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by Peter
(Devon, UK)

I’ve been interested in putting up a shed with a green roof and wondered what are the most important things re: the actual building. I am not sure what the slope of the roof should be, or if it needs to be flat? Also, what kind of membrane is the best?

Drought Smart Plants reply:

The slope of a green roof can in some cases be fairly steep – if you know much about how slopes are calculated in building codes, the ideal slope for a planted living roof would be less than a 3:12 pitch – which means that in every 12 feet of length, the roof would drop 3 feet (or whatever you’re measuring in – centimeters, meters etc.)

Any steeper than this and there is a risk of the weight of the plants especially under wet conditions sliding off (what a mess that would make!)

It’s also important to remember that snow will not slide off this type of roof, and heavy rainfall will add an appreciable amount of weight as well. This means that the actual structure must be strong enough to bear the extra weight.

A flat roof (or one that is nearly so) is most comfortable for the green roofer to work on, but once the plants are in place, this isn’t an issue.

For the actual waterproofing material that goes under the plants, I’ve had great success with epdm, or the rubber roofing used for tar and gravel roofs and pond liners.

It’s crucial not to damage the rubber, but luckily it has some stretch to it, so it’s pretty forgiving.

Make sure that it is completely covered by soil or mulch to prevent UV damage from the sunlight, and your roof will last for years.

Hope this helps with your decision making process,