We launched our blog series on ‘Designing Contemporary Roof Gardens’ earlier this year.  In this second instalment we want to take a closer look at material choices and feature selection, and the additional criteria that one has to consider when designing at height.  We have used again some examples from the series of penthouse terraces that we have designed at Chelsea Creek London for St George. 

One of the key items to consider is the longevity and sustainability of the materials that are selected.  Of course in a ground level garden one would always design (if budgets permit) using materials that are highly durable and will either mature with age or be sustainable for a minimum of 10-15 years.  The issue at stake when designing for roof gardens is that there are real practical considerations to be made about how these materials might actually be replaced in the long term.  With a new build project, one can probably use a crane that is perhaps being employed elsewhere on the build at the initial time of install.  This would be the most cost effective way of getting materials to height and also minimises any restrictions that might otherwise be in place regarding what elements can or cannot be lifted to the garden.  For example, if the only way to get materials to the roof terrace is via lift shafts or air vents, then you are going to be severely restricted in dimensional terms of what will fit and what can practically be carried.  It is far more responsible in the longer term to select premium materials at the outset to ensure longevity and therefore minimising potentially higher costs long term with any remedial work that might be required.

We also have to consider the harsher climatic conditions that are prevalent in elevated gardens.  Wind can be a major factor and creating micro climates can be as equally important for less hardy materials as it might be for planting.  It is essential to check the full detail of any material specifications to ensure suitability for a roof garden.  Furniture that we were looking to incorporate into the Chelsea Creek roof terraces had to be either heavy enough not to move even in strong winds or required semi-permanent methods of fixing via bolts under the paving or decking.  Semi-permanent so that it may be moved, on calmer days, to another location within the roof terrace.  Wind considerations are also paramount in how comfortable the terraces might be to use and therefore the creation of micro climates for the users themselves is key! 

Solid forms of fencing are just not suitable for roof gardens as they create wind tunnels and it is much more preferable to choose slatted fencing or boundaries with apertures that allow the wind to disperse.  We included slatted glass boundaries in some of the roof terraces so that the views are were still unimpeded but the slatted glass helped to minimise the full potential force of the wind, within the garden itself making it a more pleasant place to occupy.   Planting and hedging can also be used highly effectively to produce very attractive wind breaks, not forgetting of course that the plants themselves need to be able to survive the harsh climatic conditions.  We introduced very short stemmed Fagus pleached panels which, in the harsh winter winds, will allow the wind to pass through whilst in summer will offer more protection when in full leaf.

We introduced canvas awnings into many of the gardens and felt it was important to specify them with anemometers to ensure that even if they were accidentally left out by their owners they would automatically close during high winds.  The potential damage that could be caused not just to property, but also to people, if left unattended, could be significant, and is to be avoided at all costs. 

With any paving used, it is essential to ensure that it is truly frost hardy and our advice would be to slightly increase the overall depth of paving to be on the safe side.  Ensure good water runoff in all areas off any hard landscaping, since even small pools of water are more likely at height to turn into dangerous ice rinks!  Pedestal systems are often the favoured choice for paving on roof gardens and so it is also worth considering whether a woven mesh would be beneficial on the reverse of the stone.  We would always recommend checking this with the stone supplier.

Gardens at height are even more susceptible to UV damage, and as an example, canvas awnings that were specified at Chelsea Creek penthouses, needed to utilise high UV stable materials, as did any soft furnishings such as exterior curtains, and also cushions for the garden furniture. 

There is another key consideration when considering UV and material selection and that is heat and conductivity.  Whilst again this is always a consideration at ground level, it becomes even more important at height.  A garden on the 26th floor is going to heat up significantly more in direct sun than a garden at ground floor level.  Glass, steel and other metals should be used with care, and careful design planning given to where and when they are used so as to avoid any potential heat injury.

Any hardwood decking to be used is likely to weather up significantly more on a roof garden than it would on the ground.  We chose solid hardwood decking, that could be left to age back to a beautiful silver grey, should the client not be happy to be regularly applying a wood stain, to protect and retain its natural colour.  Avoid materials that allow no choice in the matter for example soft wood decking would be a complete disaster for a roof garden and should be avoided at all costs.

Glare is another consideration to be given to roof terraces, much more so than for ground level gardens.  Be aware that using very light colour stone will contribute to glare in high sun conditions so careful consideration as to placement of this to be well thought through.  Use light stone by all means but think through how this might interrelate with the interior or external seating areas.

We also introduced some beautiful ‘stone effect’ eco plastic planters that were perfect for roof gardens on many fronts.  Not only were they highly UV stable, they also didn’t require any maintenance at all and were lighter than equivalent stone planters (thus reducing the loading on the roof).  We would have to recommend choosing or designing them in smaller sizes in order that they could easily be removed from site but for Chelsea Creek we had the luxury of designing them full size! (They can always be cut into smaller sizes to remove form site as and when required in the future).

Wind and water features are also an interesting topic.  Water features at height require being wind proof and therefore fountains and water chutes are a complete no no.  We explored instead water features that trickled and ran along the surface of the feature, although one still needs to consider the greater evaporation rates that are present in roof gardens and to ensure that sufficiently sized water reservoirs are specified – or are automatically topped up from the mains water supply.

Hot tubs are a possibility for roof terraces, but in addition to the weight issue (see below) one also needs to carefully consider proximity issues to any boundaries.  Most hot tubs are in the region of 900mm above ground, and given that you can’t excavate on roofs, and with legislation stating that you need a minimum boundary height of 1100mm you cannot position a hot tub within say 750mm of any boundary, with a drop of more than 600mm.  Roof terraces are very often short on space, and are often little more than corridors, so you need to find innovative ways of solving this problem, or install a higher fence/boundary.  At Chelsea Creek one of our solutions was to introduce a hedge as a barrier between the edge of the tub and the boundary, another was to install a louvred glass trellis.  Another feature where this was of concern was with the outdoor kitchens that we designed and incorporated into the roof gardens.  With a counter top height of 900mm again we needed to position them carefully to avoid any potential safety issues.

Storage is another issue that we often feel gets overlooked.  In an average ground floor garden it’s pretty normal to have a shed for storing garden equipment, not so for roof gardens!  The ideal solution is for the developer/builder to incorporate a small storage room into the building itself, accessible from the garden, but failing this one does need to consider how and where garden tools and furniture will be stored. 

The weight of any features to be incorporated needs to be fully explored and understood.  For example a planter itself may not be that heavy but fill it with soil and install a large root-balled tree and suddenly it’s a whole different equation, and of course, when fully saturated the weight increases still further. It’s essential to know the total weight of all features to be incorporated, and to submit these to a suitably qualified Structural Engineer in order that they can calculate out the maximum loading and advise as to whether the current structure can safely support it or not.  The Structural Engineer will also need to consider any additional weighting from footfall and to calculate out the worst case scenario’s which may occur during crowded garden parties.  Never try to estimate or calculate this yourself; leave it to those whose role it is to do so.  It is also possible to specify a lighter soil mix for use on roof terraces. There is value in this for roof terraces that are primarily planted, but for extensive roof gardens, with many hard landscape features, it probably makes minimal difference to the calculations overall.

Finally maintenance and access for maintenance, is a thorny issue, and should be considered during the very early phases of design, and certainly with regard to the introduction and consideration of any features.    How will the roof garden be maintained? How will garden debris be removed from site? Is there sufficient access to remove the necessary materials?  If you are unable to comprehensively answer these questions then it’s back to the drawing board.

Images from Chelsea Creek Dockside Roof Terraces, designed by Aralia. Copyright St George PLC