Plants can help to moderate the urban summer temperatures and improve air quality by mitigating air pollution and greenhouse gases; and they are a way of promoting biodiversity and sheltering wildlife. Why then not create green roofs on the top of our urban and suburban buildings, where they can also minimize solar heat gains through the roof, and reduce air conditioning bills and make our homes more energy efficient?
That’s the idea behind green roofs…
But, what about the practical side of the idea? Which are the pros and cons of green roofs? Are they advantageous for the common homeowner?
Probably not… Anyway, green roofs can be several things and before weighing the pros and cons, it’s important to understand what's exactly a green roof.
Types of green roofs according to IGRA
The IGRA (International Green Roof Association) considers three types of green roofs for flat and low-slope roofs, each one with its own type of plants, maintenance and costs.
The simplest type of green roof is technically called extensive green roof. It involves a relatively thin soil layer and a mixture of low-growth and drought-tolerant plants that can be found in coastal areas, semi-deserts or dry mountain landscapes, that is, plants like succulents, herbs, mosses, and grasses.
Semi-intensive green roofs are another type of green roof, involving deeper soil layers and shrubs like lavender, and mostly herbaceous perennials and grasses. They do not have trees or even tall bushes.
The third type of green roof considered by IGRA is called intensive green roofs or roof gardens, which besides involving deeper soil layers may include lawns, perennials, bushes and trees.
Modular-Tray green roofs
But besides the three types of green roofs considered by the IGRA, there are also modular-tray green roofs and mini-top-roof gardens.
Modular-tray green roofs are also installed on flat and low-slope roofs, but instead of soil and vegetation layers they use pre-vegetated trays, forming a grid (see images).
Modular-tray green roofs are simpler to install and cheaper. With them, possible water leaks are easy to find and fix.
As to mini-top-roof gardens, they are a sort of modular-tray green roof or extensive green roof limited to a part of a flat-roof.
The soil, the plants, the fabrics, the filters, the drainage system or the possible reinforcement of the roof (to support the weight of green roof) have a significant cost. The different categories of roof greens considered by IGRA can easily cost $12-$24 per square foot, according to the type of system, while modular systems based on plastic trays may cost around $8 per square foot.
Like all other types of green roofs they require drought-tolerant vegetation and other plants capable of withstanding extreme temperatures, drought and high winds, and shouldn't be confused with ordinary gardens.
You shouldn't think of them as nice gardens, attracting birds or butterflies. Ordinary plants will not survive in the harsh environments of urban roofs, and their design should be addressed in a professional manner. And they too require a proper waterproofing system, carefully designed to prevent water leaks.
Pros and Cons
But why green roofs? Why should anyone build such a type of roof, a lot more expensive than ordinary roofs? And is it practical?
In many cases no, but it depends a lot on your commitment, values and climate.
The idea of improving the quality of the urban air and minimizing the heat island effect makes sense, and green roofs can provide benefits for the homeowner: extremely high roof insulation, increased protection of the roof, lower air-conditioning costs...
But you should also consider issues like the weight of the materials used on the rooftop and its impact on the roof beams and building structures, the maintenance and the survival of the plants (high winds that can blow away the plants, extreme temperatures...), water leaks and waterproofing problems (extensive to flat roofs) and drainage and maintenance costs and cares...
Bottom line: are green roofs practical for common homeowners?
Green roofs aren't cheap or easy to implement. They have advantages, but they also involve costs and downsides that should be carefully addressed. Building a green roof can be challenging.
See also: Types of Green Roofs