We get many questions on Green Roofing and Living Roofs and here is a list we believe should simplify things for you.
What is the Difference between Extensive and Intensive Substrates?
Extensive is the most popular substrate / media mix for green roofs and consists of 80-90% of inorganic materials (crushed brick/pumice/L.I.C.A.) and 10-20% of organic materials (compost).
Intensive is the choice for roof gardens (See below for details on green roofs vs roof gardens).
The term Extensive means "covering or effecting a large area"
The extensive mixture is designed for low maintenance hence its popularity as most roofs are difficult to get regular access to for watering and weeding. The extensive roofs shouldn't need any watering during the year, however this will depend slightly on your planting choice and how hot and dry the summer becomes. Use common sense as you would with your garden at ground level, if it looks like it needs watering, water it. In some commercial applications contractors install a miniature roof at ground level with the exact same elements as the higher level roof to give an indication of how it is performing.
The intensive substrate mixture (circa 20-30% inorganic and 70-80% organic) is designed more for roof gardens when access is easy and you can take care of watering and maintenance as you would a normal lawn area.
Do I need a Drainage Board / Layer?
This depends on your roof. Many experienced green roofers and designers love to argue about the need of a drainage layer on a green roof, we tend to use the popular German FLL standards in the UK however the Austrian standards that do not favour drainage/water retaining boards are very successful also. (In later posts we will serialise these two respective standards).
If you are doing an extensive roof and the roof has a slight pitch on it, circa 1 to 40 fall ratio, then you can get away without a drainage layer. This is because drought tolerant plant species such as sedums do not like too much water or sitting in water. If you are doing a roof with a fall of 1 to 5 ratio you would need to consider some kind of retention board that prevents slippage.
You can also use the drainage layer as a water storage board too. There are various types of drainage layers from gravel to specialised plastic systems.
This is unless you are creating a wetland roof in which case you are looking for more water storage and this design should be carefully approached and even consider an irrigation system to maintain moisture.
Normally commercial living roofs require a water retention/drainage layer as part of the design.
What maintenance is needed?
This depends on the type of living roof you go with.
Drought tolerant species such as Sedum's are the lowest maintenance as they can survive in drought conditions. All they need is a light hand weed once a year for wind blown seeds if you are concerned with these.
Wildflower green roofs will require weeding once or twice a year and its best to cut them back at the end of autumn and remove the cuttings to retain their appearance.
To create biodiversity in a living roof its best to leave them to do there own thing.
You can create living roofs that use grass and depending on the finish you are looking for it can require mowing. If you want the green roof to be neatly mown make sure there is easy access or alternatively look at installing a robotic mower for the roof.
What is the lowest maintenance green roof?
Extensive roofs are the lowest maintenance, they can survive with little water and a low amount of input.
What should I put on my green roof?
This is totally up to you. The most popular are Sedums or Wildflowers. Our Sedum and Wildflower mat gives the best of both systems. If you are totally unsure contact us and we can advise further.
Is Sedum the best for a living roof?
This depends on what you want from a living roof. There are advantages to all types of living roofs.
How do I know if the roof is strong enough to have a green roof on?
This is a job for a professional. Your local building control officer should be able to handle this enquiry or an experienced architect, building surveyor or structural engineer will be able to inspect your roof to see what loading it can withstand.
We can advise on the weights of different systems when completely saturated with water (the worst case scenario).
If it is a new build project you will be able to make sure that the joists and structure are strong enough for whatever living roof you want.
If its an existing building you may be limited by the structure and joists as to the loading that you can put on the roof. If the structure or joists aren't strong enough you will be able to talk to an architect, building surveyor or structural engineer to see how you can upgrade the roof to be able to take the weight of the system you want.
Do I need planning permission?
This depends on your location and its always best to check with your local planning office.
Some districts require certain criteria to be met when a green roof is installed which usually involves a border of pebbles or gravel around the edge of the living roof.
What are the benefits of a green roof?
There are many benefits of a green or living roof.
- Attracts wildlife by creating habitats.
- Wildflowers attract and help bees.
- They also catch heavy metal particles from industry.
- Provides extra insulation on the roof which keeps the building cooler in summer and can reduce heating costs in winter.
- Provides sound proofing.
- Can add value to your property by prolonging the life of the waterproofing membrane.
- Provides storm water retention – it helps release water slower than a normal roof. Helping to reduce flash flooding which is part of the governments S.U.D.S. (Sustainable Urban Drainage Systems)
- Plus it looks great!
How can I attract the most wildlife?
The best living roof for wildlife is to create as many micro habitats as possible.
This can include some of the following ideas:
· Bee hives.
· Logs and stone/bricks for critters to live under and around.
· Bird feeders.
· Wildflowers for the bees.
· Shaded areas.
You can also create other types of habitats such as wetlands. This along with meadow areas have been reduced in the wild so creating these types of habitats are vital.
Can I make a wetland living roof?
Yes, its a new way of creating a living roof. There is more strain put on the roof so you need to contact an experienced architect, building surveyor or structural engineer to make sure that you can put that amount of load onto the roof. You also need to make sure the waterproofing materials are happy to sit in water. The waterproofing options we recommend are perfect for it.
What is the best waterproofing for green roofs?
We recommend using EPDM for green roofs. Our systems carry independent certification that the membranes and essentially the joints are suitable. Many people recommend using pond liners and cheaper alternatives but a green roof is something that is built to last so why put something substandard on. The easiest systems to fit and install are the UK Flat Roofing EPDM, HERTALAN EPDM and PRO-EPDM. We always recommend using the PRO-EPDM system as this is the highest quality and thickest EPDM on the market.
How easy is it to install a green roof?
Green roofs are reasonable easy to install. It is just a series of layers and the hardest part is getting the right information to achieve the right depths and of course getting all the layers onto the roof.
Which is easiest to fit a modular green roof or a living roof kit?
For someone with no experience the modular system is the easiest to fit. It is quick and easy to install however many people also enjoy taking their time and building the living roof as you would enjoy building your own garden.
How long will a green roof last?
With 70mm of extensive soil substrate there are enough nutrients in there to last circa 30 years.
What is the difference between a green roof and a roof garden?
A Green Roof is a roof that is normally not easily accessible and you wouldn't be using the space as a garden/terrace. Usually uses extensive systems.
A Roof Garden is an area that typically has green roofing materials on but is used as a garden / terrace. You need a different build up on the roof with deeper intensive substrates and different drainage layers etc. Contact us for a roof garden build up.
The extra substrate needed for a roof garden will mean you need to allow for much more weight on the roof. Professional help should be sought from building control, an experienced architect, building surveyor or a structural engineer to work out the weight that the roof can take or can be designed to take.
How much Substrate do I need?
Sedum's can survive in drought conditions and need the smallest amount of substrate on a green roof. We recommend using 70mm of extensive substrate or more.
Wildflowers need more substrate 100mm as a minimum ideally. The more substrate you have the more variety of wildflower you will have on your living roof.
To create further biodiversity in a green roof you can mound up the soil in areas to encourage burrowing invertebrates.
Adding logs and reclaimed bricks can also add to the biodiversity of your living roof.
When checking with a professional ask them about point loads to see if your roof is ok for adding a biodiversity living roof.