The roof on the house has been just fine, although we have some weeding to do. For a shed, rather than a house, we could do something simpler, while using up left over wood and soil. To hold the soil against the slope of the roof, there is a wooden grid. To stop the soil washing away while still letting water drain, there is weed mat and holes bored in the horizontal grid members. I might have got carried away making the grid up, it really didn’t need angled pocket screws, but WTH. The grid is held in place by some stainless strapping over the roof edge and the handy nearby concrete wall.
I only want to build this once, so I want to do it right. That means making it strong enough and preventing corrosion… and the shed has a very flexible steel roof. Whoops.
The roof is going to be carrying about a quarter of a tonne of wet soil. On top of a wooden shed. In an earthquake zone. That’s not good and I don’t want all that coming down on someone’s head. Hence all the uprights in the shed have been at least doubled, there’s three more purlins, and rafters fitted every 400 mm. I used a deflection-limited approach to the structural design, which is a technical term meaning that I jumped up and down on top of it to see how much it flexed under 80 kg. It didn’t. And then after loading it up, I put a straight edge against to rafters to see how much they had bent. I couldn’t detect any bending, so I think it’s good.
As for corrosion, the shed roof is coloursteel, so half a mil steel, galvanised with lots of zincalume, and baked on paint. It’s warranted for fifteen years, even in salt spray zones. (Yes, we’re in a salt spray zone, despite being 150 metres up and three kilometres inland. We’re inland from Cook Strait, which is one of the roughest and windiest pieces of water in the world.) So it should be good, right? I mean, that’s what nearly every house in NZ uses… although it keeps plenty of people employed replacing roofs every few decades.
Coloursteel is great if it gets to dry out every so often and the rain washes the crap off it on a regular basis. Sadly, it shouldn’t be used in contact with permanently wet materials or soil, which is a bit arse for a roof that’s going to be covered in soil. The steel roof is also going to be impossible to inspect for rust without digging up the plants, leading to a possibility of the whole thing collapsing on someone. So to keep it from rusting, there’s a layer of polythene directly on the steel, doubled over so that staples holding the top layer to the wood are separated from the steel by the bottom layer of the film.
For good measure, the top edge of the poly is also siliconed against the steel, to stop water running down between them. I probably should have put a flashing over that edge, but we’ll see how it goes.
For the plants, I ended up scratching my head somewhat. The micro-climate on the roof varies from sunny to shaded. It’s in a damp spot that gets plenty of rain, but with the thin (100 mm) soil it’ll vary from moist to very dry. And then there’s huge amounts of wind, coz this is Wellington. In the end, I just used Greater Wellington’s regional plants guide and chose the toughest natives from a bunch of ferns, low cover, and sedges.
From left to right: carex secta, carex virgata, hebe albicans, coprosma prostrata, asplenium gracillimum/hen & chicken fern, blechnum discolour/piupiu, muehlenbeckia complexa/puhuehue, coprosma acerosa, leptinella diocia, carex fagillifera, pratia angulata/panekenake. And then there were some extra leptinella from the house roof of a different kind, which seems to do very well in shady spots.
These ended up spread across the roof, with the ferns to the north in the shade of the tree and the sedges to the south where it’s more exposed. I’ve no idea what will survive, but plenty of weed mat and bark mulch should help.
Mostly, I like that from above you can barely see the shed at all. From the front, the sedges stick up over the roof and the coprosma should creep over the edge of the roof-line, helping the shed to blend in. And hopefully, it’ll last a good long time. Ask me in a decade or two how it’s going.
Things I screwed up:
- If you don’t want anything to rust, use stainless fasteners, right? Nope. Stainless is good on its own, galvanised is good on its own, put the two together and you’ve made a battery. The zinc gets eaten by the stainless. The straps that stop the whole thing sliding down the roof are stainless and they’re screwed directly to the galv. Bother. I’m going to have to put some non-conductive plastic between them and replace the screws with plastic rivets.
- I don’t think the weed mat will blow away, but we’ve a 120 k storm developing today, so I guess I’ll find out. I should have put some rocks around the edge.
- I planted the tall sedges right next to the solar panel for the shed internal light. I’ve enough cable to raise the panel up by a metre or two, but that was dumb.