Sunday 8 May 2011

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.

It means, sadly, that I lose the twist. In a sense it doesn't matter; if I'm insulating the roof with sheeps wool - which again I'm now considering very seriously - there needs to be an inner skin on the roof, a ceiling, to hold the wool up. Which means the rafters are hidden, so the twist would show only in the alignment of the pillars - a bit subtle. But this process of refining the design has been one of losing one little elegant touch after another, and I mourn the passing of each and every one.

You'd think that losing 15% off the internal volume would also be a pretty big issue. Surprisingly, it doesn't seem to be. The picture shows exactly the same furniture (at exactly the same size) as in all the other 'furnished' drawings, and it all fits with adequate circulation space. The two sofas are no longer at right angles to one another - but they are still fully two metres long. The office area is now quite cozy and definitely wouldn't be comfortable for two people. And there's some awkwardness between the kitchen counter and the dining area which needs a little more thought.

Fifteen percent off the diameter is actually slightly more than 15% off the cost, because the longer rafters are more efficient of timber. And the smaller volume will be warmer for the same heat input. All this may be academic; it now seems likely that we'll be able to mill our own timber on site by the time I'm ready to build. And reducing what was already a small dwelling needs serious thought. But... simpler is quicker, and already next winter is snapping at my heels.

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