Grass roofs serve a dual role, providing a back-to-nature beauty that places a structure one-step closer to its environment, while adding insulation to that structure that lowers heating and cooling costs year round. They aren't difficult to install, as long as you work in layers, building a solid foundation on which to plant your grass seed. Once in place with established growth, they're largely maintenance free, needing only a bit of water every now and then to augment the natural rainfall, much as your ordinary lawn does. Have a structural engineer check the roof's ability to hold the weight of a grass roof before proceeding.
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Things you need
- Plastic sheeting
- Asphalt mastic
- Aluminium green build edging
- Polyester filter fabric
- Top soil
- Drip irrigation system
- Grass seeds
- Wind blanket
Cover the entire roof with a piece of thick plastic sheeting to serve as a waterproofing membrane between the grass and the roof. Attach the sheeting to the roof using asphalt mastic, brushed on in rows, and then covered with the sheet. Overlap the sheeting by 4 inches across adjacent rows, and then tape the seams with fibreglass tape.
Edge the perimeter of your pitched roof with an aluminium green build edging to maintain the position of your grass roof materials. Lay the edging out in a row along the roof perimeter with the bottom of the L-shaped railing laying atop the plastic sheeting. Secure the edging in place by driving roofing nails through the holes along the edging base into the roof.
Cover the layer of plastic with a thin layer, about 1/2-inch thick, of concrete. Use a trowel to spread the concrete over the plastic right up to the side barriers of your roof. The concrete will serve as an organic barrier, preventing the roots of the grass from growing through the plastic and into your original roofing material. Allow the concrete to dry overnight.
Add a drainage layer over the concrete consisting of an equal mix of gravel and pumice about two inches thick. Level the drainage layer out with a rake.
Place a polyester filter fabric over the drainage layer to hold the soil in place while allowing the water to drain through to the drainage layer.
Cover the filter fabric with your growth medium. Because of the need to reduce weight, use a growth medium that consists primarily of inorganic materials, between 75 per cent to 90 per cent. You can use material such as expanded slate, shale or clay, volcanic pumice or crushed clay roofing tiles. To this, add organic weed-free soil such as an organic compost or top soil. You can purchase speciality growth medium from suppliers dealing in green roof materials, or create your own mixture.
Install a drip irrigation system over the growth medium. Purchase a simple irrigation system, one that can supply needed nutrients for the first one or two years, from a garden centre.
Add grass seeds to the growth medium that are suitable for local growing conditions and able to survive with a minimum of tending. Make sure to choose low growth grass to cut down on weight and wind shear effects.
Cover with a wind blanket to protect the seeds as they establish themselves. Water daily until the grass begins to sprout, then weekly until it's fully established on your roof.
Tips and warnings
- Check with your local building inspector prior to beginning your grass roof installation to ensure that the roof is within building code.
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