How to Create a Green Roof

From reading the newspapers, surfing the internet, watching TV and spotting new “eco” buildings popping up all over the country, there can be no doubt that the environmental, aesthetic and cash-saving aspects of green roofing are fast being translated into real projects in the UK. From animal housing to garden offices, kitchens to warehouses, more and more buildings are sprouting living green roofs in the bid to support our wildlife and lower our carbon footprint.

The term “green roof” encompasses a wide range of planting styles, build-ups and maintenance programs.  Broadly speaking there are two types of green roof.

Intensive Green Roof:  A relatively deep layer of growing medium hosting a wide range of plants and possibly requiring quite a bit of maintenance.  A brilliant example of an Intensive Green Roof is the Roof Garden in Kensington High Street.  One and a half acres of garden, 100 feet above Central London, boasting 70 full-grown trees in three themed gardens and maintained by a team of professionals.

Extensive Green Roof’s:  Have a shallow layer of growing medium – as little as 2.5cm, support a less diverse plant population and need minimal maintenance – just one, maybe two feeds during the year and a spot of drain clearance.

An intensive green roof needs a pretty substantial building to support it and is usually factored in at the planning stage.  An extensive green roof however, can often be retrofitted on to existing sheds, arbours, garages or extensions.  This article aims to show you one way to create an attractive, wildlife friendly, living layer of insulation for garden structures and small buildings.  We’re using Enviromat Sedum Matting but there are several different techniques and materials and build-ups to choose from – most of them are easy to find on the Internet.

1.    BE SAFE

• Take care when working on a roof, Make sure you comply with the HSE Work at Height regulations and bear in mind too, that you may need to lift heavy materials up on to the roof, a 1m x 2m roll of Sedum Matting can weigh as much as a grown man, so installing it is not a one person job.

2.    Be sure that your building is suitable for a green roof. 

• Sedum matting, just as the name suggests, consists of sedum plants grown on to a mat for easy handling.  Whether they are on the ground or in the air, sedums need a sunny spot where the drainage is good.  
• If the roof is in a particularly exposed area, e.g. a windy hillside, it may be prone to drying out, so include some sort of irrigation system in your design – even if it’s only a hosepipe that can reach the highest point of the building.
• If it is to support a sedum roof, a structure must be reasonably sturdy. The Enviromat roof build up can weigh up to 100Kg per square metre and then you need to account for “live” loading – snow, or someone standing on the roof to spread fertiliser.
• A roof slope of between 1 and 20 degrees is best.  Any steeper and you may have problems with anchorage and/or irrigation. 
• All areas of the roof need safe access on to the roof for installation and for maintenance and the roof deck should be smooth roof deck. Corrugations or dramatic undulations can create air pockets beneath the green roof build up – and they’re not good.
• Beware too of potential drainage problems - drainpipes discharging on to the roof or a dripping tree line.

3.    Check the Waterproofing

• The Enviromat green roof build up, won’t waterproof your structure, so make sure that waterproofing is in place and in good condition before you start.

4.    Edgings

• A living green roof needs an edging, not just to hide unsightly raw edges and to protect them from drying out, but to keep strong winds from lifting the layers of matting and causing chaos.  On a flat roof, paving slabs are an ideal choice.  On a pitched roof, a timber edging can be quite effective. Whichever method you decide on, the edging must not damage the waterproofing and, it must allow excess water to drain away.

5.    Root Barrier

• Roofing felt needs to be protected from potential root damage with a layer of heavy gauge polyethylene.  The EPDM type waterproof membranes don’t need this extra layer – plants don’t feel invited to root into them.

6.    Drainage Layer

• On a FLAT ROOF i.e. a slope of three degrees or less, it’s a good idea to use drainage matting.  There are several systems available on the market; the one in our illustration is a lightweight material that simply rolls out on to the roof to ensure that the plants’ roots don’t sit in puddles – sedums won’t survive waterlogging.  Don’t worry about gluing or nailing it to the roof – that’s what the edgings are for.

7.    Always Use Water Retention Matting

• Sedum plants are drought tolerant but they’re not drought proof.  Water retention mat is a lightweight, relatively inexpensive fleece that helps ensure that the plants have enough water to stay healthy and vigorous. Water retention mat sits on top of drainage mat on a flat roof, or on top of the root barrier (or waterproofing) on a pitched roof.  On an apex roof, it pays to have a double layer of water retention matting for 30-50cm either side of the ridge where rainwater would normally run off quite quickly.

8.    Install your sedum matting

• Sedum matting is normally delivered to site rolled up and palletised.  Just like lawn turf, it will not store in the roll so it needs to be unrolled as soon as possible after delivery.  

• Plan your work so that you can avoid walking or kneeling on the plants as you install your sedum matting – in this way they will settle in much quicker.

• Sedum matting is laid in much the same way as turf – except that the rolls are larger and heavier.  Simply unroll it and push the edges up close together.  If you are using Enviromat sedum matting, you’ll find an extra flap of geotextile tucked underneath.  Unfold this and lay it out flat beside the sedum mat – the next piece is installed on top of the flap to help with anchorage.

• Use a saw or serrated knife to trim the sedum mat to size.  Better still, ask if the supplier can provide pieces in the dimensions you need – be careful not to order pieces that are too heavy for you to safely handle though.

9.    And finally…

• Make sure the edgings securely hold everything down, sprinkle on some slow release fertiliser, soak the roof with water, and admire your work.

Maintenance

Generally speaking, a well-installed, extensive sedum roof will need very little care and attention.  But it cannot be stressed enough that low maintenance is not the same as no maintenance. These plants are surviving in a very shallow layer of nitrogen-poor growing medium with no access to ground water or soil nutrients.  True, sedums are very conservative in their nutritional needs.  In fact the word sedum is taken from the Latin sedeo meaning “to sit”, presumably because these plants just seem to perch on rock faces, walls and other seemingly inhospitable places.
Nevertheless, if the client is not to be disappointed by the roof’s decline in appearance, the sedums must be fed at least once a year. Twice, if it’s been a really wet year or if the roof is steeply pitched.  At the same time, it’s a good idea to remove fallen leaves or stubborn tree seedlings and check that the drainage outlets are clear.

There can be no doubt that no matter what build-up is chosen, what planting is used and whatever the intensity of maintenance, a living green roof provides valuable wildlife habitat, insulates against heat and cold and generally improves the appearance of a building whilst helping to exacerbate flooding and offsetting some of that soil-destroying hard landscaping.  Installation needn’t be difficult or complicated – so long as the building is strong enough – and the initial expense translates into quite a low cost of ownership – did you know that a green roof will double, sometimes even treble the life of the waterproofing!  So why not give it a go? Even if it’s just a test area on top of the guinea pig hutch.

Recommended Reading:

For more information about green roofing, read Green Roof Construction and Maintenance by Lucy Huckett Or visit
www.greenroofstoday.co.uk, www.bauder.co.uk, www.Enviromat.co.uk

Q Lawns run Green Roof Information days throughout the year and also publish a Green Roof CD ROM with detailed information on designing and installing a sedum roof.  Contact Rebecca Elsey on 01842 828266 or e-mail sales@qlawns.co.uk

By Angela Lambert