Tag Archives: roof gardens

Roof garden Design: #4 – Planting for green roofs

Continuing in our series on roof gardens, this post is guest-written for us by Maggie Fennell of GreenSky, a nursery specialising in supply of plants and substrates for green roofs and which has joint research projects with University of Sheffield. Her contact details are at the end of the post.

All green roofs share certain challenges for the plants. From lightweight, extensive roofs to full scale roof gardens, they will be living in an artificial, sterile growing medium without the diverse soil ecology that supports plants in a more natural environment. Unlike ‘terrestrial’ planting, they have no access to the water table so they are completely reliant on rainfall and artificial irrigation. Add to this the extremes of temperature and wind exposure at roof height and it is clear we have a very challenging environment in which to plant.

Extensive, lightweight green roofs take on this challenge by using only the very toughest, most resilient plants which are naturally found in poor, shallow soils facing harsh conditions. This means wildflowers and hardy succulents such as sedums are most commonly used. At the other end of the spectrum intensive roof gardens provide increased structural loading and maintenance access to support a wide plant range from trees to turf lawns.

Biodiverse green roof at Sharrow School
Sharrow School in Sheffield is a world class example of a green roof that is both biodiversity-driven and visually stimulating

In between these two extremes there is growing interest in ‘semi-intensive’ green roofs which support a much wider range of plants than the extensive roofs by providing 150-250mm depth of substrate and a modest maintenance burden. Hardy perennials and some small shrubs can be used to create a variety of ecologically diverse and visually attractive ‘roofscapes’ without the heavy loading requirements of an intensive roof.

To maximise the effectiveness of this kind of planting it is important to consider two main differences from ground level landscaping. The first is the survival of plants in these harsh conditions – how can losses be reduced? The idea of forming a healthy community of plants that mimic a natural ecosystem is not one that often arises in the realm of aesthetically driven amenity planting, but on the roof it is more important. The second is the changing viewpoint. Often there is not a ‘front’ or ‘back’ view, as green roofs might be overlooked from various angles rather than from the same level.

Randomised plant placement is the normal planting method on extensive roofs requiring minimal maintenance
Randomised plant placement is the normal planting method on extensive roofs requiring minimal maintenance

The natural planting technique that results from both of these considerations is one that mixes up individual plants alongside different species in a mosaic-like form, rather than swathes of a single species. This randomised or ‘naturalistic’ arrangement of plants can seem quite foreign to landscapers used to working to precise planting schemes which carefully position plant structures and colours for a certain visual display.

One reason why the juxtaposition of varied species is helpful for green roof survival is related to how plants cope with the stresses of limited substrate depth. Different species have their own techniques for gaining the maximum moisture and nutrition from the limited supplies available on the roof. A sedum with shallow, fibrous roots will provide less competition to a neighbouring, relatively deep-rooting wild carrot. Plants can differ nearly as much below ground as above it, and the diverse root structures will exploit different levels of the substrate layer to the best effect.

This variation of plant performance also means that interspersed species show off their best colours in different seasons. The overall effect of a reasonably colourful carpet of vegetation is maintained throughout the year rather than having isolated patches that look great in spring but a bit bare in autumn.

Mixed planting on a semi intensive green roof

The wet summer of 2012 was excellent for alliums on green roofs
The wet summer of 2012 was excellent for alliums on green roofs

An extension of this idea is that species react differently to the fluctuations in annual weather patterns. A particularly cold spring, an unusually harsh winter or an unseasonably wet summer might each have an adverse effect on particular types of plants. By combining many plant genera, the chances are that whatever record-breaking weather statistic should be quoted in a given year, a proportion of the species on your roof will be performing well, or enjoying an unexpected advantage. If those plants are mixed fairly evenly across the scheme they will serve to compensate for struggling neighbours, who may get to return the favour in future years.

This demonstrates how a community of plants work together in a way that reduces maintenance while maintaining a pleasing visual effect. Another example of this is resistance to pests and diseases which might have a noticeable effect on a large mono-culture, but less impact amongst varied species.

This ecosystem approach does not necessarily conflict with a designer’s artistic aspirations, it merely recognises the increased horticultural and practical requirements of the planting scheme. Plants can still be arranged according to flower or foliage colour if desired, and deeper substrate areas can provide anchorage for taller, more structural species. It is always important to consider the client’s expectations concerning both aesthetic effect and maintenance provision when designing a successful green roof – as they can both differ greatly from a ground level garden.

For more details or advice, look on the GreenSky website at www.boningale-greensky.co.uk or contact Maggie Fennell directly at maggie.fennell@boningale.co.uk

Roof Garden Design: #3 – drainage and irrigation.

Continuing with our (intermittent) series on roof garden design. I have tried to splice these between blogs on other subjects so that people don’t get too bored, but they are packed with detail so do have a read if you are new to this area of design!

B&W copyright
Part of an irrigated roof garden in St Johns Wood, London

The centrality of water to plant growth is something that it is often easy to forget in the mild temperate climate of the UK. Unlike some parts of the world water falls abundantly from the sky over much of the country, most of the time. Temperatures in the UK also rarely exceed 30oC, which means that evapotranspiration is not a major problem. However, on roof terraces the combination of increase exposure and lack of access to groundwater means that plants frequently risk drought. There are some ‘green roofs’ that can survive without irrigation and we will return to these in later posts, but for the most part irrigation is virtually essential. Irrigation systems on roof terraces fall into two basic types. There are the larger, more complex systems with a supply tank and control panel, and the simpler ‘temporary’ systems, which run from a tap, normally with a small battery-powered

A typical tap-timer irrigation set up
A typical tap-timer irrigation set up

timer.These are cheaper and easier to install (often by a landscaper), but have the disadvantage that there is a limited amount of planters that the network will supply, even if subdivided into zones. For smaller terraces however, they are usually sufficient. It is advisable to have a manifold so as to retain a tap for washing down the paving or hand watering of odd containers. In all cases (larger and smaller schemes), it is best to run the irrigation pipes beneath the paving or decking and come up through the base of the container rather than looping up over the side. We normally specify a hole to be core-drilled in the paving to allow drainage and access for irrigation pipes without crushing; the only disadvantage of this is that it limits the scope for moving the containers in the future. However, in some ways this is a good thing as it means that designed loads to the roof cannot be easily exceeded.

IRRIGATION_HEADFor larger irrigation networks, it would be wise to seek the advice of an expert. Most of these bigger systems will run from a tank (generally situated in the building’s plant room) with a pump set and a series of solenoid controlled valves opening and closing different zones. This means that the whole irrigation system is split up into zones on a geographic basis, which run in sequence. This has several advantages: firstly it allows fine-tuning of the individual zones to water demand according to local microclimate – sunnier zones might need more water for example. Secondly, it allows individual zones to be drained for repair or maintenance. Finally, and perhaps most importantly, it reduces the amount of water demand at any one time. The advantage of this is that both the tank and the pump size can be reduced, as can the inflow – the relationship between the in and outflow is obviously the main determinant for the tank size. There are also systems which can run from a central ‘landlord’ tank with a series of ‘planetary’ sub-systems which can be either under local or central control – useful for apartment or office buildings with multiple large roof terraces.

All that water then needs to go somewhere – drainage is an important issue on roof gardens where the entire substrate is usually a concrete slab. On smaller terraces it is not such a big issue, as the roof usually drains beneath the deck or paving to a gully and downpipe at one side – you just need to allow for inspection access. I know from experience on my own roof terrace in London a few years ago that it is surprising the amount of debris that can build up underneath the decking. With that in mind it is best to either use a pedestal system or if using decking, build in some easy-access panels over key drainage positions. 

Sketch showing the different layers of build-up on a typical intensive roof.
Sketch showing the different layers of build-up on a typical intensive roof.

On larger roof gardens, it is not uncommon to drain through the slab to suspended drainage systems below, although architects generally try and avoid this where they can. However, the limiting factor is the size of the roof – or more precisely, the distance from the centre to the edge. There has to be a reasonable fall for below deck drainage to work well and over large distances this can become prohibitive. Where you are intending green build-up – lawns or larger planting beds for example – it is vital to make sure that the media used are free draining to allow water to move freely to the outlets. For extensive or semi-extensive green roofs, a drainage board (‘egg-crate’) is often specified. This allows an element of storage, particularly where the growing medium is thin due to weight or other restrictions. The growing mediums used with these drainage boards are different from other media – see the (upcoming) post on green roofs for more detail. Small intensive roofs tend to use containers. Aside from the soft areas, water will also need to be drained form the paving. Using pedestals, this will happen at deck level. There are also gullies which allow drainage water to be collected at both paving level and slab level. These are particularly useful in wet-laid paved areas where most of the water needs to be taken from the surface, but inevitably some will find its way down to the slab.

Washed sand and rootzone mix being installed over a roof slab at the Lancasters. You can also see in the centre how the path is built up separately in blockwork and type 3.
Washed sand and rootzone mix being installed over a roof slab at the Lancasters. You can also see in the centre how the path is built up separately in blockwork and type 3.

For larger intensive roofs the issues and solutions are different. Fifteen years ago we constructed a roof terrace using standard topsoil in layers up to 800mm thick. Within three years the profile was showing signs of poor drainage and soon we saw water in evidence at the base of the inspection tubes we had built into the scheme. As a result of this, on deeper build-ups (intensive gardens) we now use layers of graded washed sharp sand followed by sand-dominated rootzone mixtures. This allows a robust, free-draining growing medium that remains well aerated and maintains its structure. It also encourages deeper rooting ensuring that plants are less reliant on irrigation and more on water stored in the deeper layers of the build-up.

Buzon paving pedestalsWhere paving is required on intensive (or extensive) roofs, there are a number of solutions. First is to lay the paving traditionally on crushed stone and mortar bedding. Additionally, we quite often specify hidden block work walls beneath the edges of the paving. The advantages of this are threefold  it separates the hard and soft landscape build-ups, it supports the edge of the paving firmly. It can also create a handy void in which to run services. However, the more normal method is to lay the paving on pedestals. These are plastic discs which are supported on a cylinder and base. they raise the paving up to a given level – anything up to 900mm. They have many advantages, not least that they allow the water to drain freely beneath them, with easy access in the event of any future problems. They also mean that paving can be laid absolutely level.

The next two blogs in this intermittent series will look at planting on intensive and extensive roofs. The extensive one will be written by a guest writer, Maggie Fennell of GreenSky, so look out for that one. The intensive one will be by Bowles & Wyer’s very own planting specialist, Stuart Wallace.

As usual, drop in any questions below and I will get back to you.

Roof Garden Design: #2 – Loading and waterproofing.

Buildings occupy a central position in much of our work – most of what we do as landscape designers is affected in one way or another. I started this short series because I felt that whilst there was a lot written about the generalities (urban design, cityscape etc.), there is surprisingly little written about the ‘specifics’. Roof gardens seemed the natural place to start. In the last post – Roof Garden Design: #1 Exposure and Screening  – I explored the unique microclimate of rooftop spaces and what mitigation strategies are available to us as designers. In the next article I will look at another aspect of the cultural properties of these spaces – water; but before we get on to that, there is another more basic issue (literally) to deal with – the roof itself.

When I first started out practising in landscape design about thirty 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: I would make a small jump into the air. I weigh getting on for fourteen stone (about 87kg or 190lbs).

Typical beam and block construction  (courtesy of carter-concrete.co.uk)
Typical beam and block construction (courtesy of carter-concrete.co.uk)

The small jump allowed me tell instantly whether I was on a timber roof (which would flex alarmingly), a lightweight concrete structure such as beam and block (which had a small but discernible flex) or a loadbearing concrete slab (which would not noticeably flex at all). On timber roofs, a look of alarm used to spread across the client’s face, which was as good a demonstration as any other of the need to take load bearing ability of the structure seriously!


Typical timber roof construction (image courtesy of greenspec.co.uk)
Typical timber roof construction (image courtesy of greenspec.co.uk)

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 etc. 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.

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 water proofing. 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, which is copper foil sandwiched between geotextile (www.water-lines.co.uk/rootx) before proceeding with the rest of the build-up in planted areas.

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. In the next post, I will explore some of the options in terms of build-up, as well as looking at drainage and irrigation. In the meantime, as always if you have any questions (or criticisms!) leave a note below.



Roof Garden Design: #1 – Exposure and screening

Buildings and landscape are inextricably linked, especially buildings and gardens. Most of our work is on, in, surrounded by or surrounding buildings of one form or another. Because of the centrality of this relationship, I wanted to explore the relationships between buildings and landscape more, starting with roof gardens, but also covering living walls, courtyards and other built landscapes. So… here is the first of a series of pieces, the first few of which are on roof gardens and terraces. Do leave a comment if you have any queries.


view3Over the years we have done many roof gardens. I have also been asked to speak on the subject on several occasions, include twice in the autumn last year. The popularity of roof gardens has grown in recent years. There are several factors behind this, but one of them is the increase in property prices and density of development, which has put a premium on outdoor spaces in the city. When I bought my first property (a maisonette near Elephant & Castle, London), the only outdoor space it had was a small roof terrace. The first thing we did was plant it out. It was a magical space, only a couple of floors up, but fantastic to have a garden up at rooftop level. This was very much in my mind when I wrote the section on Roof Gardens on our website, which begins: “A roof garden can be one of the most exciting and unusual outdoor spaces – or, if you don’t get it right, one of the most unpleasant! A well-designed roof garden makes great use of extra space and offers a secluded refuge, high above the city below. It has its own microclimate and special consideration of sun, shade, wind and exposure is required.” These spaces present tricky technical challenges, but also offer unique opportunities. I intend to break this series of blogs into sections on the various aspects of roof garden design – design of small and larger spaces, exposure and screening, drainage, irrigation and water proofing, plant selection etc.

Exposure and screening.

This scheme (below) was one of the first large roof terraces that we designed, for a building in London’s financial quarter overlooking the Thames. It was never built, but for me it encompasses many of the key issues and values of roof garden design. Roof terraces are extreme environments – sunny, windy, dry – generally very exposed. Not unlike a seaside microclimate. The inspiration for this roof terrace drew on that further, with bleached timber decking, weathered oak raised beds, beach pebbles and a planting palette that was based on foreshore and seaside species – kale, allium, thrift, grasses, cardoon and others. There was even a coin-operated telescope! The point here is that the planting worked with the environment rather than against it. There is no point in designing lush woodland planting to go on the top of a ten storey building. I will go more into plant selection in a later post, but it is a useful starting point.


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. Maybe clients do not want a beach theme? One can hardly blame them! However there are other alternatives. Firstly, look at mitigation. 56860007

Trellis fixed to balustrade (11th Floor)
Trellis fixed to balustrade (11th Floor)

On this roof terrace we incorporated pergolas and screens to lessen the effect of wind. These were designed in early on so that the shoes for the pergola could be incorporated into the water-proofing 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 (U-shaped clamp brackets – see left), or by having freestanding trellis panels that are held by the weight of containers, using a steel frame. This is a trick we often use. Perforated metal or timber screens are much more effective at dissipating wind than glass or solid screens. This seems counter-intuitive, but it is true. A solid screen creates more turbulence. In terms of shelter, it offers something like 1 x height in front of the screen and 2 x height to the rear of the screen, measured at floor level. Beyond this is turbulent air, often with quite a sharp boundary between the two. With a perforated screen of something like slatted trellis, this increases to 2 x height in front and 5 x height to the rear, with the optimal permeability about 40% ‘hole’ to 60% solid.

View from terraceIf 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 of sufficient size to stop them blowing over. This roof terrace in London’s West End has a strictly limited palette of materials and planting, but perhaps the most striking thing when you look in a little more detail is that there is virtually nothing above the parapet level in the scheme. Even so, this hasn’t stopped the planting below parapet level being used effectively to sculpt the space.

Shade options for roof terraces
Shade options on a Mayfair roof terrace

Shade options for roof terraces 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.

In the next two posts, I will consider roof loading, water-proofing, build-ups, drainage and irrigation. In the meantime, if you have any questions just pop them in below and I’ll get back to you.


Garden Design for Small Spaces


It may seem perverse to link the words ‘small’ and ‘space’ together, but unfortunately for most people who garden in central London, this is an all too familiar conundrum. Even more perversely, it doesn’t seem to make much difference how big your house is, the spaces aren’t necessarily any bigger – you just get six of them. However, even the smallest external space has potential. The real joy is that with access to light and water it is possible to enliven the flatness of the urban environment. Few things are more cheering than spring bulbs bursting through soil, or an exuberance of foliage and flower on a hot day.

Unfortunately the gritty reality is that basement lightwells, balconies and roof terraces can be daunting places to try and establish a garden and you should use all the weapons at your disposal. Let’s start with some practical considerations. How is the space to be used? Can some or all of it be easily seen from inside? What is access like? The links with internal spaces are often vital and should be exploited as much as possible. Make the most of views from important windows by placing pots, sculptures, specimen plants or other features on the same axis. Try and draw the viewer out into the space by giving hints of something just out of sight.

Try and create drama in a small space. Mirrors can add depth and mystery, especially if partly veiled by foliage. Use lighting, particularly uplighting, to accentuate features such as pots or sculptures. Strong textures (which work well in confined spaces) are much emphasised by carefully placed lighting. Try luxuriant foliage or slatted trelliswork against white or brightly coloured stucco walls. Water and light combine well together. Water features can be very dramatic in confined spaces, and these days there are all sorts of possibilities that take up very little space. The sound of trickling water can add to the ambience of a small terrace.

Lighting not only adds drama, but also extends the period during which you can use a garden. We often use firepits to add a strong focus; they are a great gathering point on cooler evenings and allow use of the garden well into the autumn. For roof terraces or smaller spaces, there are options fuelled by gel or by gas. Hot tubs are worth considering too; there is nothing quite like lying on your back in a hot tub looking at the stars – we did a garden in London’s West End last year with a hot tub that had a view of Green Park! There are even wood-fired ones for the more adventurous souls.

Don’t forget the surface you walk on. If you don’t want the upheaval of lifting the existing paving why not lay timber decking or thin porcelain tiles over the top.

Planting will always do better if it is rooted directly into the ground, but in many situations this is not possible. Give some thought to containers, as these are an important part of the ‘furniture’ of your external room. Don’t automatically go for terracotta, we often get timber containers made up, or you could try lead or ceramic. If working to a tight budget, found objects such as old zinc galvanised baths, buckets, or even lavatory pans can be wonderful. We have also used sections of air-conditioning ducts before as planters.

Horticultural considerations are of paramount importance. Remember the three basic needs of plants: water, light and nutrients. Consider installing a simple irrigation system -many of these are available over the counter at garden centres in kit form or can be installed at a reasonable cost by a competent gardener. Drainage is also important. Light is at a premium in courtyards and deep lightwells so choose plants carefully. Generally speaking, green-leaved plants will put up with lower light conditions than variegated or coloured foliage types. Reserve silver and grey leaved plants for high light positions. In really dismal conditions, rely on foliage rather than flower and pick plants well adapted to such conditions such as ferns. Pockets of colour can always be introduced with bedding plants. Compost should be of good quality and, if possible, replaced on a regular basis (every 2-3 years for example). On roof gardens you will also have to consider exposure. Use permeable structures, such as a close mesh trellis of horizontal battens, for shelter rather than solid screens such as glass; the more solid a windbreak is the more turbulence it will create. Seaside plants are well suited to these conditions. Combine them with decking and beach pebbles for a maritime feel. All in all, remember that the more you put into a small garden the more you will get out of it.