Hampshire County Council

Buildings Land and Contracts Panel Item

24 October 2003

Notice of Motion: Green Roof Technology

Report by the Director of Property, Business and Regulatory Services

1

Summary

1.1

The following decisions are sought:

That approval be given *

2

Reason(s)

2.1

*

3

Other options considered and rejected

3.1

*

4

Conflicts of interest declared by the decision-maker or a member or officer consulted

4.1

Not applicable

5

Dispensation granted by the Standards Committee

5.1

Not applicable

6

Reason(s) for the matter being dealt with if urgent

6.1

Not applicable

Approved by:

..........................

Date of decision:

.........................

Councillor T K Thornber

Hampshire County Council

Building Lands and Contracts Panel Item *

24 October 2003

Notice of Motion: Green Roof Technology

Report by the Director of Property, Business and Regulatory Services

1

Introduction

1.1

Councillor Bill Wheeler raised a notice of motion at the County Council meeting on 24 July 2003 regarding Green Roof Technology. This report provides a response to the Notice of Motion and highlights a range of issues associated with green roof technology.

2

Notice of Motion

2.1

The following Notice of Motion was raised at the County Council meeting on 24 July 2003:

"This County Council being concerned with the future of both the urban and rural areas of Hampshire and the development issues of air and water quality and storm management asks:

That our architects look into Green Roof Technology to see if it would be suitable for:

(a) buildings that we are designing for public use;

(b) influencing other architects building within our county

The benefits to the County would be:

(1) increased areas for insects and bird life to thrive;

(2) to help control the water flow in the long term leading to fewer

incidents of flooding".

3.

Background

3.1

The County Council's architects have, where appropriate, specified green roof technology on a limited number of buildings over the last ten years or so. A good recent example of a suitable and relevant use was on Admiral Lord Nelson School (now owned and managed by Portsmouth City Council) designed before LGR in 1997. This project employed green roof technology as an essential element of the design to obtain planning permission on this environmentally sensitive site.

4.

Definition of Green Roof

4.1

The term "Green Roof" can, and has been used in the past to describe a variety of constructions ranging from fully subterranean accommodation overlaid with conventional soil structures, through to containerised planting schemes placed upon conventional roofing solutions. Even more conventional concepts such as thatch or timber shingle roofs can be broadly considered as "green roofs", under the parameter that they are principally constructed of naturally occurring and sustainable materials.

4.2

Within the context of this report, the term "Green Roof" has been used to describe an elevated and waterproofed roof structure, which is essentially covered with a veneer of soil and some form of planting, to provide additional and perceived aesthetic and environmental benefits.

Green roofs can be broadly categorised into two major types:-

(a) Intensive Roofs: The creation of more formal landscaped gardens including provision of small trees, shrubs and low planting, which are intended to be accessed by the building occupants for leisure purposes. Intensive green roofs impose significant additional loads onto the building structure, typically weighing in excess of 1 tonne/M².

(b) Extensive Roofs: Primarily installed for aesthetic and environmental benefits, with access limited for the purposes of maintenance only. Typically, extensive green roofs will have between 50mm and 100mm of soil and will exert loadings of between 100kg - 200kg/M².

4.3

Plant selection is one of the most visually important factors of the green roof composition. Many options are available and their success is dependant upon a whole variety of factors. The most common selections incorporate selected grasses, which serve to stabilise the soil structure with their complex root structures and varieties of "sedum", which possess large, fleshy leaves that can store water and which are extremely hardy, even in the most extreme conditions. Recent initiatives have seen the development of moss covered roofs. Moss has good properties for the storage of water, but needs to be kept shaded or it will rapidly fail in hot, dry conditions, adding significantly to fire load across the roof covering.

5.

Advantages of Green Roof Technology

5.1

The dense development of urban conurbations can give rise to barren deserts of concrete and tarmac, which prevent rainwater from naturally infiltrating the sub soils. As well as being considered visually unattractive due to the general loss and lack of greenery, these surfaces absorb and store energy from the sun during the day and release it during the night. The resultant effects are increased storm water run off and drainage problems, significant differences in temperature levels across urban and open areas, recognised as the "urban heat island effect", and a general loss of habitat for wildlife.

5.2

Green roof technology can help to address these problems by restoring bio-diversity and offers the following perceived advantages:-

5.2.1

Water Management: Rainwater falling onto open land is either absorbed into the aquifers, or is taken up by plants and returned to the atmosphere by transpiration from their leaves. There is virtually no surface water run off. However, in densely developed urban areas, this trend is significantly reversed. Only small amounts infiltrate to the aquifers or are processed through transpiration. It is estimated that upto 75% can be discharged as contaminated surface water run off and this needs to be managed with sewers, storage and treatment processes, or it will cause flooding and will be discharged, untreated, into streams and rivers, where it will cause pollution. It is proffered that green roof installations can help to ease this situation by absorbing and storing a significant percentage of the rainwater in the soil and plant layers, where it can be naturally processed by filtration, evaporation and transpiration.

5.2.2

Energy Efficiency: The provision of a green roof layer serves to provide a dynamic insulation layer, which will help to retain heat loss from within the building during cold weather and defend against solar gains during extremely hot weather. In essence, the associated layers of a green roof build up act as a storage layer, which modifies the response of temperature effects, leading to lower atmospheric ozone production.

5.2.3

Fabric Protection: Inextricably linked to the energy efficiency benefits outlined above, the provision of a composite green roof build up serves to protect the building fabric from direct exposure to the ravages of extreme elements such as the wind, rain, ultra violet degradation and thermal variations. Such aspects may serve to prolong the service life of the building fabric and reduce life cycle costings.

5.2.4

Urban Ecology: Green roofs can help to provide green space and wildlife habitat which is both of visual amenity and benefit to the inhabitants of urban developments. In addition, the existence of plants can help to improve air quality by increasing humidity, filtration of dust, reduction of temperature and replacement of oxygen.

6.

Disadvantages of Green Roof Technology

6.1

Whilst there are many advantages of green roof technology, it is important to consider and review the disadvantages.

6.1.2

Imposed Loads: Irrespective of whether "intensive" or "extensive" roof solutions are adopted, green roof constructions impose greater loads than more conventional roofing systems and therefore require stronger supporting structures and foundations.

6.1.3

Roof Leaks: Roof membrane failure in green roof systems has been a major cause of reluctance to adopt this form of alternative construction and modern technology has endeavoured to reduce the incidence of failure. However, failure of the waterproof membrane due to faulty materials and workmanship, or root damage, remains as a significant problem.

6.1.4

Maintenance: Compared against conventional horticultural schemes such as manicured lawns and planting beds, extensive green roof solutions can be considered as relatively low maintenance. However, when compared against conventional roofing solutions, the input required is exceptionally high. Particular attention is required during periods of initial establishment, when seeds and plants will require regular irrigation, weeding and feeding with an appropriate release fertiliser. Once established, maintenance levels can be reduced, but regular weeding, trimming, irrigation during drought conditions and general husbandry, is an essential legacy. In order to comply with the statutory requirements of the CDM regulations in respect to roof access etc. additional cost would be attracted in the provision of sophisticated safety access equipment.

6.1.5

Fire Load: Unless a scheme is provided with an expensive automatic irrigation system, green roof planting can die back in periods of extreme drought and present a serious additional fire load and arson target. This is particularly relevant to the use of grasses and mosses, which are highly dependant upon regular irrigation to sustain chlorophyll levels.

6.1.6

Wildlife Infestation: Whilst promoters of green roof concepts herald the attraction of birds and butterflies as a benefit of this form of environmental technology, green roof systems are not necessarily selective in their attraction properties and can serve to entice some forms of wildlife which may be considered as a nuisance by building occupants.

6.1.7

Cost Considerations: Due to the composite nature of the roof build up, additional structural requirements and labour intensive construction and maintenance elements, green roof installations are generally considered to be more expensive than alternative conventional roof constructions. "Spons" price guide 2003, advises that initial cost for a landscaped roof finish would be in the range of £134 -£163/M² compared with £26 - £30/M² for a conventional three layer, high performance felt system. These cost estimates are for the roof finish only and make no allowance for any additional structural requirements to support the additional loads.

6.1.8

European Climate: Although generally described as "moderate", The European climate can range from periods of extended drought and soaring temperatures of 30⁰C, through to periods of extreme cold and frost, with temperatures well below freezing. Planting sustained by green roof systems are significantly more sensitive than conventional planting regimes which have more diverse and abundant resources to sustain their development, therefore, the appropriate selection is critical and limited in diversity to only the most hardy of species groups.

7.

Influencing other architects

7.1

In influencing other architects' buildings in Hampshire, the real issue is securing the confidence of clients who are commissioning new buildings to use this relatively new technology. As indicated elsewhere green roofs have a higher capital cost and are more onerous in maintenance terms. They also create a potential fire risk if not properly maintained. Issues like this have led to green roofs being used relatively little and perhaps only where driven by aesthetic or visual criteria. By contrast, in Germany, where there has been legislation and financial support in favour of green roofs, a significant proportion of new buildings are constructed with them.

8.

Conclusions

8.1

Green roof technology is an area which is well understood by the County Council's architects who have, where appropriate, put this to good use, albeit in a limited way. The design of a new dormitory at the Minstead Study Centre is currently being completed and it is intended to include a green roof along with other sustainable initiatives.

8.2

There are advantages in providing a better habitat for insect and bird life as well as potentially helping to reduce surface water run-off. In relation to the County Council's estate, the area of a buildings roof is usually a very small proportion of the site area and therefore the composite effect is very small. For example the school estate has significantly more site than building area and introducing improved drainage arrangements to playing fields and play courts would have a bigger impact on flood risk. Provision of habitat and landscaped areas greatly improves opportunities for bird and insect life to thrive.

8.3

The selection of appropriately robust, economic and visually pleasing

roofing materials are always a key issue on any new building project.

Should future opportunities arise where costs, site and design parameters

are suitable then Green Roofs will be considered.

Recommendation

That the Buildings, Land and Contracts Panel advise the Executive Member for Policy and Resources that a recommendation be made to the County Council that Green roof technology be considered for future projects where costs, site and design parameters are appropriate.