Green Roof FAQ

How much maintenance does a green roof need?

A green roof is a designed and engineered part of a building, and so can have as much or as little maintenance as desired. The nature and content of the growing medium can dictate whether a roof needs just two checks each year, or intensive year-round care, like a garden.

How much does a green roof cost?

As with any construction process, cost is dependant on specification. There are many variants which need to be considered when specifying a green roof. However, indicative costs are between £60m² and £100m² for extensive roofs and £100m² to £140m² for semi intensive or intensive roofs. Wildlife/biodiverse roofs are not necessarily significantly cheaper than extensive green roofs.

Does a green roof weigh more?

A green roof weighs more than a conventional roof due to the extra weight of substrate, vegetation and their water holding capacity. This extra weight can be easily factored into the structural design of a new development. Existing buildings can be retrofitted with a green roof, but may need to be reinforced to take the additional weight.

Do I need to consult a Structural Engineer?

It is always advisable to consult with a Structural Engineer regarding any structure where people will pass underneath or inhabit the space beneath.

How do green roofs affect the life span of a roof?

The lifespan of standard roofing material in good condition is at least doubled by the addition of a green roof, thereby reducing the need for repair or replacement.  The green roof protects the membrane from corrosive UV rays and also from the extreme temperature fluctuations which occur throughout the day.

How do green roofs affect water drainage from the roof?

International research indicates that even thin (80mm) green roofs reduce annual roof run-off by  50% - 90% depending on the system and climatic zone. Green roofs therefore make a significant contribution to urban drainage and flood alleviation schemes. Moreover, the rate of discharge following heavy rainfall is delayed, reducing some of the problems associated with storm surges. With an increasing need for new developments to limit their stormwater run off, the Environment Agency now highlight the use of green roofing in their May 2003 publication “Sustainable Drainage Systems (SUDS) – an introduction.”

How do green roofs reduce the need for energy?

In the UK, more energy is spent artificially cooling buildings in the summer, than heating them in the winter. Green roofs reduce a building’s internal temperature during the summer months, reducing the need for air conditioning in the summer.  As a result green roofs also reduce CO² emissions.

Are green roofs a fire risk?

Intensive (deep) green roofs have relatively deep soils that hold a lot of water and so offer resistance to fire. Extensive (thin) green roofs can easily be designed to provide adequate fire resistance by adopting the following precautions in European FLL guidelines:
*  substrate should be at least 30mm deep and contain a maximum of 20% organic material
*  the forms of vegetation must constitute a low fire load (i.e. they provide a low volume of combustible matter that can act as a fuel to feed a fire)
*  0.5m wide gravel fire breaks should be provided around any openings penetrating the roof surface and around the edges of the roof 
*  for larger roofs gravel fire breaks should be provided at 40m spacing.

Will plant roots grow into the original roof?

It is important to include a root resistant layer in the build up of the green roof system. Roofing manufacturers sell specialist materials for commercial buildings, and pond liner material is more than satisfactory for DIY installations.

How do green roofs affect wildlife?

New developments lead to a loss of habitat.  Green roofs create green space in urban areas to maintain species vital to local  biodiversity and can be designed to address issues highlighted in Local Biodiversity Action Plans (LBAPs). In particular, green roofs have been shown to favour many rare invertebrates found on brownfield sites, as well as ground-nesting birds such as skylarks. Please see the Wildlife Roofs section (link)

How do green roofs affect the quality of life for people?

Green roofs contribute to a greener urban environment and improved quality of life for communities in high density developments as they can be used for social and leisure activities if accesible.  It has been shown that patients in hospital rooms overlooking green space recover more quickly and require fewer painkillers or nurse attention.

Does a green roof need planning permission?

It is always advisable to contact your local Planning Department and building control team to explain what you are aiming to do, especially if it is on a permanent structure such as an office, dwelling or garage. The Planning Department will then advise on what your next steps should be, in most cases where green roofs are installed on existing buildings, planning permission is not required.

Where can I buy seeds to plant my own green roof?

We have teamed up with Pictorial Meadows to supply a specially blended green roof seed mix developed by Dr Nigel Dunnett of the University of Sheffield. The seed mix offers a range of plants which look impressive and are perfectly suited to the rigours of roof-top life.

Is access to the roof needed?

Every green roof requires some degree of maintenance to ensure it is kept in top condition, so providing provision for access at an early stage is always beneficial. Often green roofs are used for recreational or research purposes and require regular access.

According to the British Standards Institution, proper access has to be provided to the roof area of all new-build developments, and inspections must be carried out twice a year. Health and Safety regulations require anyone within 2m of a roof edge to wear a safety harness. Brackets, man safe systems, edging or fall nets can all provide attachment points and protection from falls. 

What guarantees do manufacturers offer?

The major green roof manufacturers offer 15-25 year guarantees on waterproofing when it is installed by an approved contractor. Some may also offer warranties for the green roof’s plant element.

What is the typical U-Value of a green roof?

Green roofs are not included in U-Value calculations as they are considered to be continually saturated. The U-Value will be taken to be the same as the roof without the extra green roof layer. The roof will however benefit from increased protection from heat in the summer and some protection from cold in the winter.

Is there any funding available to install a green roof?

There is currently no governmental funding or incentives available specifically for green roof installation in the UK.  However, it is sometimes possible to win funding for outcomes such as habitat replacement/introduction or urban agriculture, and then apply it to a roof space.

Do green roofs leak?

No, a green roof does not cause leaks.  Only poor installation or incorrect specification of the 'standard' roofing materials will result in leaks on green roofed buildings.

When retro fitting a green roof on an existing building, extreme care must be taken not to damage the existing roofing material. Always ensure that a 'root-resistant' element is present, either as a chemical within the waterproofing or as a separate layer.

Can green roofs grow on a slope?

Green roofs can be grown on any pitch of roof, including vertical walls, however, roofs of greater than 9.5° generally have specific design requirements in order to retain the water and substrate across the roof surface. On roofs of up to 45° this can be achieved with the installation of a series of cross batons or grids which must be more closely spaced as the roof gets steeper Minimum falls of 1:60 are recommended to allow free draining.  On extremely steep roofs, irrigation may be required.