Green Roof

There are three main types;

• Extensive
• Intensive
• Semi-intensive

Extensive green roofs have a thin growing medium and are planted with sedums that retain moisture. They usually require no maintenance, except for some initial irrigation and are generally less costly to install.

Semi-intensive (semi extensive) green roofs have a greater depth of substrate to allow planting of grasses and medium size plants.

Intensive green roofs have a deep growing medium, which allows the use of trees and shrubs. Some city parks are in fact intensive green roofs and are on top of large buildings. The depth of the growing medium requires extra loading requirements within the holding structure and requires a complex irrigation system for maintenance. They are generally quite costly and require extra structural design to the building.

The first consideration in creating a green (or brown) roof is to ensure that the structure can support the extra weight of the soil and plants, especially when wet.

Expanded clay pellets are very light and allow the substrate to drain freely. It is a perfect medium for growing alpines and grasses. This is best mixed using a concrete mixer if it is a large amount.

You will need planks to spread your weight, whilst doing the planting. Bulbs are perfect for a spring flower display with many alpines flowering in the summer. The grasses tend to self seed and will eventually create a more naturalistic planting. They also add movement in the breeze. Sedums spread and fill all the gaps over time. Semi-intensive roofs (semi-extensive) offer a more bio diverse habitat than simple Sedum roofs.

Brown roofs usually use spoil from a local garden or building construction and are left to colonise freely to create a biodiversity habitat. The bare substrate is just left to let nature take its course. Log piles, gravel and even small pond areas. This is probably the least expensive way to create a living roof.

Flowering meadow seed mix is also an inexpensive way to create a living roof. A substrate low in nutrients is required for this. I would recommend a 90% recycled crushed brick and 10% compost mix. The most important factor for a newly planted roof is to irrigate for the first season. A small pump from a water butt and a drip system is good.

For my own green roof project at home, on a new extension an EPDM liner was glued to the ply board. A root barrier and membrane was then laid. I am currently experimenting with used car tyres which will hold a reservoir of moisture on a slope and create a framework to stop the soil slipping. The top of the tyre is cut out to create a container. You can make a timber grid that will stop slippage but will need a drainage layer with moisture pockets.

There are plenty of products on the market, most are recycled plastic. I am also using different substrates. For example as part of my business I collect spent sloe berries (a by product of organic liquor manufacturing) these are then left to compost. I have found that the hard seed takes a long time to decompose so is perfect for light weight compost with good drainage. I am mixing this with recycled type 1 for sedums and alpines.

For drainage around the perimeter, recycled aerated block topped with gravel is used. Waste from the building process will be spread to create a brown roof. I will be introducing a few wildflower plugs from the garden to get it started; the rest is up to nature!

By Mark Tolfree of Gabriola Garden Design

Click Projects below to view details

GRFGGD001 - Green Roof Shelter Over Seating