Green roofs, or living roofs as they are also known, can be classified according to type of usage, construction factors and maintenance requirements, into three different types; extensive, semi-intensive and intensive green roofs.
Extensive roofs are thin lightweight roof systems that typically have a planting media measuring 6 to 20 cm deep, while heavier, thicker green roofs are known as intensive roofs and their growing media ranges in thickness from 20 cm to 100 cm. Semi intensive green roofs, as their name states, are a combination of the aforementioned two types. They have a thinner planting media than an intensive roof but thicker than an extensive one, and the whole system ranges from 120 to 250 mm. An extensive roof weights about 60 to 150 kg/m2, a semi intensive from 120 to 200 kg/m2 and the heavier amongst the three, intensive roof, weights more than 180 kg/m2 and up to 500 kg/m2. Apparently, intensive roofs are the most expensive between the three systems, while the extensive roofs have the lowest cost.
Intensive green roofs can support the growth of a wide range of plants, including shrubs and small trees while the extensive green roofs, can typically only accommodate a small selection of drought-tolerant plants with shallow root systems. Mosses, sedums, herbs and low-growing grasses are common choices for extensive roofs. Due to that, extensive roofs have low maintenance needs and they don’t require irrigation except during a heat wave. On the other hand, intensive green roofs need high upkeep and should be irrigated regularly.
The table below, briefly presents the three types of green roofs.
(Classification of green roofs according to type of usage, construction factors and maintenance requirements. Source: BabakRaji, Martin J.Tenpierik, Andyvan den Dobbelsteen. The impact of greening systems on building energy performance: A literature review. Renewable and Sustainable Energy Reviews, Volume 45, May 2015, Pages 610-623)