So I already made a shed 10+ years ago but I want to get rid of it, it’s too big and blocks too much light, and Tomoko wants to convert it into a greenhouse or at least a place to overwinter some plants. At the same time we need a place to put our bicycles and other stuff that shouldn’t be left outside but doesn’t need to be in the house. Along the side of our house we’ve got a long area that’s fenced but isn’t really doing anything besides being a place to put our garbage bins. If I can partially enclose this area with a roof it’ll be better for the garbage bins – less snow will get on them, and I can use it to store the bicycles, lawnmower, and other big garden-type stuff that ought to be protected from the rain and snow. Tomoko, worrying that this might not be aesthetically pleasing said I should make the roof a green roof. Challenge accepted!
The basic structure will be straightforward enough. I’m going to use my existing 6×6 fence posts as support for the rear and one side of the structure and then place more 6×6 posts in the front. The fence posts are roughly 8 feet apart and in the front I’ll space the new ones 4 feet apart. At the top of the posts I’ll run 2×6 lumber as beams and I’ll have 2×6 joists running across them every 16 inches. Above that will be the roof which’ll be 3/4″ plywood. As our ground slopes from the front to back and the fence follows the slope the roof of the shed will also be sloped which will allow the roof to shed water.
In my reading about green roofs they all mention that they will be much heavier than a normal roof because of the added weight of the growing media as well as the additional water and snow it’ll retain. In my initial planning I was going to make the beams and joists out of 2×8 lumber but it feels like overkill. The joists are only going to span 4 feet. I bet 2x4s would probably be strong enough but a bit of over-engineering is OK.
For the green roof I am going to border the roof using 2x8s. Put down a layer of polypropylene underlayment, then 45 mil pond liner, another layer of the underlayment, then gravel for drainage, landscape fabric, and then soil on top of that. My soil depth will be around 4-5 inches so I’ll be limited in what will grow on it. The plan is for a mix of sedum, thyme which we’re already growing, and whatever else I can find that has shallow roots and won’t be asking for too much attention.
The good thing about a storage shed like this is that it has a fairly low-stakes failure mode. If the green roof leaks then there’ll be some dripping on the garbage bins or bicycles but there isn’t a floor that’ll get damaged. If the actual structure isn’t sound I’ll be able to see some bowing before the whole thing comes crashing down on the bicycles. Even then I only really need to worry about Tomoko’s.
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