Residential Rooftop Urban Agriculture: Architectural Design Recommendations
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The rooftop space in a dense urban environment can account for much of the city’s surface area. The rooftop’s potential is usually underutilized and unrealized [74,81,94]. Urban areas with limited open spaces, green spaces, and high land values and densities can use rooftops for intensive farming [87,95,96,97]. Usually, apartment building rooftops are vacant and unutilized spaces that can be utilized as farms for cultivating food [84,85,97,98,99]. Applying innovative materials such as efficient waterproofing membranes and lightweight growing media, along with innovative cultivation methods such as soilless systems and efficient irrigation systems, facilitates food cultivation on rooftops [95]. Vegetables, herbs, small fruit trees, and berry bushes can be cultivated on rooftops in dense urban areas [85,91,96,100]. Rooftops usually have the essential physical and climatic characteristics to support plants [94]. Rooftops usually receive sufficient sunlight [98,101]. Rooftops are free of pests such as snails and slugs [74,101]. Rooftop farms are safe from vandalism in community gardens [74]. Various fruits and vegetables can be cultivated on rooftops [102]. Leafy vegetables are the principal fresh vegetables cultivated in urban agriculture, comprising rooftop farms [26,79,82,103].
The following factors are vital and should be considered before setting up rooftop farms: (1) Climate: The globe’s climatic zones can be categorized as tropical, arid, temperate, cold, and polar. Open-air farming is possible in temperate and tropical zones but limited in arid, cold, and polar zones [76,104]. By utilizing innovative cultivation methods, food can be grown in controlled environments such as greenhouses, modular production units, or indoor environments in arid, cold, and polar climates [104]. (2) Sunlight: The amount and duration of sunlight the rooftop receives are crucial factors. Most vegetables, such as tomatoes, peppers, eggplants, cucumbers, squash, peas, beans, beets, lettuce, kale, Swiss chard, carrots, and such, require a minimum of six to eight hours of direct sunlight per day [26,93,98,100,105,106,107,108,109,110,111,112,113,114]. (3) Wind: In dense urban areas, rooftop container gardens, raised beds, and green roofs under an open sky are exposed to the elements. Rooftops can be exposed to constant wind, which is usually stronger than that at ground level. Wind can damage plants by tearing off their leaves and breaking their branches. Furthermore, blossoms might be torn off before they can be pollinated. Wind can result in soil dryness and the dehydration of plants. Wind can be filtered or deflected by constructing bamboo, reed screens, trellises, walls, or tall parapets [26,79,81,92,95,98,100,101,105,107,110]. (4) Shade: rooftops near taller buildings are not ideal places for setting up rooftop farms due to neighboring buildings’ cast shade [81,92,115]. (5) Water source: Proximity to a water source, such as an outdoor faucet on the rooftop, is essential [87,95,98,100,106,116]. Besides tap water, there are other irrigation sources like well water, recycled rainwater, or graywater [76,92]. To effectively water the plants, irrigation systems should be installed on rooftops [87,95,98,100,106,116]. (6) Electricity: electric sockets should be installed on the rooftop [98]. In addition to on-grid electricity, solar panels can be installed on rooftops [76]. (7) Extra load: The rooftop garden’s infrastructure must be considered an extra load placed upon the roof structure. The roof structure should be able to sustain the additional load. When saturated, the growing media installed on a rooftop could weigh between 960 and 1600 kg/m2. The farming infrastructure’s load can be reduced using lightweight potting soil, containers such as plastic or fiberglass, and hydroponic systems [26,84,98,100]. (8) Access to the rooftop: Access via an elevator or staircase is essential. Any rooftop garden must have easy access via a staircase or elevator to carry equipment and maintain the garden [26,74,84,87,95,98]. A staircase or elevator might be shared with the residents, or due to privacy issues, an external staircase might be constructed [27,92]. (9) Safety: a rooftop garden should have a parapet around the edge [92,95]. (10) Limited rooftop space: Rooftops usually house vital mechanical systems and equipment, such as chiller plants, water tanks, solar water heaters, lift motor rooms, TV antennae, and pipes. Therefore, there might be insufficient room for cultivating food on rooftops [115]. (11) Building codes and legislation: The rooftop design should comply with building codes. Residential rooftop farms operating in cities might be required to obtain licenses and permits from local municipalities and undergo regular inspections. Ensuring permission for the rooftop farm might be an issue [65,81,84,87,92]. (12) Proper insulation: the roof should be properly insulated and waterproofed to avoid leaks in the roof [102]. (13) Logistics: transferring soil and other equipment might require a crane, which can be challenging [26,79]. (14) Installation and maintenance cost: Transferring the entire cultivation materials and infrastructure to the rooftop should be considered. Conventional cultivation methods require less installation cost than high-tech greenhouses [76]. (15) Social parameters: if the purpose of the rooftop farm is to host visitors, then sufficient space should be facilitated to gather a small to medium number of visitors. (16) Finishing surface: The rooftop finishing surface should be covered with tiles or a wooden deck that functions as a walkable platform. Rooftop surfaces in regions that receive extreme rain and snow should be covered with anti-slip and frost-resistant tiles. (17) Walking paths: paths free of any barriers should be designed on the rooftop. (18) Lightning protection system: a lightning protection system might need to be installed on the rooftop. (19) Grouped mechanical systems: to clear the rooftop surface for farming, exhaust vents, mechanical systems, and equipment should be grouped together and installed in one corner of the roof. (20) Managing rainwater runoff: to manage rainwater runoff effectively, the slope of the rooftop should be considered. (21) Privacy: Farming activities such as the passage of farmers, equipment, technicians, inspectors, and visitors might conflict with the residents’ privacy. Residents might object to such activities [92] (Figure 1).
Food can be cultivated on rooftops via the following three methods: (1) open-air rooftop food production, (2) “low-tech” rooftop greenhouses, and (3) “high-tech” rooftop greenhouses. Open-air rooftop food production usually utilizes conventional soil-based cultivation methods such as rooftop container gardens, rooftop raised beds, green roofs, and vertical rooftop gardens. In this method, both horizontal and vertical surfaces on rooftops are utilized for cultivating food. Soilless systems, such as hydroponics, can also be installed on open-air rooftops (Figure 2). “Low-tech” rooftop greenhouses utilize simple cultivation methods in greenhouses without utilizing mechanized systems for controlling the indoor climate (Figure 3). “High-tech” rooftop greenhouses utilize soilless systems such as hydroponic systems in controlled environments by utilizing mechanized systems for heating, cooling, shading, and lighting (Figure 4). Open-air rooftop food cultivation, such as container gardens, raised beds, green roofs, and vertical rooftop gardens, is possible in temperate and tropical regions. Low-tech greenhouses are popular in temperate climates such as the Mediterranean region, while high-tech greenhouses can be applied to arid, cold, and polar zones [104].
3.2. “Low-Tech” Rooftop Greenhouses
Cultivating food in greenhouses on rooftops is another approach to urban agriculture. Greenhouses are often installed on rooftops in dense urban areas due to a lack of land and land costs. Cultivating greenhouses requires exposure to natural light, which is difficult to get at ground level, which makes rooftops an ideal location for installing greenhouses [26,79]. Rooftop greenhouses can be implemented on buildings such as residential blocks within dense urban centers [50,166]. Rooftop greenhouses consist of a greenhouse constructed on the roof of a building [25]. Vegetables, fruits, and aromatic plants are usually cultivated in greenhouses either by following conventional farming techniques or in soilless culture systems like hydroponic systems [166,167].
A greenhouse is a structure that houses crops and shelters them from the elements [168,169]. It is built from a frame covered by transparent glazing. It uses sunlight to create a favorable environment for crops to grow. It is possible to extend the growing seasons in greenhouses [168]. “Low-tech” rooftop greenhouses are conventional greenhouses that do not require climate control systems. Various crops can be cultivated in such unconditioned greenhouses all year around in moderate climates [33,170]. Rooftop greenhouse structures can be categorized into the following: (1) cold frame, (2) attached greenhouse, and (3) A-frame greenhouse. (1) Cold frame: this is used in spring and fall to extend the growing seasons (Figure 13). (2) Attached greenhouse: This is a lean-to structure attached to a south-facing wall. Attached greenhouses are usually made with glass windows. The construction cost is usually lower than for freestanding greenhouses [168]. (3) A-frame greenhouse: A detached structure that stands apart on the rooftop. It is usually made with glass windows [100,168,171] (Figure 14).
Greenhouse structures can be made from galvanized steel, aluminum, plastic, wood, and PVC [35,168,171]. Urban rooftop greenhouses are often exposed to strong winds; as a result, they should be constructed as stable structures with strong cover materials that can withstand wind forces [33,88,168,171]. A greenhouse structure should be sheltered from the wind by setting up barriers. Exposure to wind can contribute to greenhouse heat loss [171,172]. Galvanized steel is an ideal structural choice. It can withstand strong winds and snow loads [168]. Glazing is an essential element of any greenhouse. It facilitates the entry of sunlight into the greenhouse. It is attached to the frame and is the most expensive component of the greenhouse [173]. It can be made from glass panels, plastic sheeting, polyethylene films, acrylic sheets, translucent fiberglass, polyvinyl chloride (PVC), copolymers, and polycarbonate panels or rolls [7,168,171,172,173].
The following points should be considered when designing a rooftop greenhouse: (1) Orientation to the sun: the east-to-west orientation is preferable to the north-to-south orientation. (2) Climate: low-tech rooftop greenhouses can be installed in temperate and tropical regions. (3) Shadow: neighboring buildings or mechanical systems might cast shadows on the greenhouse. (4) Building codes: The rooftop greenhouse structure and cover should comply with the local building codes. Selected materials should comply with fire safety laws (avoid inflammable materials) and load-bearing laws (snow and wind loads). Fire safety laws and load-bearing laws should be considered when designing open-air rooftop food production, “low-tech” rooftop greenhouses, or “high-tech” rooftop greenhouses. (5) Hosting building equipment: due to a lack of available space on the rooftop, the building’s mechanical system might be installed inside the greenhouse. (6) Light condition inside the greenhouse: The selected greenhouse cover should allow maximum natural light transmission. Attention to proper lighting conditions inside the greenhouse is essential since crops’ growth depends on it. (7) Natural ventilation: installing side and roof vents is crucial for facilitating indoor air movement [169].
3.3. “High-Tech” Rooftop Greenhouses
As the globe experiences rapid urbanization, cultivating food in “high-tech” rooftop greenhouses can be regarded as an alternative secondary source to conventional soil-based farming [29]. They can address the growing concerns regarding urbanization and food security [50,104,174]. In dense urban areas where vacant land suitable for cultivation is limited, utilizing innovative high-tech technologies that require minimum cultivation space offers tremendous opportunities for space-confined cultivation [25]. Situating farming systems such as “high-tech” greenhouses in line with soilless cultivation systems on and in buildings can be considered building-integrated agriculture (BIA) [30,35,175].
“High-tech” greenhouses can supply a significant quantity of fruits, vegetables, herbs, and medicinal plants [50]. “High-tech” rooftop greenhouses can be considered a form of controlled-environment agriculture (CEA) in cities. CEA focuses on applying innovative, high-tech cultivation methods in controlled environments in and on buildings. In “high-tech” rooftop greenhouses as controlled environments, high-performance soilless cultivation methods such as hydroponic systems, grow lights, and climate control systems that are operated by computers are installed. In this method, high-quality vegetables and fruits can be cultivated on a large scale all year round. Crop yields can be increased, while production costs can be reduced [7,33,76]. Controlled-environment agriculture aims to maintain the growing conditions to optimize crop cultivation [36,176] (Figure 15). If the purpose of the greenhouse is to produce vegetables, fruits, and herbs all year round, then it should be equipped with the following systems: (1) heating, (2) cooling (ventilation), (3) shading devices, and (4) lighting devices.
(1) Heating: the purpose of heating is to stabilize the greenhouse temperature. Propane heaters, electric fan heaters, gas or oil heaters, solar heaters, and radiant heat lamps are heating options to warm the greenhouse [168,173]. (2) Cooling (ventilation): To avoid overheating, a proper ventilation system is necessary. Airflow can be provided through doors, side vents, and roof vents. Vents can be either hand-operated or automatically operated by connecting them to a thermostat. In “low-tech” greenhouses, vents are usually operated manually, while in “high-tech” greenhouses, vents are operated automatically. Exhaust fans are usually installed on greenhouse roofs to draw the hot air outside. Exhaust fans are more effective than vents since they provide more indoor airflow. Portable oscillating fans can also facilitate internal air movement. Fogging and pad-and-fan systems can effectively lower the air temperature in greenhouses [7,26,104,168,171,172,173]. (3) Shading devices: Providing shade is essential to decrease the solar radiation load reaching the plants. It protects the plants from overheating and burning during the summer months. Shade can be provided by installing external blinds, internal blinds, and a shade cloth. Shading devices can operate manually or automatically [168,171]. (4) Lighting devices: As mentioned before, plants require a minimum of six to eight hours of sunlight daily. Supplementary lighting might be required during autumn, winter, or extended periods of sunless days [9,168,171]. In high-tech greenhouses, incandescent/halogen lamps, fluorescent light tubes, metal halides, high-pressure sodium lamps, and light-emitting diodes (LED) are highly utilized [7]. Light-emitting diode (LED) grow lights can be installed in greenhouses with a life expectancy of eight to ten years. LED grow lights are specifically manufactured for cultivating crops in controlled indoor environments to enhance crop yields and reduce production costs. The advantages of LED grow lights are their compact design, light quality, low thermal energy generation, low energy cost, and durability [7,30,76,168,171]. Automatic systems are installed to control the greenhouse microclimate via controlling heating, cooling, shading, and lighting devices [9].
3.3.1. Soilless Cultivation Methods in “High-Tech” Rooftop Greenhouses
Farming without soil can be considered an innovative method for cultivating food. It includes hydroponics, aeroponics, and aquaponics [177]. Various fruits, vegetables, herbs, pharmaceutical plants, sprouts, and microgreens can grow in hydroponic systems in rooftop greenhouses [25,85,177,178,179]. By utilizing hydroponic systems, the crop yield is maximized compared to conventional soil-based methods [82,85].
Hydroponic systems equipped with grow lights can be installed in regions that do not receive sufficient sunlight [177]. In this system, crops can be cultivated in deserts and infertile lands, such as mountainous regions [179,180,181]. Hydroponics is a suitable cultivation method in congested urban centers where there is a shortage of vacant land for cultivating food [177,181,182,183,184].
Plants in hydroponic systems are grown without soil. The plant’s roots are submerged in a nutrient-rich solution [177,178,179,180,181,183,184,185,186,187]. The amount of nutrient solution is calculated; therefore, the amount necessary for the plant’s growth is delivered to the roots [181,183,186]. Plant roots can be supported and maintained in hydroponic systems by peat, moss, perlite, rock wool, vermiculite, sand, gravel, or soil pellets. Hydroponic systems can be categorized into the following types: (1) the wick system; (2) the drip system; (3) the ebb and flow or flood and drain system; (4) the nutrient film technique (NFT); (5) deep flow technique (DFT) pipe system; (6) the floating raft system; and (7) the column system [177,178,180,184,185]. The mentioned systems equipped with grow lights can be assembled on rooftops in greenhouses to cultivate crops intensely. Drip systems, the nutrient film technique (NFT), deep flow technique (DFT) pipe systems, and column systems are well-known hydroponic systems used on residential rooftops [85].
In a drip system, water and nutrient solutions can be delivered to plants using little hoses and drip emitters [177,180,185,188]. This system pumps water and nutrient solutions directly from the tank to each plant’s root area in the right proportion [177,184,188,189]. Typically, plants are cultivated in a growing medium so the nutrient solution and water drip down gradually [177,180]. Water and nutrient solutions can be recycled or drained in this system [180,185].
In the nutrient film technique (NFT) system, water and the nutrient solution are continually pumped from the reservoir to the channels known as gullies [177,180,184,185,186,188]. Plants are arranged in the net cups and placed in channels, and their roots are suspended in passing water and nutrient solutions [177,178,180,183,186,188]. The roots of the plants must be constantly kept moist. The plant roots are always in direct contact with water and nutrient solutions; therefore, the roots are more susceptible to fungal infection. In this system, water and nutrient solutions can be recycled several times. The NFT system is totally dependent on a water pump to deliver water and nutrient solutions to the plant’s roots [177,180,183].
In a deep flow technique (DFT) pipe system, water, and nutrient solutions are pumped through PVC pipes. Plastic net pots are put in the holes drilled in the PVC pipes. All the plants are put into plastic net pots. The plant’s roots touch the shallow nutrient solution and water stream in the PVC pipes. This system’s pipes can be organized horizontally or vertically on several levels. Water and nutrient solutions are transferred from one channel to the next via drip lines. This system is suitable for cultivating low-growing crops in multiple zig-zag vertical plains in interior spaces [178,180].
In the column system, crops are cultivated in light growing media supported by the column. The column should be able to support the weight of the pots, growing media, plants, and irrigation system. Using a column system makes intensive crop cultivation per unit area possible. One column can be installed in each square meter. On a rooftop of 100 m2, it is possible to install 100 columns approximately. The height of each column is 1.7 m, and each column hosts 32 plants [85].
The advantages of cultivating plants in hydroponic systems in comparison to conventional soil-based agriculture are as follows: (1) No soil is required [180,185]. (2) Compared to conventional farming, plants grow more quickly and have smaller roots because the nutrients are sent straight to the roots [177,179,185]. Intense cultivation in small areas in any location is possible since plants can be grown closer to each other [177,179,180,185]. (3) Hydroponic systems enable increased productivity per acre, high-density cultivation, higher quality harvests, and effective nutrient, water, and aeration controls [76,85,177,178,179,180,181,186]. Hydroponically produced plants are fed a balanced diet, making them healthier than their soil-grown counterparts [183,186]. (4) Hydroponic crops can be grown all year and are called off-season because weather changes do not affect them. Harvesting locally multiple times per year without utilizing the existing arable land is possible [85,177,181,183,185,186,187]. (5) Labor can be minimized in hydroponic systems [85,118,177,178,181,183,184,186]. Various conventional soil-based agriculture procedures such as weeding, spraying, watering, and plowing are eliminated [177,181]. (6) Water consumption is reduced in hydroponic systems. Water and nutrient solutions can be partially or entirely recycled [35,85,118,177,178,180,181,183,185,186,187]. (7) Compared to traditional farming, larger yields can be achieved since more plants per unit can be cultivated [26,85,92,177,183,184].
The principal disadvantages of soilless farming can be categorized as follows: (1) Hydroponic systems require technical knowledge and operational skill [177,179,181,183,184,187]. Constant monitoring of the system is necessary. (2) The high upfront cost of purchasing and assembling the system might burden the user [85,177,179,183,184,187]. (3) In hydroponic systems, water-borne diseases can spread from one crop to the next since the plants share identical nutrients [177,181,183,184,187]. (4) Oxygen shortages can hinder production, which might cause crop failure [177]. (5) The EC, pH, and optimal nutrient solution concentration level should be constantly monitored [177,180]. (6) To keep the system operating, lighting and energy supply are essential [177,179,181,187,188]. As an example, the amount of energy consumed to cultivate one kilogram of lettuce by utilizing hydroponic systems in a greenhouse in Arizona, USA, is approximately 82 times more than cultivating one kilogram of lettuce using traditional methods of cultivation in the same region [190].
3.3.2. “High-Tech” Rooftop Greenhouses: Benefits and Challenges
The benefits of installing “high-tech” greenhouses on rooftops can be considered as follows: (1) In comparison with conventional soil-based agriculture, higher yields are possible [26]. “High-tech” greenhouses facilitate the cultivation of high-quality nutritious food throughout the year within urban areas [26,29,36,90,104,191]. (2) Rooftop farming promotes local food production [7,25,30,166,167]. (3) “High-tech” greenhouses can expand growing food in climates and locations unsuitable for cultivation [26,29,104,192]. (4) They can assist city dwellers to reconnect with food cultivation in urban centers [25]. (5) They improve the roof’s insulation; therefore, the energy requirement for cooling and heating the building is reduced. They can optimize the energy efficiency of the building [7,25,166]. (6) The internal environment of rooftop greenhouses is precisely controlled; therefore, the consumption of urban resources such as water and electricity is minimized [29,36]. (7) It is projected that future rooftop greenhouses will link their heat, water, waste, and CO2 to the metabolism of buildings to optimize resource use [7,25,166]. (8) Graywater and rainwater can be recycled for irrigation [7,25]. (9) They provide precise lighting conditions, humidity, and temperature that lead to high yields [193]. (10) Compared to conventional farming, less water for irrigation is needed [181,183,187]. (11) Food can be cultivated in the minimum available space [29,36,194]. (12) Rooftops, as unused or underutilized spaces, can be converted into productive grounds [26].
The principal challenges of installing “high-tech” greenhouses on rooftops can be summarized as follows: (1) The initial cost of establishing, equipping, and implementing a “high-tech” greenhouse is usually higher than conventional “low-tech” greenhouses. (2) The energy consumption is higher than conventional open-field agriculture. As an example, in order to cultivate one kilogram of tomato in a “high-tech” greenhouse situated in Washington State (WA), approximately 231 times more energy is required than cultivating the same amount of tomato in open-field farming in the same state [195]. (3) The variety and quantity of vegetables and fruits that can be cultivated are lower than in conventional open-air farming. (4) Labor that is specialized to maintain a “high-tech” greenhouse has to be trained. (5) The high labor cost, especially in the developed world, can be challenging. (6) Local zoning laws can be a principal challenge, preventing the installation of greenhouses on rooftops [7,21,25,26,30,76,92,196,197].
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