RoofHarvest

Intended users

Urban farming startups
City planners & sustainability consultants
Architects & green building engineers
Nonprofits promoting food and climate justice
Developers pursuing LEED / Living Building Challenge credits

Need you’re trying to provide a solution or support for

In the face of pollution and climate change, LEED implementation in the AEC industry is increasingly prominent to promote a sustainable built environment. One way to achieve LEED points is through the use of green rooftops, as urban rooftops are a largely underutilized resource. As cities face increasing pressure to address food insecurity, climate resilience, and sustainable infrastructure, rooftops present a powerful opportunity. However, designing multifunctional green roofs is complex and time-consuming. Designers must juggle various priorities, including maximizing food yield while capturing solar energy (for building usage) and reducing runoff by retaining stormwater, all while not exceeding roof load limits. This tool enables designers to explore rooftop layouts and understand the trade-offs between yield, energy, water, and structural considerations, facilitating a holistic approach that addresses all design objectives.

Inputs

Variable Input	Description	Range	Purpose
Length_A	Length of main (longer) wing of L-shaped roof	40-60m	Adjusts footprint and area
Width_A	Width of main wing	40-60m	Adjusts footprint and area
Length_B	Length of offset wing	5-30m	Adjusts footprint and area
Width_B	Width of offset wing	5-30m	Adjusts footprint and area
PV allocation (%)	% of roof area dedicated to PV panels	0-50%	Balances food vs. energy production
Irrigation type	Efficiency of water delivery	Drip (90%), Spray (70%), Manual (60%)	Affects water consumption
Sun exposure (%)	% of area with full sunlight	60-100%	Impacts yield & PV energy
Path spacing (%)	% of roof area reserved for access for garden and panel maintenance	5-25%	Ensures accessibility for maintenance

Constant Input	Value	Description
Location	San Francisco, CA	Sets rainfall & solar radiation
Rainfall intensity	25 mm/hr	For stormwater retention calculations
Structural weight limit	150 kg/m²	Max roof load to avoid overloading
Crop type	Leafy greens (lettuce, kale)	Standardized yield/water data
Yield rate	2.5 kg/m²/month	Based on typical leafy green crops
Water usage rate	5 L/m²/day	For irrigation demand
Soil & water load	70 kg/m²	For live load calculation
Solar radiation	4.5 kWh/m²/day	San Francisco average
PV efficiency	17%	For solar energy output

Underlying logic of the model you’ll implement

1. Geometry Calculation (Rooftop Shape)

Form L-shape by:
Creating Rectangles A and B from Length and Width inputs
Polycurve the points together
Constraint: Area ≤ 1800 m²

2. Solar Exposure Analysis

Run Revit Solar Analysis for roof surface to map sun values to grid points (UV coordinates)
Threshold sun values to allocate zones for each category

High sun (> 4.0) → PV
Medium sun (2.0 - 4.0) → Crops
Low sun (< 2.0) → Access paths

Potentially apply smoothing or clustering (e.g., region growing) to create usable geometry areas

3. Crop Area Metrics

Crop Area = Sum of grid cells tagged as “Crop Zone”
Crop Yield = Crop Area × 2.5 kg/m²/month
Irrigation Demand = (Crop Area × 5 L/m²/day)/(Irrigation Efficiency)

Where irrigation efficiency depends on the type of irrigation

Crop Load = Crop Area × 70 kg/m²

To calculate live structural load for safety verification

4. PV Panel Metrics

PV Energy = PV Area × 4.5 kWh/m²/day × 365 × 0.17
PV Load= PV Area × 15 kg/m²

Assumes standard rooftop solar panels (with racking and ballast)
To calculate live structural load for safety verification

5. Stormwater Retention Metric

Stormwater Retained = Crop Area × Rainfall Depth × Retention Coefficient × 1000

Rainfall depth (San Francisco): 0.025 m
Retention coefficient (for intensive green roof): 0.8

6. Structural Load Check

Avg Load (kg/m²) = (Crop Load + PV Load) / Usable Area
Check Avg Load ≤ 150 kg/m² ⇒ Pass/Fail

7. Compare outputs for optimization

Choice	Goal	Pros	Cons
Crop	Maximize	High yield, high retention	Heavy load, less energy (PV area)
PV	Maximize	High clean energy	Less food & water retention (less crop area)
Irrigation Demand	Minimize	Saves water	May constrain yield if too aggressive
Stormwater Retention	Maximize	Minimize irrigation needed, improves resilience	Requires more crop area, reduces PV
Total Load	Minimize	Stay within load capacity for safety	Limits productive area

Outputs