Lightweight soil is a growing medium composed of mineral and organic materials. The percentage of mineral and organic materials is designed based upon the type of vegetation planted. Another name that is used for lightweight soils are “growing medium” or engineered soils.
Advantages
Provides a stable substrate for the vegetation
Excellent moisture and nutrient retention
Does not compact and drains excess water quickly
Lighter in weight than typical clay based soil

Soil Mixes
It is important to have a good quality soil mix which is specifically designed for green roofs. The soil must be:

Lightweight – Standard topsoil may weigh around 1,600 – 1,800 Kg/m3 when saturated. This is a major weight element in any green roof system and will obviously add a significant loading onto the roof deck when installed. By using a specially designed lightweight soil mix, saturated weights can be reduced to 1,200 Kg/m3 or even 1,000 Kg/m3. By reducing the soil weight, the roof deck does not have to be as load bearing, so reducing costs of construction.

Free Draining – It is important that green roof soils are free draining so that “ponding” of excess water does not occur in times of heavy rainfall. Any standing water on the surface of the soil is not only detrimental to the planting, but also adds a greater weight loading on the roof deck. Therefore it is important that water filters through the green roof system quickly to the drainage layer below, where it then moves across and off the roof deck and off via the drainage outlets.

Water Retentive – Due to the often increased evaporation rates from green roofs, it is important that the soil mix used is able to retain water to supply the plants when needed.
           
Slow to Breakdown – Roof garden soils which contain high amounts of organic matter, e.g. peat, will breakdown more quickly, therefore topping up of planting beds (with organic matter) may be necessary. This may be logistically quite difficult on roof decks. Therefore by using more mineral based soils, which only break down very slowly, the need for topping up will be reduced if not eliminated.

Contain Nutrients – Fertilizers can be added in the production of the soil mix, or after it has been installed on the roof deck. The fertilizer will need to provide at least a base level of nutrition to help the plants establish and grow in the early stages. Subsequent applications will then be required throughout the life of the green roof.