How to add greenery to an existing metal roof?
I have an already existing small-incline metal roof over a garage. The roof itself is in very good condition, but I would like to transform it into a green roof - hopefully as a DIY project. Can I build the green roof on top of the existing metal? What layers should I add?
Is it safe to assume that the waterproofing is done by the existing metal roof? Do I still need the root-barier membrane, or should I skip directly to the HDPE membrane?
Thank you very much!
Under the right conditions I would say yes, it is possible to transition a metal roof to a green roof, but it is less than ideal and you will probably face some challenges. Yes, the metal would be a perfect root barrier and waterproof membrane, no question. But the big issue that comes to mind for me is that you are making a pretty notable change to the load of the roof.
So a few questions, what is your climate zone - are you in an area that sees significant snowfalls in winter? If so, does the snow slide off or stay on? Assuming this is a relatively new structure, the roof trusses or rafters may have been designed with a certain load in mind, so if you were to add several inches of dirt, which would also prevent snow from sliding off, you would be adding significant weight to the roof that may be beyond it’s design capabilities.
If it’s an old structure that was built before there were structural standards to adhere to, then it gets even more dodgy. So my first step would be to have an engineer look at it to see if such a transition is even safe.
If it can handle the load structurally, then you’re down to the logistics of it, and how to make the dirt stay. If this is anything more than a 2/12 pitch, I don’t think I would even bother trying, the dirt would just slide down. If it were a very low pitch (which isn’t often the case with metal) then maybe you could add snow guards spaced out to contain the dirt in sections, but still it will likely will slide over time even with guards in place.
Your best bet really would be to remove the metal – which is almost always reusable (and 100% recyclable if not), reinforce the roof structure as needed, add sheathing and the membranes seen here in our DIY Green Roof installation video. And remember, the steeper the pitch the more difficult it becomes. The roof line in that video is a 2/12 pitch to give you a frame of reference.