I am building a small shed with a heather/heath roof using low growing varieties that reach 10-15cm.
I plan to use 4 separate 60x40cm Euro trays, lined with soil barrier, so that I can lift these off individually to move the shed for maintenance. The trays will fit into a ‘box’ on the roof. The 22 litre trays would give a soil depth of 9-10cm once lined. I have left over astroturf that I'm using as liner which will ensure drainage and I'll add some pea gravel to this - giving 9cm of actual soil. This will weigh 20-25kg - I only weigh 40kg so that’s about my limit on a ladder.
I can control the soil and drainage, and I live in the Scottish Borders where heather grows beautifully. It’s a sunny position, sheltered very slightly from the prevailing wind, on a roof pitch of around 5 degrees. I have easy access for watering, feeding and weeding.
The overall height from the floor to the top of the heathers is limited to the height of the fence (as we don’t want to lose the view of the hill behind). So to optimise the shed space I need to minimise the green roof height. Adding deeper boxes later would not be trivial as I would need to shorten the shed.
Is 10cm sufficient substrate depth to support a well-tended 10-15cm heather/heath? Or will they want to root deeper to withstand the wind? I plan to mix Erica carnea and Calluna.
The standard advice of digging ground out for 30cm seems like it cannot apply equally to those that are tall and those that hug the ground. If 10cm is not sufficient, what’s the minimum soil depth I can get away with?
Edit: I am now intending to use a deeper euro crate that gives me 13-14cm soil depth over the drainage. The rest of the question remains - is 13-14cm enough?