Roof Top Garden

After along time away it's time to share my experience of putting grass on a concrete slab and building a roof top garden....

The key to a successful roof garden is to ensure that you can satisfy the following:
 - have a strong enough slab to support the weight
 - have a sealed slab to ensure you don't create a damp problem below
 - have adequate drainage so you don't create a swamp
 - have adequate watering so it doesn't dry out

You need a free-draining soil which holds water - an oxymoron if I have seen one!

So why build a roof top garden?

Easy - lack of space, great insulator and grass feels damn good between the toes!

Following are the stages I went through to get it right...

Step 1. Design
I contacted Earth & Water to get a basic design and also to provide me with some VersiCell and Geotextile Fabric. Earth & Water can provide the whole installation service however I decided to build it myself to keep costs down.

"VersiCell is a lightweight, high strength modular drainage cell designed primarily for sub-surface drainage and waterproofing membrane protection or to form tanks for underground storm water infiltration or storage." "VersiCell has a 98% void area to total surface and the highest possible absorption rate anywhere along the module, ensuring continuous discharge."

For the soil mix and the soil strata layer FytoGreen provided me Hydrocell flakes and also the Hydrocell RG30 foam layer.

The design looked like this - with and added layer of solid Hydrocell RG30 foam on top of the Geotextile Fabric:

And in plan:

Step 2. Decking
The first physical work was to build a wooden deck around the grass area and give the required 250mm elevation above the slab for the soil and grass. Reticulation had to be built and pre-plumbed under the deck and an Edge Board was added to give the soil something to butt up against.


Step 3. Slab
Because the structure was in excess of 10 years old it was obvious that the slab was free draining and the roof had adequate draining off of the roof. The slab itself was well over engineered and was 350mm thick.

Step 4. Waterproof Membrane
To keep costs down I opted for a double layer of heavy duty builders film to provide the drainage. The membrane was duct tapped at the edges and penetrated underneath the Edge Board ro ensure that all run off made it out and under the deck.

Step 5. VersiCell Layer
The VersiCell layer is laid directly on the waterproof membrane and forms free drainage.

Step 6. Geotextile Fabric
The Geotextile Fabric must be laid on top of the VersiCell in order to stop and soil fine penetrating  and blocking, the drainage. The fabric must be laid up the sides of the Edge Board and tacked into place.

Step 7. Hydrocell RG30 foam layer
The Hydrocell RG30 foam layer acts as a water reservoir for the garden.

Step 8. Soil
The soil mix was as follows and was mixed using a cement mixer:
 - 0.6m3 of 12mm cracked pea gravel (18.8% v/v)
 - 1.4m3 of the Landscape Mix (44% v/v)
 - 1.2m3 of the Hydrocell flakes(37.5% v/v).
Total 3.2m3 which is 25% greater than the measured volume, but you need to account for particle integration loss and compaction. Weight is about 2T.

The soil was then carried bucket at a time up two flights of stairs to the roof. Thank you friends!


Step 9. Compact
Compact the soil with a roller


Step 10. Turf
Lay the turf on the soil and water in. I opted for Buffalo as it's hardy and with the extended exposure I think it's needed.


Step 11. Have a beer and watch it grow

Update:
Three months later and we are moving into summer. The grass is growing extremely well and the room below is now noticeable cooler!