Designing a Roof Garden - Proprietary Systems

Designing a Roof Garden - A Designers Guide

There are many proprietary roof garden systems which are available in Europe, many of which originate in Germany. As a rule these roof garden systems claim to be founded on scientific principles yet there have been no outstanding technical break troughs. These systems seek to minimise the weight and depth of the drainage layer by using open sheets of expanded polystyrene or convoluted moulded plastic forms.

Some of the drainage systems are designed to retain a small amount water e.g. some convoluted plastic sheets can contain up to one litre of water per square metre. This is the equivalent of two or three days evapo-transpiration loss. The value of such small quantities of water is not clear but the manufacturers list this as a bonus for their systems.

There would appear to be some confusion on the part of those who market some proprietary roof garden systems. Some start with moisture matt, which retains a small amount of water. This is then covered by a convoluted plastic drainage layer. What happens is that the roots pass through the gaps in the plastic sheets and fine fibrous roots spread through the moisture matt.

Almost all companies which provide proprietary waterproofing systems insist that their root barrier is used between the membrane and the garden. As a rule the root barriers tend to consist of a layer of thick roofing felt lightly bonded to the membrane itself or a sheet of very thick polythene. The point where root activity is at its maximum is around the outside of a planter and particularly in the corners.

However no special techniques are available for protecting these weaknesses. The thick polythene systems usually rely on overlapping adjacent sheets by a metre or so to prevent roots getting under them. The problem of corners and up-stands is conveniently ignored.

The open granular drainage layers under traditional roof gardens are surprisingly free of roots, presumably as they were an open desert from the roots point of view. At the Scottish Widows Headquarters in Edinburgh, Scotland the landscape lies over a layer of lytag up to 1m in depth which in turn lies over a leca drainage layer. Almost no root activity takes place in the lytag layer due to its hostile nature. It would seem that an important lesson from the past has been forgotten.

If you have any questions please feel free to contact us