Site location & sewershed
Set this first — CSS vs MS4 changes WQv credits, rainfall depth, and site requirements across every section.
Click the map or search an address to detect CSS/MS4. You can also toggle manually.
Purple Roof Designer
Orifice detention calculator and WQv / RRv / Vv volume credits per NYC DEP Table 4.1.
Detention & Orifice
Inputs
Regulatory & physical results
Compliance by drainage area
Purple roof tributary · 0 sf
| Required | Purple provides | Still need | |
|---|---|---|---|
| WQv | 0 cf | 0 cf | — |
| Vv | 0 cf | 0 cf | — |
| RRv | 0 cf | 0 cf | — |
DEP requires each practice to manage its tributary area. Vv uses Table 2.7 RD (—).
Volume Credits (WQv · RRv · Vv)
Purple Roof layers
| Layer | Depth (in) | Porosity |
|---|---|---|
| Soil (ET) | ||
| Mineral wool | ||
| Honeycomb | ||
| Drainage (fixed) | 0.2 | 0.9 |
Purple Pavers layers
| Layer | Depth (in) | Porosity |
|---|---|---|
| Mineral wool | ||
| Honeycomb | ||
| Drainage (fixed) | 0.2 | 0.9 |
Combined summary (cf)
| Credit | Purple Roof | Pavers | Combined |
|---|---|---|---|
| WQv | 0 | 0 | 0 |
| RRv | 0 | 0 | 0 |
| Vv | 0 | 0 | 0 |
Combined roof area: 5,000 sf → feeds site planner
ET (soil) credits WQv + RRv always. Detention credits Vv always; WQv on CSS only (Table 4.1).
Site Planner
Per drainage area DEP compliance — purple roof tributary plus required ground SMP on remaining area.
Site inputs
Also set via map on the Purple designer tab.
Contributing drainage area from Purple designer
Total site minus purple controlled area
Purple roof + pavers — set on Purple designer tab
NYC Stormwater Manual Table 2.8 — runoff coefficient for remaining ground area.
0 = no net new paving
Advanced — connection type
Affects RD used for site Vv requirement (Table 2.7).
Site compliance & indicative ground sizing
Calculating…
Ground / remaining · 0 sf
| Required | Ground SMP | Still need | |
|---|---|---|---|
| WQv | 0 cf | 0 cf | — |
| Vv | 0 cf | 0 cf | — |
Target volumes for tank/bioretention below: WQv 0 cf · Vv (RD) 0 cf. Plus QDev ≤ QDRR on this area.
Ground SMP sizing (required when ground area > 0)
Size tank and/or bioretention to meet the ground drainage area “still need” volumes and QDRR. For planning — confirm with engineer of record.
Adjust design parameters
Adjust design parameters
Ground SMP comparison — value of Purple Roof
Indicative ground detention and bioretention for three roof scenarios — same tank depth and bio settings as above.
| No roof SMP | Traditional green 4″ ET soil · C=0.70 |
Purple Roof current design |
|
|---|---|---|---|
| WQv to treat | 0 cf | 0 cf | 0 cf |
| Vv to treat | 0 cf | 0 cf | 0 cf |
| Detention tank | 0 cf | 0 cf | 0 cf |
| Tank footprint | 0 sf | 0 sf | 0 sf |
| Bioretention footprint | 0 sf | 0 sf | 0 sf |
Technical Background
Orifice flow derivation and equivalent Qi rationale (Excel Sheet 3).
Section 1 — Origin of the 0.35 equation constant
1a. Empirical derivation of Cd = 0.07
The coefficient of discharge (Cd) for the Purple Roof orifice control box was determined through direct stage-discharge measurements in a controlled laboratory setting. Rainfall was simulated on a 20 ft × 4 ft Purple Roof cassette equipped with depth sensors throughout its 2-inch honeycomb storage layer on a 0.2″ low-transmissivity detention drainage layer. Fitting observed discharge to the standard orifice equation yielded Cd = 0.07, calibrated for peak-flow conditions at ~2 inches of storage.
Laboratory methodology video ↗
1b. Step-by-step derivation
| Step | Expression | Note |
|---|---|---|
| Standard orifice | Q(cfs) = Cd × A × √(2g × h) | Foundation equation |
| Empirical Cd | Cd = 0.07 | Lab stage-discharge regression |
| Gravity term | √(2g) = √(64.4) ≈ 8.025 | g = 32.2 ft/s² |
| Orifice height | Fixed 0.2 in gate | Centroid at 0.1 in = 0.00833 ft |
| Head | h = (dmax/12) − 0.00833 | Head in feet |
| Orifice area | A = (w/12) × (0.2/12) = w/720 sf | w in inches |
| Substitute | Q(cfs) = 0.07 × (w/720) × 8.025 × √h | |
| Convert to gpm | × 448.83 | DEP workbook uses gpm |
| Consolidate | (0.07 × 8.025 × 448.83) / 720 ≈ 0.3501 | Rounds to 0.35 |
| Final | Q(gpm) = 0.35 × w × √((dmax/12) − 0.00833) | Used in compliance calculator |
Section 2 — Equivalent Qi (DEP workbook compliance)
2a. NYC DEP assumes linear drain behavior
The DEP Chapter 4 QROOF formula is linear: it uses Qi (gpm/in), number of drains, and storage depth, then divides by 449 for cfs. Conventional roof drains are approximately proportional to depth; the Purple Roof orifice follows non-linear √(head) physics.
2b. Equivalent Qi at peak design depth
Define maximum ponding depth dmax as a fixed design parameter. Calculate exact flow at that depth with the orifice equation (QGPM per drain). Dividing QGPM by dmax yields an equivalent Qi (gpm/in) that reproduces the same total system flow when entered into the DEP workbook. This is exact at the governing design point—not an approximation.