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Creating a living roof - a roof that is made up of grass and/or shrubs planted in a layer of soil - represents one of the biggest construction challenges. The supporting structure has to be incrementally stronger to carry the extra load of soil and plants. And the roof also has to be extremely carefully waterproofed because, once the 'garden' is in place, there is little chance to fix any leaks.

However, a living roof is also one of the most environmentally friendly elements of a building. Properly designed, it can reduce to almost zero the physical footprint of the building, since the area of soil and vegetation on which the house stands is now on the roof.

The 'normal' way of creating a living roof involves the use of a heavy rubber membrane such as a pond liner. I am struggling to find an alternative that fits in with the principle of using environmentally friendly, low-embodied energy materials that can be easily recycled or degraded at the end of the building's life. Part of the challenge lies in trying to find an impervious material that will not rot or decay or be damaged by the roots of the vegetation growing above it!

I welcome any suggestions on how to waterproof a living roof with 'natural' materials!
living roof
Living roofs not only provide great insulation but can replace the area of ground that was disturbed to build the house, helping to reduce its environmental footprint!

Insulation...
My choices are between Cellulose or - my current preference - Rice hulls.

Cellulose is well-known, readily available and accepted within the building industry. However, while it benefits from being recycled and having low embodied energy, it does suffer from VOC off-gassing due to the chemicals used in the printing process.

Rice hulls (the discarded shells of grains of rice), are completely natural, untreated and yet highly resistant to fire and mold due to their high Opaline silicate and Lignin content.

Tests have shown that rice hulls are almost incapable of supporting a fire and easily exceed the safety requirements for use in construction.

Furthermore, at 90% relative humidity, the equilibrium moisture content of the hulls did not rise above 15% which means that they would not support mold growth. And, with only 10% nutritional value, they are less succeptible to insects than most other forms of natural material.

Although not well-know by building inspectors, there has been extensive testing done in accordance with ASTM standards which I am confident will reassure most inspectors that this is a very safe and effective form of insulation.

Unfortunately, although commonly available in California, rice is not indigenous to BC so would require considerable transportation.

A very informative article about a house built with Rice hulls by Paul Olivier can be downloaded here. (linked with kind permission from Paul Olivier)


© the natural building site (June '11)