If you want to grow anything on a roof terrace, it is of paramount importance that you first ask a structural engineer to check that your roof would be able to withstand the extra weight of a garden before you start constructing it. You also need to make sure that your freehold will allow you to have a garden on your roof.
You will need to think about access to the garden for people and anything you will need to carry up there like furniture, plants, and of course, water. You may want to run a pipe up to an automatic irrigation system which will save you time in the long run.
Before you start creating a garden you must make sure that the roof has adequate water-proofing. Any plants, soil and containers you have will benefit the environment in terms of helping to prevent flash flooding as they absorb water during downpours but you must ensure that any alterations or additions to your roof will not cause damage to your house.
Some water-proofing layers such as bitumen and asphalt can be compromised by root penetration so use a pond liner material to protect an existing layer of roofing material. Ideally the protective layer should be in one continuous sheet, but if this is not possible make sure there are good overlaps of 20-30cm.
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