Designing a Roof Garden - Loading

Designing a Roof Garden - A Designers Guide

There are only two conventional ways of constructing roof gardens. The first is to have a free-draining roof, where surplus water is removed. The second is to retain water on the roof. This has an additional weight penalty of up to 100 kg/m, although with thicker profiles this becomes proportionately less significant.

All the modern proprietary systems seek to reduce the weight of the drainage layer. Unfortunately the drainage layer is a small part of the total weight of the profile. Further effect sometimes goes into reducing the weight of the substrate but the benefit of this, from a horticultural perspective, is very questionable.

Most commercial buildings are designed to receive a crowd load of 5kN/m, plus a dead load of 1 or 2kN/m for furniture etc. If people have access to a garden then it must be assumed that they will congregate at the same density that they would on a crowded office floor. In other words the weight of the garden is in addition to the 5kN set aside for the live load. If it is possible to guarantee that people will not congregate on a garden then the live load component can be removed from the calculation.

Most floor slabs are designed to take a uniform load. However, point loads can sometimes be placed over columns and on the line of beams. Structural engineers usually suggest that they should comment on proposals. This is a very laborious way to progress matters. It is better to have a drawing marked up with the capabilities of the slab and to design within these constraints.

Even if a slab is designed to take a uniform load it is usually possible to adopt a swings and roundabout approach. For example if there is paving of minimal depth or weight then the adjacent soil may be deeper. Similarly large blocks of expanded polystyrene can be used to infill large voids in a way that produces tree pits whilst at the same time allowing a reduction in weight.

If there is a major change in level between two sections of a building or if there is a need for a slope then slabs of extruded polystyrene can be used as a lightweight void filler. The slabs should sit directly on the structural slab so that they are stable. Drainage can be provided by chamfering the lower corners of the slabs.

The slabs seldom need pinning together as the weight of the soil holds them in place but it sometimes makes installation easier and safer. Once the slabs have been placed it is usually advisable to cover the formation with a geotextile to keep the voids open. It is important not to exceed the angle of repose of the substrate.

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