THE JOURNAL FOR THE SOCIETY OF GARDEN DESIGNERS

How to design a roof garden, part 1

In this roof garden project, designer Katharina Nikl built lighting into the containers. Photo: Katharina Nikl


Designer John Wyer explains everything you need to know about creating a roof garden


A well-designed roof garden makes great use of extra space and offers a secluded refuge, high above the city. These spaces present tricky technical challenges, but also offer unique opportunities. So what do you need to know before taking one on?


Roof load & weight

Let’s start with the most obvious – loading. When I first started out practising in landscape design about 30 years ago, I was frequently asked to look at roof terraces on older, existing buildings. The first question I always used to ask was what the roof structure was made of (translating to what load it might be able to take). Clients frequently didn’t know, and were sometimes a little dismissive of the necessity for care here.

As we were generally standing in the centre of their roof space, I would conduct a simple test: a small jump into the air to check for bounce. Of course, this is no substitute for proper calculation and advice from a qualified engineer, but it is a starting point in knowing what sort of load the roof is likely to be able to withstand.

On new buildings, it is generally possible to get information from the structural engineer as to what load the roof has been designed for. This will generally be expressed in kilonewtons (kN). One kilonewton roughly translates into 100kg of force-load, so 4.5kN/m2 designed load approximately equates to a 450kg load per square metre.

Engineers often make an allowance for live load on top of self-weight or dead load, to take account of the weight that people, furniture and such exert. So in calculating paving weights this has to be maintained, but for larger freestanding planters, these can (with the engineer’s approval) include the live load allowance. Note that the calculations need to allow for wet compost and the weight of the plant itself rather than dry compost.

Most structures can take more weight around the perimeter than in the centre, but be particularly careful on structures of a beam construction to find out which direction the beams run in. It is possible to safely load quite heavily across the ends of a number beams, but is dangerous to load along the length of a single beam, even if it is along the edge of the terrace.


Waterproofing

The state of the waterproofing is also very important: there is little point in starting out on an expensive roof terrace if the waterproof membrane is old and in need of renewal. This needs to be tackled first by the client.

There are many different systems of waterproofing. On larger roofs, it is common to use hot-melt continuous systems. On smaller roofs these are not always economic, although there are traditional asphaltic systems. I would also recommend that a root barrier is installed. Roots can attack organic compounds such as asphaltic roofing or mineralised felt.

Even if the membrane is a continuous sheet, roots exploit weaknesses and joints, which can cause leaks in the future. Chemical root barriers are therefore better than physical ones. Most roof companies have a standard product for just this sort of situation, but if not, lay a proprietary product above the drainage layer, such as RootX, before proceeding with the rest of the build-up in planted areas.

Something else to keep in mind is that most roofs these days are ‘warm roofs’, meaning that the insulation is above the roof structure rather than below. On larger roofs, it may be covered by a thin concrete layer, but frequently the insulation needs to be held in place by the layers above.


Environmental factors

Another big issue is exposure. Roof terraces are extreme environments – sunny, windy, dry – generally very exposed, and not unlike a seaside microclimate. The extreme exposure of some spaces means that the design is necessarily limited, and this requires some careful footwork in terms of design and detailing.

Pergolas and screens lessen the effect of wind. Where possible, design these in early on so that the shoes for the pergola can be incorporated into the waterproofing for the roof terrace. Even if this is not possible, it is always possible to fix screens in one way or another – sometimes by using temporary fixings to balustrades, or by having freestanding trellis panels that are held by the weight of containers, using a steel frame. Perforated metal or timber screens are much more effective at dissipating wind than glass or solid screens. If screens are not an option, try and keep everything possible below the balustrade. There are things that will survive fairly radical exposure – olives, for example, or tamarisk. Beware of the ‘windsail’ effect of trees and make sure the containers are big enough to stop them blowing over.

Shade is an important consideration. Roof terraces can be exceptionally hot in the summer if the sun is out. Consider designing some sunny spaces for lounging/sunbathing and more shaded areas for dining. This can be done with parasols, although be warned – these blow around in high winds. Alternatively, you could consider more permanent screens fixed to pergolas, or even boom-mounted shade sails, which retract when the wind is too strong.


Find out about the issues of irrigation, drainage and soil and paving on roof gardens in the second feature in this series, here