The Green Roof Garden

▲ Video of the Green Roof in April/May 2017

The Green Roof Garden lives on top of one of our garden buildings (the office-workshops). It provides an alpine landscape of drought tolerant plants that are visible from the first floor of our house, and to a lesser extent from the rest of the garden. I use a ladder to regularly inspect it up-close too. The photos on this page are from 2010-11, eighteen months after planting.

Environmental ideals

Green roofs are promoted for their environmental benefits, chief of which is the fact that they provide habitats for insects and birds and make up for the ground lost by the foot print of the building. Many of the plants suitable for green roofs are also attractive to bees and butterflies for their nectar. Other environmental benefits include a significant reduction in water run off as the soil slows the passage of rain. A more contentious benefit is the insulation factor – more likely to be significant in cooling the building in summer rather than keeping it warm in the winter.

A linked factor is that a properly constructed green roof actually protects the roofing material (butyl rubber in this case) by preventing UV from reaching it.

Gardening value

For me the main benefit is the opportunity to develop a distinct garden area where I grow plants in a community that is totally different to any other area within my property. The literature on green roof growing is limited but growing. Most designs rely on Sedum species as these are extremely drought tolerant and can exist with only 2″ of growing medium and little care or attention. I have gome for something a little more adventurous – with 3 to 5″ of growing medium (deeper at the front edge) – this increases the range of plants it is possible to grow, but also increases the maintenance as more weed species try to establish themselves too.

I am trying plants that I would expect to survive on the green roof, but do not appear in the literature at present including the larger Sedum species, dwarf bulbs, alpine herbs and even ferns. It is quite surprising what survives and what does not – for example, lysimachia nummularia is associated with damp-semi-shade in most garden books, yet it is thriving in this drought prone environment! By reading posts under the ‘green roof’ category you can follow the progress of this experiment.

Maintenance

I weed the roof a couple of times per year, add or remove plants as the mood takes me. Unlike many green roofs I provide a modicum of irrigation as this significantly increases the range of plants you can grow: in practice this amounts to little more than putting the hose on it for half an hour after a couple of weeks with no rain in the summer.

Construction

The main details of the construction can be found here.

An impostant point to bear in mind if you are planning a green roof is to make sure the structure can take the additional weight: 4in (10cm) of saturated soil adds 100 kg per m². On a building the size of mine this is approaching two tonnes! I had to reinforce the rafters by bolting ¼ in steel plate to them to prevent bowing – you could achieve the same thing by using deeper rafters but new planning rules limit the height of garden buildings close to boundaries to 2.5m, so you may not have room!