Designing a Roof Garden - Hard Landscape

Designing a Roof Garden - A Designers Guide

Low Density Porous Concrete

If there is a need for an area of hard landscape to be placed over a drained layer then a bed of low fines lytag concrete (1:1:7) can be placed on top of a proprietary drainage layer. Low fines concrete is very strong but needs to transfer its load directly to the underlying slab.

If correctly formulated and handled this material can be porous but the interstices are not large and will block with time if they are not used correctly. Making and handling low fines concrete can be problematical as the aggregate tends to absorb water between mixing and placing. Experimentation and practical experience are essential.

These mixes should not be used to provide a drainage layer in isolation. A contiguous bund of mix can form a dam and obstruct the free movement of water to the drainage points. This means that a drainage layer is still needed across the roof. For this purpose a plastic egg crate system is probably best.

It can be laid underneath the mix so that some of the cement slurry fills the cones to provide a solid base. However, to support paving the mix can be placed directly on a separator laid on the roof. There are examples where thick beds have been successfully placed on top of a leca drainage layer.

Ponds

If there is a need to separate a water feature from planting then a wall can be formed which is connected directly to the roof slab below the membrane. With a free draining planted area there is not a standing head of water. However, with a water feature there is a positive pressure permanently acting on the membrane.

This will find the smallest flaw. As a result a supplementary membrane is to be recommended, to provide belt and braces protection. If an informal design is required then there is no need for apprehension as the perimeter wall can be hidden under soil and/or cobbles (fig 6). It is also possible to produce a water feature by building walls of lightweight concrete on top of the membrane and then using a secondary membrane to line the pool.

Planter

A soft informal edge to a planter can be formed by trapping a geotextile vertically between topsoil and cobbles. This simple approach is very successful (fig 7 and plate 6). Around the outside of a planted area the soil can be less than 150mm thick and rise over less than a meter to a depth of 450mm with out any significant visual impact. Hollow pre-formed grc and stainless steel hollow box forms have been used to produce light weight kerbs. They use the weight of the substrate to hold them in place (fig 8). These offer a low weight solution to retaining soil. Alternatively paving can be placed on top of slabs of extruded polystyrene and the resultant voids filled with topsoil.

Formal planters can be formed from materials such as brick and timber. The advantage of timber structures is that they be placed on top of a flexible insulation layer. They also have the advantage that they can be easily relocated if access to the membrane becomes necessary. Having said this extruded expanded polystyrene slabs are surprisingly strong and it is possible to construct planters insitu.

Engineering bricks with a cement sand mortar are probably the best materials to use. If soft facing bricks are to be used then tanking will be necessary to prevent water penetration. If a strong waterproof render is required then GRC can be very useful as it has structural properties of its own. As a rule hard landscape elements are heavy and expensive to construct. If it is possible to design out the need for them then it is advantageous.

Artificial Rocks

Roof gardens must be the classic location for the use of artificial boulders. They weigh a fraction of real boulders and if correctly handled can look more realistic than nature. There are many ways in which it is possible to create rockwork. The one way which will not create a good effect is to use natural stone. Peoples perception of nature can be very different to reality.

A basic effect can be achieved by applying a sand cement render to a steel mesh which is fixed to a frame or armature. However, the quality of the final product is very workmanship dependent. The best effect is achieved by taking a silicone rubber mould from a natural rock surface and spraying glass reinforced cement (GRC) onto it.

The resultant panels are then secured to a metal frame. The backs of the panels are then packed with mortar. Finally the joints are filled with mortar which is worked insitu until it blends with the adjacent panels. The effect is brought to life by spraying with diluted UV stable emulsion paint. Individual points of interest, such as algae and lichen, are added by hand.

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