.
• Avoid walking on wool; use insulation knives; wear PPE (fibers can irritate skin).

Step 6 – Growing media

• Confirm structural capacity with the EOR.
• Spread evenly within +1″ of final grade; allow 10–15% settlement.
• Compact and water thoroughly; avoid disturbing lower layers.

Step 7 – Vegetation

• Water media before planting; replace damaged plants.
• Install per layout and manufacturer; saturate after install.
• Follow specified density/pattern for coverage.

Step 8 – Initial watering and maintenance

• Test irrigation for coverage and pressure.
• Begin watering and maintenance immediately after planting.
• Initial inspection within ~4 weeks; then routine care: watering/fertilization, weeding/debris, drain inspection and cleaning.

Hydrologic performance is supported by peer-reviewed work. A key finding for detention-style green roofs (including Purple Roof) is that the SWMM Bioretention LID control is often the more appropriate representation than alternatives, because it models drainage through a single restricted outlet (orifice-like behavior) consistent with how the system actually releases water.

• Insights into green roof modeling using SWMM LID controls for detention-based designs — Journal of Water Management Modeling
• Two Green Roof Detention Models Applied in Two Green Roof Systems — ASCE Journal of Hydrologic Engineering

For product context, case studies, and the wider Sempergreen offering, visit Sempergreen USA. For the Purple Roof concept and dedicated resources, visit the Purple Roof site.

• Sempergreen USA — US green roof systems, projects, and company information.
• Purple Roof — Purple Roof concept and technical marketing resources.

The TR-20 design tool is a browser-based single-system hydrologic model for a Sempergreen Purple Roof assembly. It routes rainfall through layered storage (soil, retention wool, honeycomb detention), orifices, and optional paver and contributing areas using SCS TR-20 with a Type II hyetograph.

• Storm data: Pick a site on the map to pull NOAA Atlas 14 depth–duration data (via the project’s Atlas proxy); choose return period and storm depth, and set simulation length (24–72 h).
• Outputs: Interactive hydrograph, peak flows and depths, and a printable PDF summary with a shareable URL. You can export a HydroCAD-compatible .hcp snippet for the modeled system.
• Best for: Engineers sizing or checking one Purple Roof “train” and communicating results to reviewers—not a full multi-basin site model.

Units: USA or metric display for inputs and reporting; the exported .hcp uses English units as required by that workflow.

The Site planner uses the same TR-20 engine at the parcel scale. You assign areas and curve numbers to multiple surface types (hardscape, landscape, untreated roof, treated roof), then view a combined site hydrograph instead of a single subcatchment only.

• Purple Roof option: When enabled, the treated-roof leg can route through honeycomb detention so you can compare peaks and timing with a conventional green-roof assumption.
• Storm data: Same map-based Atlas 14 fetch and design-storm controls as the TR-20 design tool.
• Best for: Landscape architects and engineers exploring how surface mix and an optional Purple Roof change site runoff relative to simple CN-only thinking—still a lumped TR-20 representation, not a full 2D drainage network.

Shareable links encode your scenario so colleagues can reopen the same assumptions.

The SWMM dashboard runs continuous simulations with EPA SWMM (in the browser via WebAssembly) driven by hourly precipitation and air temperature from Open-Meteo ERA5-Land at a point you select on the map or enter as coordinates.

• Climate: Choose a calendar year or custom date range; load the hourly series in the browser (no local server required for climate preview).
• Simulation: Run SWMM applies that rain gage to the bundled continuous model and returns long-period hydrographs, LID comparison metrics, and summary tables.
• Exports: Download rainfall and temperature .dat snippets and a full .csv for offline use after you have loaded a dataset.

Deployment note: Full Run SWMM requires the Node server (node server.mjs) or an equivalent host running the project API. Static hosting alone supports the UI and climate loading, not the WASM run endpoint.

Chart “metric” mode rescales displayed flows and depths for review; the underlying SWMM input stays in native project units.

No. All graphs, numbers, and downloads from these tools are informational and preliminary. They are provided “as-is” to support early design conversation and peer review—not stamped engineering, not a permit package, and not a substitute for jurisdiction-specific codes, standards, or construction documents.

• For engineers: You remain responsible for model assumptions, boundary conditions, and independent verification before relying on results for bidding, compliance, or construction.
• For plan reviewers: Treat outputs as applicant-supplied supporting material. Confirm inputs, storm sources, CN and area assignments, and whether the simplifications (lumped TR-20, continuous SWMM template) match your agency’s expectations.
• For landscape architects: Use the tools to explore scenarios and graphics with the project engineer of record; planting and drainage details still require coordinated civil design.

Professional judgment: Consult a licensed professional engineer responsible for your project before using any result as a basis for design, permitting, or contractual performance.