Going straight
The singlespace roof has a slight twist, and I love it. The inner triangle is three and three quarter degrees off square from the middle hexagon, which again is three and three quarter degrees off square from the outer ring. It's that subtle twist that makes the roof so uneuropean, so quirky.
There's a reason, of course. The reason is that I couldn't get rafters long enough to span the ten metre diameter internal space that I wanted; and I didn't want to have to make a very complex joint at the top of each pillar. But I've been spending the last week working very hard on working out how to make my dwelling simpler to build, lower carbon and, ideally, cheaper; and one of the questions I've asked myself is how big a single space could I build with the rafters I can get.
The answer is that I can get 4800mm rafters at 200 x 50mm cheaply — just as cheap per metre run as 3600mm rafters. Given that the rafters cannot go right to the peak of the roof and that the gradient is shallow, two 4800mm rafters will actually span almost ten metres. But that's the full span of the roof. The walls come inside that span. If I'm going to use straw bale — which I'm now thinking of very seriously — each wall is 600mm thick, and allowing 150mm for eaves that takes 1500mm — or 15% — off the inner diameter, and consequently off the floor space; down from 78.5 square metres to 56.75 square metres.
Wool gathering
Singlespace, as described in the essays up until this one, is a building with a concrete floor and wall, and extruded polystyrene as insulation. Both of these materials involve a lot of embodied energy, and hence are far from carbon neutral. They're also not local — they don't occur naturally on site, but need to be transported in. I need to use these materials because I've embedded the house into the hillside, and it's a damp hillside.
But, as Pete pointed out, I don't need to earth-shelter the walls. I could have a walkway round the back of the house. Then it could be drained much more conventionally, and the wall wouldn't have to resist the pressure of either earth or water. So the wall could be much lighter. If there were problems with it, access to repair it would be easy.
More significantly, it could be timber — and timber does occur on site (although if I plan to build this year I won't have seasoned timber of my own and will have to use 'imported' timber). Even 'imported' timber has a far lower energy cost than concrete.
East wind
It's in the nature of this place, up on its high ridge, that it lives in the wind blowing in off the grey Atlantic. Our winds are westerly or southwesterly 70% of the time. Being on the western side of the ridge, my croft takes the full force of them. That's the main reason why I'm designing my croft house to be earth covered, sunk into a natural declivity in the ground. But I don't yet have a croft house; I don't yet have planning permission. So I've built a temporary shelter, my summer palace, which is essentially just a platform in the trees with a crude tent over it. And because the prevailing wind is in the southwest, I've built it in the northeast corner of my wood.
All the time I've been planning and building the summer palace, the wind has been in the west, and the wood has given it good shelter. Today, it was virtually finished. Today, I moved the last of the furniture into it. Tonight I would have moved in completely, but that I have to go to Edinburgh at the weekend, and I didn't want to leave the cats alone in a place they weren't familiar with...
Tonight, according to the met office weather station four miles away, it's blowing force nine. From the East.
Planking the roof
Radial planking
Planking the roof of the cone poses interesting problems. You can plank a cone radially, and every plank is straight. My rafters run radially, but that doesn't have to be a problem; I could put short purlins between the rafters to fasten radial planks to. I can even envisage some rather pretty joinery for those purlins. It isn't impossible.
But every plank would have to be cut longitudinally — identically, so I could make a jig and it wouldn't be a big deal, but it's a significant job. More significantly, radial planking also would not strengthen and stiffen the roof, and I'm still concerned by my friend Pete's comment that the roof could be floppy in strong winds. The tensile bracing which Pete suggested and which I've shown in the last drawings will help, but...
Transport
One of the things I need to consider about the croft is moving around. Not moving me around, my feet or a bicycle do that. And for getting my groceries home, well, a bike or my feet or at worst a wheelbarrow will work. But while building my house I'm going to need to get a fair bit of building materials in, and once it's build I'm still going to need occasionally to move heavy or bulky stuff around.
The planners, of course, will not want a house which cannot be reached by road. But the planners are not me, and I do. At present, you can get a normal car over the hill to the croft... in dry weather. In normal weather, you can't — not because it will sink in, but because you can't get traction. The hill is too steep and you can't get grip.
So what are my options?