Green Roof & PV

From the Winter 2012 copy of Green Building Magazine (volume 22, No3).

Two green roof studies. One, carried out in Berlin, where green roofs had been installed in the 1980’s. They varied the PV arrays to find the most efficient:

  • Over 5 years, PV panels over a green roof Vs a bitumen roof has 6% higher yields.
  • If over a green roof AND on a swivelling stand that tracks the sun, then 10% higher.

So green roofs boost PV productivity AND as they insulate the roof, they decrease the amount of energy needed for heating and cooling.


The second study looked at planting schemes. The most popular current choice is sedum, du to high resistance to draught. Sedum was compared to Stachys byzantian, Hedera hibernica and Bergenia corifolia.

Stachys byzantina (commonly known as Lamb’s Ear) outperformed the other species in terms of leaf surface cooling, cooling the substrate beneath it’s canopy and even the air above it’s canopy during short intervals over hottest periods, when soil moisture was not limited.

The conclusion was to not choose the best plants for a shallow substrate, but those that provide the best all round environmental performance, including bio diversity. This may involve deeper substrates and some form of irrigation.

Advantages of a Green Roof


  1. Reduced energy needs. A living roof acts as an insulator, reducing the energy needed to heat and cool your home or building.
  2. Reduced greenhouse gases. Living green plants convert carbon dioxide to sugars, producing oxygen as a byproduct.
  3. Reduced urban heat island effect. The cooling effect of evapotranspiration and the lower Solar Reflective Index* of a living roof result in lower overall heat given off by the roof surface. (*SRI: a measure of the energy a material absorbs, then releases as heat.)
  4. Enhanced stormwater management. Slick, impermeable roofs shed water quickly and efficiently, contributing to both higher and faster peak runoff and flooding in densely developed areas. A green roof’s plants and soil slow both the rate and the energy of runoff.
  5. Enhanced water quality. Plants and soil in a green roof absorb and break down pollutants in rainwater. The slower flow rate of stormwater equals less erosion and subsequent sedimentation downstream.
  6. Added habitat. A living roof provides shelter and food for local birds, bees, butterflies and other fauna.
  7. Improved value and curb appeal. This is a no-brainer — just look at the pictures!
  8. Improved quality of life. Admit it: You’re happier when you’re surrounded by beauty … and I’d argue that most ordinary roofs fall in the category of blight rather than grandeur.

Rooftop Hydroponic AND Fish Farm anybody ? had an article on this prototype system that combines hydroponics and a fish farm into one unit for all year round veggies and a few fish.

The prototype Globe/Hedron “is a bamboo greenhouse designed to organically grow fish and vegetables on top of generic flat roofs. The design is optimized for aquaponic farming techniques: the fish’s water nourishes the plants and plants clean the water for the fish,” according to designer Antonio.

Rainwater harvesting

If there is going to be rainwater harvesting:

  • “Green / grass” roofs may mean little roof water run-off.
  • It seems that people tend to either go for a Grey Water recycling or a Rain Water harvesting system, not both. Have to cost these both up.

Anyway, if there is going to be a rainwater harvesting system, it would seem that putting the storage tanks for this on the west side of the house could be ideal.

  • This side of the house should have little if any people traffic.
  • The roofs could all be sloped to drain to the west so that the drain pipes could flow down to a single rain water harvesting tank.

Eco Building Products: Spring health check for green roofs

Spring health check for green roofs

An extensive green roof has the potential to bring many benefits – such as saving money on air conditioning, supporting wildlife, increasing the value of a building and improving its appearance – but none of these benefits apply to a green roof that is poorly maintained.

Most green roofs are low-maintenance, but ‘low-maintenance’ is not the same as ‘no maintenance’. A little bit of TLC carried out this spring need not be time-consuming or expensive and will pay dividends.

Enviromat offers a competitively priced green roof maintenance service, but if you prefer to care for your green roof yourself, here are some reminders of what needs doing this spring:

1. Do a quick check to make sure that any waterproofing you can see is in good order, that fall restraint systems are in place and in good condition, and that walkways, etc are as they should be.

2. Remove any rubbish or fallen leaves from the roof.

3. Ensure all drainage outlets are clear and that rain can run off freely.

4. If you have pebble edgings on your green roof, pull out any vegetation that may be growing through them.

5. Check that you do not have any unwanted vegetation on the living part of the roof. Be on the lookout for tree seedlings and pull them out – their roots could damage the waterproofing.

6. If you have any bald patches on the roof, now is a good time of year to replant these areas. On a sedum roof, simply break pieces off healthy, well-established plants and push them into the growing medium. Keep them well watered and they will soon take root.

7. Apply fertiliser. Enviromat recommends using Nutrifusion Spring/Summer feed. Follow the manufacturer’s instructions carefully.

8. If no rain is forecasted, give your green roof a nice long drink of water. This will activate the fertiliser and help new cuttings to establish.

Green Roof

The Silver Spray plan includes some flat roof areas that will have a “green roof”.

There were a wide variety of these at the  Ecobuild show. It seems there are lot of options in terms of:

  • What plants are in the green roof. All the way from tall grasses, shrubs to lower and thinner sedum matting.
  • The base system that the green roof sits on and in. These range from egg cup looking sheets to matting that the plants grow into.
The main reasons, beyond aesthetics seem to be the  biodiversity increase, sound and thermal insulation and rainwater absorption (with slow release).

I asked Robert from ra-studio about his experience with green roofs:

I have done a couple, one in Cornwall (Sea house) and one on the Lancashire moors!

Green roofs are often used to combat heavy rainfall, and stop massive water run-off into the surface water sewers.  With more and more people removing garden spaces (lawns etc) and replacing with hard paved terraces, the green roof system acts as a way holding the water back and allowing it to drain into the RW sewers far more slowly.

We used a Bauder roof at Sea House.  Bauder do basically 2 types of green roof system – an intensive and extensive system.
The intensive system is a fully blown grass roof system that allows you plant lawn, shrubs, trees etc up there, and due to the thickness of the soil (normally a min of 250mm thick), it is quite a heavy build-up.
The extensive system is basically a sedum mat that is the thin / lightweight solution.  It uses sedum / succulents in the form of a sedum mat (approx 25 – 30mm thick), and although the plants on the roof will take up some water it is far less that the full grass roof.
There is a drainage layer under the sedum mat, and any excess water that the plants don’t take up, is released into the drainage system.  See Bauder’s website

Roof Insulation

Under the green roof, there will still be thermal and water insulation. This a lot of other products were at the Ecobuild expo, “Stone Wood Slabs for a flat roof”.