Lifestyle

Raise The Roof

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How it works

Most roofs will be able to support a hat of greenery but you may need to provide additional strengthening if your growing ambitions are big. Saturated roofs can exert upwards of 100kg per square metre, so you can see that a frail timber shed might need bulking out! However, for a simple sedum roof (more on this shortly) no additional reinforcement should be needed.

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Soggy roots can cause irreparable damage to roofs, so it’s essential to start by laying down a waterproof barrier over the surface of the roof to protect it. Thick polythene or butyl pond liner works well here and will serve to keep roots from growing into the roof. Sections of barrier need generous overlapping in the direction of water flow to remove any possibility of leaks. In most instances there then needs to sit a layer of drainage – either open-pore drainage matting or for bigger projects on strong roofs, leca (expanded clay) pebbles. Smaller roofs with a gentle slope, such as those on the average garden shed shouldn’t require this drainage layer.

Raise-the-roof-2On roofs with a reasonable pitch there then sits a water-retention mat (you want the water to run off eventually but not before the plants get their fill). This may be in conjunction with a permeable membrane to stop the growing medium from coming in contact with the drainage material and clogging it up. The final layers are the growing medium and plants.

Flat roofs or roofs with a modest pitch will need little more than a retaining edge, usually of timber, to prevent slippage of the different layers. Roofs with a steeper pitch will need the growing medium boxed up into a lattice of planting pockets to stop it all sliding away.
Bear in mind access. While some roofs are very low maintenance they still need the occasional check-up, to water in plants as they establish, remove weeds until plants have knitted together and to rake off autumn leaves.

Most smaller garden structures, including shed roofs, shouldn’t require planning permission, but it’s always worth checking for larger roof areas like garages. Give the local planning office a ring to confirm what you have in mind sits within the rules. The Peterborough planning and building department can be found at www.peterborough. gov.uk/planning_and_building.aspx or by calling 01733 453410.

What to plant

Pic-CREDIT-HarshLightThe easiest green roofs are planted up with succulent plants such as sedums (stonecrops) and houseleeks. These fleshy plants store moisture in their leaves so that in times of drought they have plenty of water to tap into. This remarkable property makes them incredibly hardy and tolerant of the hostile growing environment that most roofs offer. You can buy ready-planted rolls of sedum from companies such as Enviromat (0333 4564526, www.enviromat.co.uk). The rolls are unfurled and laid exactly like a carpet. Alternatively buy plug plants of these succulents or propagate your own from cuttings (this is very easy to do and will save you a bundle).

Roofs playing host to wildflower plants and herbs will need a growing medium of at least 10cm depth. The growing medium is usually a mix of soil-based compost with additional drainage material. Again, many suitable plants are available in plugs to space out across the roof. In very little time the plants will grow out to knit together and create that desired carpet of greenery. Norfolk-based British Wild Flower Plants (01603 716615, www.wildflowers.co.uk) has an excellent selection of native plants well- suited to rooftop living, all delivered as plug plants.

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Once your green roof has established little more needs to be done – there’s no regular watering or feeding needed, the exception being for herbs that are regularly picked. What you will need
to do is tip the balance in favour of your plants by removing any weeds that appear, a job that’s probably only necessary in the first year. Pick or rake off leaves in autumn to keep the plants from becoming smothered.

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