Frequently Asked Questions
about Green Roofs

Vegetation substrate

7.  Is it possible to use regular topsoil on green roofs?

Regular topsoil is not recommended for use on either extensive or intensive green roofs.

The most important characteristics of a green roof substrate are good water permeability and high maximum water capacity. Both of which are often low in topsoil. Some topsoil may have better water permeability and retention capacities than others; however, the variability in quality is high and relying on topsoil is risky. Other problems for vegetation when using topsoil is dryness; topsoil gets especially hard when dry and can damage plant roots. The issues mentioned above can be amended by adding organic content, however, this could compromise the fire resistance of the substrate.

In comparison to the mineral substrates which are normally used for extensive green roofs, topsoil has low water permeability, low water retention, much higher weight and less suitable chemical and physical properties. In addition, the contamination from weeds and pests creates competition for drought resistant plant communities which are used on extensive green roofs.

2. What can I use instead of topsoil?

An alternative to using topsoil are the substrates which are specially engineered to compliment green roof vegetation. The engineered substrates are designed for the extreme conditions of the roof (increased wind, frost-and-thaw-cycles, full sun exposure, ...) as well as accommodate for the lost natural characteristics that a soil would provide at ground level.

The substrates are tested to ensure that they meet FLL guidelines, this means that they ensure a long life for the green roof. The substrates have been used for many years and for millions of sqf of green roof build-ups. They can also be adapted to compliment particular local climate conditions.