How to design a roof garden

pcI admire our urge to green any outdoor space available, looking to create gardens in the trickiest of places. There is something particularly special about roof gardens in the city; gardens in the sky, high above street level. As I walk around London I find myself looking up to see if I can spot tell-tale signs of a great roof garden — foliage and flowers dangling tantalisingly over the edge of a wall or balustrade.

Roof gardens can sometimes be retro-fitted on to old buildings, or be integrated during the construction of an extension, while many new-build properties include balcony spaces and larger terraces with reinforced concrete roofs that can take a decent weight. The approach to a roof space is always practicalities first; work out what can and what can’t be achieved before getting your hopes up.

Pots, compost and plants are heavy, and if pots get blocked up and filled with water they get even heavier — and that’s before you’ve considered the issue of how many people will be on the terrace along with any garden furniture. Planning permission, structural engineers, weight loading and how and where any structures (handrails and balustrades) can be fixed is paramount; you will most likely need professional advice in some way or another. Overload a roof and it can get dangerous for obvious reasons, so my advice is to not attempt to create what estate agents call “an unofficial roof terrace”. I know of some and have heard of many more that have been ordered to be taken down, which means that time and plenty of money are wasted.

Things to consider

There are some great products, including adjustable pedestals designed to “float” tiles, decking structures and regular-sized paving slabs over the existing roof so that water can drain through and no cement is needed, meaning the top layer of the roof isn’t affected or penetrated. A protective layer will need to be put out on the roof first. Artificial grass has come on in leaps and bounds, and there’s a range of looks to choose from. It’s is easy to install and, even if only for a temporary solution, can look effective and fun while greening the space up.

Containers and compost
Look for containers made from lightweight materials, such as plastic, fibreclay, fibreglass, zinc or galvanised steel. Plan carefully, measuring out the area, and think about how the containers can be placed for maximum planting space. Consider having planters made up (using wood or galvanised steel) to slot perfectly together, with some taller and some shorter, to create a tiered planting effect. There are also fabric planters (large lightweight bags) and even good old growbags, which can supplement the main planting, fit in a few gaps and are ideal for a few salads, herbs or veggies.

Plastic imitation terracotta and lead planters may sound naff, but some are thoroughly convincing and lightweight. Stick to one type of finish for a cohesive look and, if the weight loading allows, try to get at least some big planters too, so you can add height into the planting. Use a drainage layer to avoid waterlogging and use lightweight compost (some come ready-made for roof terraces and have Leca — light expanding clay aggregate — included in the mix).
Screening and windbreaks
Filtering wind with trellising and planting is far more effective than solid barriers, which are constantly buffeted and form damaging eddies on the garden side. For windbreaks consider olive trees (surprisingly tough and hardy), Arbutus (strawberry tree), Escallonia,Phormium,Viburnum tinus and Griselinia.
Silver birch, tamarisk and Cordyline australis do well and help with height but won’t create much protection from the wind. Once you have created a buffer, plants such as dwarf pines, Abelia,Cotinus,Phillyrea latifolia, rosemary, lavenders, Phlomis and cistus will help to build up a framework. Perennials such as agapanthus, Centaurea,Crocosmia, pinks and pasque flowers cope well with exposure and do well in containers, as do ornamental grasses such as Stipa gigantea and the tall Miscanthus sinensisvarieties. Any plants that don’t do well in harsh winds (bamboos, acers, etc) should be avoided.
It may sound obvious but, before you go shopping for plants, containers, furniture and so on, make sure you know how you’re going to get them up to your roof — including measuring the access. I have known elements that have fitted neither the lift nor the stairwell being embarrassingly sent back. A more substantial job may require a specialist contractor and a winch, or even a crane, which would mean co-ordinating deliveries with military precision and getting a licence to block the street; it can be done, but at a cost.
Regular watering is essential; containers can dry out in day or two. Some people manage with hoses dangling out of windows, but if you can install an outdoor tap somewhere close then do. Drip irrigation kits and simple timers can be fitted easily and these days are reliable, good value and often more efficient than watering by hand — and mean you can go away for a week or two.