Designing a Roof Garden - Processed Substrates

Designing a Roof Garden - A Designers Guide

Perlite

This is a natural alumina silicate which when crushed and heated to 1000C expands to give a white, lightweight aggregate with a semi-closed cellular structure. Perlite is stable, with a density of 130 kg/m (8 lb/ft), a low water retaining capability and no CEC function. As a result it should only be used to lighten peat based substrates.

Expanded Polystyrene

In the production of raw polystyrene beads a foaming agent may be added. This is later activated by heating with steam, so that the beads expand to give soft spheres. Expanded polystyrene is a totally inert material with a closed cell structure which does not decompose. It seldom constitutes more than 25% of the total volume of a substrate and is used to reduce water retention. They do not materially effect the physical properties of the substrate unless they are present in large quantities. Even then their main use is to reduce density.

Vermiculite

This is a complex metal silicate which is found naturally as a thin plate-like material called mica. The graded mineral is exfoliated by heating to 1000C for one minute. This causes moisture trapped in the mineral to convert to steam so that it expands to twenty times its former volume. In this form vermiculite has a high porosity, a good air/water relationship and a bulk density of 80 kg/m (5 lb/cu. ft).

It is available in a number of grades, the most common being 4 - 6mm in diameter. It has a high CEC which compares with that of peat but over a year or two it tends to collapse with a resultant loss of voids. As a result Vermilicite should not be used in substrates which are intended for long term use. Vermilicite from certain sources can contain magnesium limestone which will raise the pH of a substrate and these should be avoided.

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