Grass roof wooden framed workshop, condensation advice.

[TechTroglodyte]

Hello,
I’m building a workshop in my garden and in my usual over ambitious style I’m going to be putting grass on the roof. To keep weight down, I’m planning on using clay hydroponic pebbles and honeycomb matting to keep it from rolling off.
Now, under the pebbles, I’m planning a sheet of root matting and then a couple of layers of thick DPM, then 18mm structural ply, and on the inside of the roof, insulation boards.

Here’s the question, am I going to get moisture collecting between the DPM and the ply as neither are breathable? If the answer is yes, how do I avoid it? I have some house wrap which I’m going to be using for between layers in the walls, would that work as a sort of vapour layer?

Thanks in advance, and please try to refrain from saying “don’t do that” or “pay for a builder” I’m committed to doing this myself and I’m going to make it work if it kills me :D

[dewaltdisney]

I am sure that you have your reasons but at my age, you always think of heading off a future problem and this screams out future problems. I am not sure what the gain is here it seems overly complex and unnecessary. I would pitch a conventional roof and focus on internal insulation to make your workshop usable all year round. Do not lose sight of the purpose and intention of the building. A grass roof will prove to be a nuisance. Thank me in five years when it would have to come off.

DWD

[Someone-Else]

Moisture will collect on the underside of the DPM, the only way to prevent it is to have air flow. But more importantly, how are you going to get a lawnmower up there?

[TechTroglodyte]

What about a layer of bitumen on the ply to stick the DPM down to? No air, no condensation right? Like you would with shed felt.

Unfortunately I’m “at the age” where I want to make cool stuff, and I have a family of bees that are going to love the wild flower mix I have for it. I dug up a 1.5m deep hedge to put the workshop there, and I’m still feeling guilty about destroying that habitat for our hedgehogs… Anyway, I have my reasons for wanting to do it, was really just asking for some advice on avoiding newbie mistakes while I do it :)