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Can a green roof and solar PV system work well together?

Posted by Jason Ramsdale on 20 January 2016 at 10:05 am

What is a green roof?

A green roof is a roof fully or partly covered in vegetation. It is a layered system. You'll need a strong, waterproof, roots-resistant membrane at the bottom, then a growing medium like soil, then finally the vegetation on top. In some climates they also include an irrigation system to water the vegetation but this is not needed in the UK.

Benefits of a green roof:

  • Provides insulation helping to keep your home warm in winter. Helps to keep home cool in summer through evaporation and transpiration
  • Reduces noise and air pollution
  • Increased biodiversity, provides a habitat for invertebrates and ground nesting birds
  • Helps to lower peak rainfall runoff; reducing the risk of floods
  • Provides protection for your roof against wind, rain and sun
  • It looks nice!

What type of plants are used?

The most common and suitable choice is a sedum roof; sedum is a type of plant that stores water within their leaves making them very drought resistant. As they grow across the ground rather than upwards they can be easily installed as “mats” and require very low maintenance.

Are green roofs compatible with solar PV?

Absolutely, research has shown that the cooling effects of green roofs can improve the power output of PV systems significantly. [1] Plants take in water through their roots and release water through their leaves through a process known as transpiration. The process of transpiration, like evaporation, requires an input of heat. This means as your green roof transpires it can lower the air temperature around the panels, potentially improving their performance by as much as 10%.[2]

A combined solar PV and green roof also has the potential for increased diversity as they will provide variations in moisture, sunlight and wind exposure. This can lead to a greater biodiversity when compared with a fully exposed green roof with no PV.

As soon as our blog went live, one of our recommened installers Stratford Energy Solutions Ltd tweeted their approval along with a recent case study:

Broughton Community Pavilion green roof and solar PV

This green roof and solar PV hybrid is part of a new eco-friendly community centre in Broughton, Milton Keynes. We thought it looked great and so have added it to the blog!

Are green roofs only for buildings with a flat roof?

No, while it may be easier to imagine a roof-top garden on a flat roof, there are products on the market that can allow for green roofs on roofs with up to 45 degree slopes (e.g. Optigreen). For really steep roofs an anti-slip system can be used which has plastic sills to hold the soil in place.

Are there incentives available to install a green roof?

Biodiversity and habitat funding streams might help groups attract grants to support a green roof on a community building. However, homeowners are likely to have to fund it themselves.

Do green roofs damage or protect the existing roof?

Providing that the green roof installation includes a root resistant layer, to prevent roots penetrating into your existing roof, it should protect your existing roof from the elements.

Sources:

[1] Koehler

[2] The Green Age

Photos:

PV and green roof W.L. Talbert

Sedum anglicum Joan Simon

 

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