In our last extract from the Sustainability Lounge debate the business case for building vegetation on tops of structures is debated

A lively discussion on the pros and cons of building green roofs took up a chunk of the second day of Sustainability Now. Here's a flavour of the debate, kicked off by expert Alun Tarr from Blackdown Horticultural Consulants.

The opportunity

Alun Tarr: It has been estimated that over 200,000,000m² of UK urban roof space could be greened/browned/planted without significant structural change. The environmental benefits could be huge and multidimensional and without 1m² extra land take...why aren't we doing it!!

Phil Clark: Good point Alun. How much would this cost approximately?

Alun Tarr: do you want a m² price or for the whole lot!!....There would be discount for quantity!! Based on 1000m² low pitched in SE Britain around £40-50.00 per m² S+F excluding the waterproofing and sub layers.

Elizabeth Waters: Hmmm. I'm not convinced green roofs wouldn't significantly affect the building structurally, they are very heavy when compared with Aluminium or similar. When you say "urban" is that housing and non-housing? And there are added maintenance costs to consider...

Alun Tarr: Hi Elizabeth. We plant on top of 1.2mm standing seam roofing as one option. We are talking about systems with SATURATED (worst case) loadings of under 100kg per m² i.e. in the area of paved/ballasted roof build ups. With regards maintenance, this will be either to do with maintaining roof function (drainage etc) and with regards green roof performance which will be driven by how the client wants that roof to perform - long and shaggy...low maintenance, tight and manicured...high maintenance. Maintenance issues can be long and convoluted but essentially this should not be a major stopper on implementation.

Elizabeth Waters: Thanks for the clarification Alun. One of the projects I've worked on recently has just declined a green roof in favour of Aluminium and greywater harvesting on the basis of loading and maintenance costs. The dead-loading on the walls virtually doubled, and meant that we had to double the materials to compensate. I don't know if we were just poorly quoted, but we seemed to be getting similar results from each of the Contractors invited to Tender on the project.

Alun Tarr: In past times we used to see a significant number of proposed green roofs drop out during the project development stages. I would be really interested to know how many dropped out at early concept stage because ill-informed engineers and QS's made the wrong call. What is exciting is that as each benefit of implementation of a green roof installation is fully taken on board that green roof becomes more and more enmeshed in the whole project infrastructure.

Phil Clark: What kind of statutory requirements are out there for green roofs?

Alun Tarr: With regards stuatory requirements/ green roofs as a 'tool' to improved building performance this will be driven by Storm water attenuation benefits of green roofs (EA recognised benefit and inclusion/allowance within site run off calculation. Habitat loss mitigation - creation of roofscapes that actually provide meaningful habitat for targeted species and policy that links individual roof space together. Reduction in building Carbon Footprint - Green Roofs very significantly reduce heat sinking in hot/Summer climates...reducing Air Con requirement is already perceived in Mainland Europe and certian areas of the US as a major benefit. big current drivers in the Uk will be storm Water attenuation and habitat loss mitigation....interesting that 10 years ago main driver would have been 'reduction in viusal impact'!!!

Della Pearlman: I would be interested to see independent research on green issues - e.g.costs vs.payback, whether some technology is just "green wash" e.g. wind turbines in urban areas with little wind, or whether there is a cost benefit involved. Sorry to keep on about costs, but in the long run clients need to see the financial imperative as well as the green reasons

Alun Tarr: I would agree Della. Care needs to be taken with this approach as the raft of benefit can be missed if involved parties are only making descisions on single issues

Melanie Thompson: Of course, as fuel costs go up the payback on green measures will improve, so it's not cut and dried.

Cost conscious

Elizabeth Waters: Alun, if we (being the Consultants or Architects) wanted to keep down the cost for the Clients, and, as you say, without the benefit of specific technical knowledge on green roofing, how would you recommend we proceed? At the moment, our mindset is take half the money from the cost of a green roof and plant more trees around the site, with a similar effect to the green roof and ground level ecology, but without the knock on costs. Most of our clients are reluctant to employ a specialist consultant, again, due to the increased cost.

Mark Elton: But that would miss many of the benefits of the green roof in the first place such as stormwater retention, summer overhetaing etc. Offsite greenery still wouldn't give the benefits to the property that a green roof would ie prolonging life of roofing membrane, summer overheating.

Alun Tarr: The ground based planting/landscaping wouldn't improve the buildings thermal performance or limit the instant storm water run off from the bare roof. Bare roof is wasted resource. I agree wholeheartedly that altruism is a dangerous element to have to rely on and it must be linked to good evidence and where possible economic benefit...even though that economic benefit may be due to 'green roof it or no go'. With regards keeping the cost down elizabeth, often the cost goes up with Lightweight systems as the desire is for instant green solutions which tend to be expensive. I would always advocate plug planted or seeded roofs rather than pre-grown blanket build ups. although they take longer to develop they are usually cheaper, more diverse and provide better habitat.

Gardener's question time

Mark Elton: Alun, I have two small green roofs on my house at home, now 2 years old. It seems as thought the sedums get smaller each winter. Is this because they need feeding or because birds are eating them? Incidentally they are bright red with yellow flowers at the moment - not green at all!

Alun Tarr: With regards your Sedum roof let me have some info on the build up. A problem with Sedum roofs is the general belief that they are bullet proof and can survive on nothing. birds do not eat them although they sometimes pull away at random stems. Let me know what the build up is and where the roof is situated (bit more than 'on my house' would be helpful).

Nick Reilly: Well designed ground level ecology can also provide major passive heating and cooling benefits depending on the specific site. If you can't justify both, the ground level work may have more potential to give you an energy pay back?

Alun Tarr: Been good speaking to you all. Have to log off now. If any further dialogue is of interest or more in depth answers required pelase contact me through our web site