I am guessing many of you will be familiar with the www.naturalbuildingblog.com website?

Along with Dr Owen Geiger (I think he is something of a legend) we are looking at collaborating on a self build natural low impact house design but it needs to meet UK regs. It's very early days on this project. Eventually when the project has some legs I would like to see it open sourced for everyone to collaborate on it. The intention is to make it as easy and cheap to build as possible and for novices to do the as much of the work as possible.

This gives a brief overview: http://www.naturalbuildingblog.com/wiki-natural-house-proposal

I am not from a building background so I have several knowledge gaps! Owen is a asking me things that I simply cant answer so I hoping with the wealth of knowledge here I can get some answers. (Owen is from the USA so he is also not familiar with UK codes) The initial design idea is for a straw bale roundhouse, probably with a reciprocal roof, but the best way to do the roof is a puzzle. This is what he has said:

Making a green roof on a reciprocal roof is not my preference, however it avoids a lot of complexities as outlined below. Here's one alternative to show how things can quickly add up to a lot of work. Things become even more involved and expensive in order to meet code:
factory trusses? double pole roof to provide roof insulation: TJIs? Compression ring? Operable skylight for ventilation? Knee wall to support the upper TJI rafter? Ceiling material? Roof ventilation? Insulation type? Roof penetrations for pipes and wood stove? Purlins? (pallet wood?), Access to install the insulation? Blow in? Metal shingles? Are there affordable metal shingles in the UK?

http://www.showroom411.com/Media/DirectoryEntries/paradign%20shingles.jpg

Pallet shingles probably won't work on the roundhouse because each board would have to be tapered to conform to the curve. It seems like too much work and probably not accepted by code anyway. It would be good to hear from someone with firsthand experience on this. Right now it seems like metal shingles nailed on pallet wood or recycled wood purlins is the best approach. Again, we need to find out what the codes allow. In the US, you can't used recycled wood for structural members unless it's been specially inspected and graded by government approved inspectors. Of course, this wipes out any savings.

Thanks for reading, if you've managed to get this far on this long post.
Hope someone can help.

Jason

I think the internal diameter of the roundhouse will be around 7 metres

not sure if this helps any, but a few years ago we were invovled in a refurbishing of a horsemill, which sounds like it would be similar, that was of a simlar-ish radius and used softwood sarking bent around the curves diagonally, with a vapour permeable underlay (hence non-ventilated) with diminishing courses of natural slate nailed directly through the membrane into the softwood (which is traditional practice for slating up here in scotland).

AFAIK this roof was a cold roof, so insulated horizontally at ceiling level, but the same principal should work with insulation on the pitch easily enough. I wasn't involved in this particular job (i work for the membrane supplier BTW) as it was before my time here, but i don't think building control had any issues with this, and this was at a time when VPU membranes were much less widespread then they are now. some pics here that should give you the gist of it:

https://drive.google.com/folderview?id=0B3BrYh8nb7HSdF9xU3dETXppdFU&usp=sharing

If you want it to be cheap then making a round building is the wrong way to go IMHO. Make it rectangular instead. There are of course various straw bale buildings in the UK, and lots of other natural houses, some of which have building regs approval, so I'd suggest the first step is to look at some of those and talk to the people who have built them. Perhaps pick one of those approaches as a starting point.

I would suggest learning a lot more about what has already been done and works in our climate and legal environment before choosing a technology or shape of house to build. Be aware that 'natural' and 'low cost' don't automatically go together either. It's possible to build a low-cost natural house but most low-cost houses don't use 'natural' components, and many 'natural' houses are quite expensive.

Note that we don't have 'building codes' in the UK, we have building regulations, and that difference in name marks a quite significant difference in approach. The particular regs that apply also depend on whether you are in England, Wales, Scotland or NI (or Ireland for that matter).