Monday, November 29, 2010

Lightweight, Deeper Soil Rooftop Gardens for Permaculture

We planted flat-leaf parsley, Petroselinum crispum; and sage, Salvia officinalis on a small, 3' x 12' ( 0.9 M x 3.7M) green roof section this weekend.

However we incorporated two different design criteria.

Roof Permaculture System Design
1. We used multiple layers of the root mat to create a six inch thick soil-growing layer, and

2. We overlapped the mat to create permaculture type swales.

The system is on a sloped roof with a root and water-proofing membrane over asphalt shingles.

Organic flat-leaf parsley and sage was obtained from Judy's nursery stock and planted.

A well drained soil mixture was used.

Photos will be posted as the roof-top food crops continue to grow.
Roof Permaculture - Food Growing on Green Roofs

We are using our same system that has successfully passed hurricane wind simulation of 130 MPH.

As always, email your questions and Happy Green Roofing!



Jorg Breuning said...

Interesting project. Do you need all the black plastic stuff exposed to the elements? I heard from studies that these material won't last very long when exposed - only 5-10 years. Especially in Florida the sun intensity is extremely high.
I saw that wind testing was done on a flat panel (not on a sloped) and it was also done without the heavy rain that typically come with these events.
What were the criteria for choosing the mentioned plants? jbi

Kevin Songer said...

To answer your questions Jorg - the black plastic waterproofing membrane is to be trimmed from around the edges of the bed frames in real life. These are par of our ongoing trial roof beds and I like to see how UV exposure impacts the material - so we leave a little exposed. Now, the black polyethylene weave is covered with the planting medium and is not exposed to the sun in field applications.

The weave offers support for the plant roots on severe slopes and holds the soil in place until the root systems develop. The weave also affords a measure of drainage.

You are absolutely correct, any plastic exposed will degrade over time. Installations should have all components covered.

The wind testing was done on a 3:12 slope which is typical of Florida roofs.

The highest uplift shear from a windstorm comes during periods of no rainfall. During a hurricane you will have periods of hard wind and rain, then periods of wind only. The rainfall holds down objects to a certain degree while 130 MPH wind speeds - well - I was talking to a roofer yesterday about three large HVAC units that were ripped off the roof of a Holiday Inn in Clearwater, Florida during a storm event.

Rain also adds weight to the plants and soil, further holding the material down. Though the hurricane simulator has water spray capability for leak testing, the wind shear and uplift testing is done without the water.

Interestingly, during tropical storm Fay several years ago, I filmed a brief clip of 60 MPH gusts on a green roof - the clip is here on the blog archives - I'll post a link to it again.

Your discussion brings up several important factors though - roofing stone ballast and aggregate are being discussed for elimination within many roofing and building codes due to the potential for shotgun like damage in a high wind event, if the aggregate is picked up by the wind. Soil composition and engineering, I believe is very important to the future of green roofs for arid, dry and windy climates like Florida's. Would love to hear your thoughts and experiences on soils for extensive green roofs.

As to the criteria for plants, I usually suggest the following: 1. requires little or no additional irrigation - let nature irrigate with dew, fog, recycled rainwater and more. Potable irrigation water, even if it is a backup system is anti-sustainable when used on a green roof - in my opinion. 2. green roof plants must have a low stomata to leaf surface area ratio because the hot, dry winds here will desiccate a plant in a matter of hours on a roof, 3. must be resilient to the many fungi proliferating during the rainy season here (sedums most always fall prey to Southern Blight during the summer humidity), 3. need to be low in volatile oil content, 4. should be capable of withstanding hard freezes, 5. typical leaf surface temperature in the summer on a Florida green roof plant reaches 140 degrees F to 150 degree F (60-65 degrees C), and more!

I like the agaves, yuccas, alliums, some succulents, optuntia, few native grasses (watch the leaf litter) and then the annuals, food plants and fun additions to a green roof - mixing structure with color and food.

Thank you for your comments and I look forward to reviewing your websites!