Designing a Roof Garden — Drainage Layers — Designing a Roof Garden — A Designers Guide

Designing a Roof Garden - Drainage Layers - Designing a Roof Garden - A Designers Guide

Designing a Roof Garden — A Designers Guide

In many natural locations the underlying strata is fractured and porous which means that drainage is seldom a problem for plants. In other cases a high natural water table acts as a horizontal barrier to root development. The result is that plants are very adaptable.

It is rare to find drainage problems with roof gardens. This is because Architects and Engineers are only too aware of the problems which have been associated with flat roofs in the past. Most flat roofs are now designed with falls, and drainage outlets are detailed for maximum efficiency. With a roof garden it is necessary to ensure that rain water is able to flow through the drainage layer and that the drainage points cannot become blocked.

Water has to pass sideways to a drainage point. A fall of 1:30 is more than adequate to carry water to the nearest drainage point. The gulleys should be located at no more than 30m intervals so that the maximum distance for lateral movement does not exceed 15m although 30m can be made to work. In reality gulleys are often positioned in a way that suits the pipe runs in the ceiling void below rather than the garden. Any area of roof should always have at least two drainage points in case one gets blocked.

If the area is predominately hard then the paving can be placed on spacers directly on the membrane or insulation layer. Drainage then becomes a simple matter of laying the finishes to falls. If the landscape is to contain hard and soft elements then the drainage layer must be contiguous across the area. Traditionally, clean gravel was used and more recently low density materials such as leca and lytag have been used.

There are several proprietary products on the market which are formed from plastic and which perform a very similar function to aggregates in that they create open voids. In many ways they are more efficient as the void to solid ratio is higher, as is their cost. In studies of established gardens it was found that drainage materials do not offer any real resistance to the lateral movement of water, although this could be very different during a 100 year storm which is what a bare roof is designed to handle.

There are four main approaches that can be adopted for draining substrates and these are described below:

a) Traditional Granular base

The traditional approach is to have a free-draining roof, which has a gentle fall to encourage run-off through a granular base material. The drainage layer is then formed in ‘Leca’, ‘Lytag’ or gravel. Lightweight aggregates are usually favoured, but pea gravel appears to be slightly more water retentive than ‘Leca’. ‘Lytag’ has the advantage that it can absorb more than 15 per cent of its own weight in water.

Unfortunately it is difficult to free this product from the dust which is associated with its manufacturing process. ‘Lytag’ has a very high pH of 8.0 to 9.0, which is unacceptable for most plants and certainly deters root development in the drainage zone. Its highly alkaline nature makes it unacceptable for use with a flooded drainage layer, particularly as it can combine with lime which is released by cementation compounds to produce calcareous deposits in the drainage system.

‘Leca’ is strong, inert and admirably suited for use as a granular drainage material. On balance it may be that non-calcareous pea gravel may be preferred for thin substrate profiles, while ‘Leca’ is to be recommended for the deeper ones. The drainage layer only needs to be deep enough to remove excess soil moisture. Studies of established roof gardens have shown that a drainage layer of 50mm thick is often adequate.

However, a layer 100mm should be more than adequate. A layer greater than 150mm is seldom of benefit. The only need for caution is the requirement of the drainage system to handle the worst storm in 100 years. Aggregates always create some hydraulic resistance and so impede the flow of water across a roof. As a result lengths of flexible perforated land drain can be used to link drainage points. These routes are also useful if cables or flexible pipes need to be threaded across the roof at a later date.

b) Plastic ‘Egg Crates’ and Plastic Formers

A number of proprietary systems employ plastic egg crates which can be laid on the membrane or insulation layer to form drainage voids. The deeper sections come in sheets measuring 2m x 1m. The thinner sections come in rolls sometimes with geotextile fabric bonded to their upper surface. Some proprietary systems use three dimensional plastic cubes formed into a sheet. In some cases plastic cellular grass reinforcing systems have been inverted although their long-term effect on the membrane must raise doubts about their suitability.

c) Expanded Polystyrene Slabs

Designing a Roof Garden - Drainage Layers - Designing a Roof Garden - A Designers Guide

Sheets of open pore expanded polystyrene slabs or profiled expanded polystyrene slabs have a low density but small interstices.

d) Enkamat

This consists of a light open filamentous plastic mat reminiscent to a giant pan scourer. Some of the heavier grades are marketed with a geotextile fabric lightly bonded onto their upper surface. This product was first used as a vertical drain against a retaining wall or basement. The thinner sections were devised to prevent soil erosion along water courses etc. This material has been used as a drainage layer under thin roof gardens.

e) Inclined Base

If a roof is inclined there is no need for any identifiable drainage layer other than a thick hydroscopic geotextile. The optimum slope is uncertain, but seems to be between 1:3 and 1:5. This approach has a considerable weight advantage. One problem with steep roofs is that the substrate can work its way down the roof. A heavy plastic mesh, such as Netlon can be fixed along the ridge of the roof, and run below the geotextile to avoid this problem. A second sheet can be placed over the surface of the substrate in more exposed locations.


Many different ways have been devised to handle drainage points. These range from large manhole covers 600mm square through to tiny covers 150mm in diameter. Sometimes drainage points have been totally hidden below the substrate. This might have certain aesthetic advantages but is hazardous as if it is only possible to inspect the drainage points from below. Drainage points should be capable of being inspected from above and accessed from below. Often gulleys and their associated pipework require cleaning after the construction work is complete.

Rodding points and silt traps should be placed below every garden. Being able to view the rate at which water leaves a roof is vital when calibrating the irrigation system. All drainage points should be surrounded by pre-formed manhole rings and covered with a lid. Pre-cast concrete manhole sections have been used in the past. Today 500mm diameter plastic manhole rings are widely used. Lids may need to be lined with extruded polystyrene to guard against frost action. Where possible all hose points should be placed adjacent to drainage points in case there is a leak.

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