Our green roof is an interesting element of our build that deserves a whole post of its own.
2021-01-01
Richard Keech
Recap
Since very early on in the design process, we had the idea that the garage roof would lend itself to a green (i.e. planted) roof. The recessed elevation of the garage meant that the roof of it would be visible from the kitchen and the east deck. So, we imagined looking across a vegetated area, west to the horizon would make for a significant feature to the design.
I had previously met a fellow name John Hassall who, when I met him, worked for a small outfit called Do It On The Roof which specialised in green roofs and green walls. When I reached out to John for this project he was working freelance and trading as Greenly. John was immediately interested and was more than happy to give us a costed proposal for a green roof.
Eco Pillows. One of the challenges with a green roof is the structural implications arising from the extra all-up mass of the garden, including plants, soil, water, etc. To help with this John proposed a system called Eco Pillows. This is a light-weight, modular system for hosting the plants. It also has the benefit of allowing the planting to happen off-site. The planted-up Eco Pillow modules can have time to mature offsite, and be dropped quickly and easily into place onsite when ready. The nominal static structural wet load from the garden using the Eco Pillow system is 60kg/m2.
Roof and waterproofing. The other part of the green roof is the underlying roof material and waterproofing. Based on negotiation between us, the designer, and John, we decided that the garage roof would be made of plywood, which would then have a waterproofing layer. The builder would be responsible for the un-waterproofed roof, and John would be responsible for the waterproofing, as well as the supply and installation of the planted up roof and watering system.
The plantings were all intended to be low bushy plants or ground cover. The initial planting schedule was proposed as part of the first landscape design.
John gave us a costed proposal which was acceptable. The plan was that he would sub-contract the water proofing layer, and would supply and install the rest himself.
Plants
John’s final design changed the planting schedule from the initial one shown above. The plants we actually used were as follows
Bidgee Widgee (Acaena novae-zelandiae)
Creeping Saltbush (Atriplex semibaccata)
Running Postman (Kennedia Prostrata)
Trigger Plant (Stylidium graminifolium)
Creeping Boobialla (Myoporum parvifolium)
Petite Marie (Dianella revoluta dwarf)
Diamond Head (Xerochrysum brachteatum)
Fairy-fan flower (Scaevola aemula)
Planting up offsite
In April, with the home construction well underway, we joined John at the Melaleuca Nursery, Inverloch, to observe and assist the offsite planting up of the EcoPillow modules.
The planted modules were then able to grow and stabilise in the modules under the watchful and helpful eye of the Melalueca Nursery staff until called upon.
Waterproofing
The roof of the garage was initially topped with 19mm MDF sheeting (Yellow Tongue). This was later finished using two layers of bitumen tanking. This was installed (flamed on) in mid June. The result is a durable, waterproof coating on which the green roof modules could be installed. This drained into a large box gutter at the east end.
Installation
In late October, John Hassall did the main installation of the green roof modules, including plants, onto the garage roof. These are lightweight, and held in place with stainless-steel wires running the width of the roof, fastened into the sides of the parapet.
The modules covered only about half the roof area. The balance of the roof area was covered with a coarse mulch, and trimmed around the outside with river pebbles. Then the whole lot was covered with a sand-coloured jute matting, which serves to stop the mulch blowing away. A side effect of the use of the jute mat was to give the whole thing the appearance of a sand dune. This was a plus because it is visually in-keeping with the dune systems in the vicinity.
Watering
The green roof includes a fixed watering system. Plants are watered daily on a timer. Water is delivered to the higher (west end) and passes to an absorbent mat underneath the EcoPillows. Excess water runs off to the box gutter at the west end.
The end result
The net result exceeded our vision for the green roof. Looking out across the roof helps create delightful and surprising sight lines to the south west from both the kitchen and the back deck. It’s even partially visible from the adjoining public space, so it generates some interest.
The plants are in a very exposed location, and the winds are severe. So things haven’t grown much, if any, since they were put up in October. We’ll need to keep a close eye over summer. The result is the apparent density of plants on the roof is less than we’d expected because of limited amount of spreading.
The green roof was a bit of a gamble, but it’s paid off, and fits well as part of an exciting and adventurous home.
