Ielts 2026 Reading Question 19

Section A Green roofs — deliberately vegetated layers installed on building rooftops — have become a prominent element of contemporary urban planning aimed at mitigating environmental pressures associated with densifying cities. Two principal types are recognised: extensive green roofs, characterised by shallow substrates (typically 3–15 cm), low maintenance and predominance of drought-tolerant, low-growing species such as sedums; and intensive green roofs, with deeper substrate (often more than 15 cm), greater water-holding capacity and the capacity to sustain a wider range of plants including grasses, shrubs and small trees. Extensive systems are lighter and less expensive to install and maintain; intensive systems deliver more diverse habitat and recreational potential but require greater structural support and ongoing care. The modern resurgence of green roofs in Europe and North America dates from late 20th-century environmental awareness and municipal policy experiments, though vegetated rooftops have a long antecedent in earlier vernacular building traditions. Section B Green roofs provide multiple ecosystem services that are measurable and relevant to urban sustainability objectives. By intercepting rainfall and retaining water within substrate layers and vegetation, green roofs reduce peak stormwater runoff, alleviating pressure on combined sewers and water treatment infrastructure. Typical retention rates reported in urban studies range from moderate to substantial depending on substrate depth and climatic conditions; for example, shallow systems often retain a significant portion of small to moderate rainfall events, while deeper systems perform better during prolonged storms. Vegetated roofs also influence building energy demand: by providing thermal mass and evapotranspirative cooling, they can reduce summertime cooling loads and, in some climates, moderate winter heat loss. At neighbourhood scale, arrays of green roofs contribute to mitigation of the urban heat island effect by lowering surface temperatures and modifying local microclimate. Beyond physical services, green roofs function as novel habitats within the built environment. Accumulating empirical evidence indicates that, under appropriate design, green roofs can support assemblages of invertebrates, pollinators and, to a lesser extent, birds. Many studies observe greater species richness on green roofs compared with bare or conventional roofs, though community composition tends to differ from ground-level habitats. Roofs with greater structural complexity and diverse native plantings often host more taxa. However, lawns of non-native or monocultural plantings support fewer species and may primarily favour generalist species. Section C Effective design to maximise ecological benefits requires attention to substrate composition and depth, plant species selection, and connectivity with surrounding greenspace. Substrate depth is a primary determinant of water availability, thermal buffering and root-zone volume; deeper substrates allow a wider range of species and increase habitat heterogeneity. Plant choice should prioritise native species adapted to local climatic extremes, particularly those providing floral resources for pollinators across the growing season. Structural complexity—variation in plant height, presence of woody elements where feasible, and inclusion of microhabitats such as bare patches or stones—promotes niche diversity and increases the likelihood of colonisation by specialist species. Connectivity influences the colonisation and long-term viability of rooftop communities. Green roofs that are isolated by long distances of inhospitable urban matrix exhibit limited immigration rates, particularly for less mobile invertebrates and birds. Conversely, roofs close to parks, street trees or other green roofs can act as stepping stones, facilitating dispersal and gene flow. Empirical research suggests that proximity to source populations correlates with higher species richness and occupancy rates on roofs. Despite their potential, green roofs are not ecological equivalents of natural habitats. Constraints such as shallow substrates, exposure to wind and temperature extremes, and limited resources mean that rooftop communities often have simplified trophic structures and may rely on continual immigration from ground-level habitats. Long-term monitoring remains relatively scarce; the limited longitudinal studies available highlight that successional trajectories on roofs can be protracted and influenced heavily by maintenance practices. Section D Policy instruments and building regulations have been important in scaling up green roof implementation. Municipal incentives, mandatory requirements for large developments, and grants for retrofitting have driven adoption in several cities. Yet several implementation challenges persist. Initial installation costs and the need for structural reinforcement deter uptake, especially in retrofit contexts. Maintenance regimes are essential for ecological functionality; without periodic management, invasive plants and debris accumulation can degrade performance. Standardisation of design and monitoring protocols is also lacking, complicating comparisons across projects and synthesis of evidence for best practice. Future directions emphasise multifunctional roofscapes that integrate biodiversity objectives with water management, renewable energy (for example, photovoltaic panels combined with vegetation), and social amenity. Research priorities include improving understanding of species-specific responses to rooftop conditions, optimising substrate blends for both hydrological and ecological goals, and developing decision-support tools to guide planners in balancing costs and ecological outcomes. With careful design and policy support, green roofs can be a valuable component of urban nature networks, contributing to resilient and liveable cities.