I thought I'd write a bit here about what we learnt when lime rendering the construction's strawbale wall in October/November 2011.
The roundhouse was originally built by a group called 'Shift Bristol' and they put a lot of hard work into making it. They were advised initially by Tony Wrench, who has made his own roundhouse in Pembrokeshire (see www.thatroundhouse.info). Unfortunately, Tony couldn't be there for the whole build and time was really tight for the project, so inevitably some things didn't work out as they should and the roundhouse was left not quite finished.
Since a lot of the people who helped on the build have moved on, they won't get to learn what we found out by trying to finish the jobs off. In writing this, I hope that they and other people will be able to avoid some of the mistakes we encountered and it will help make their future lime rendering projects easier!!
How the strawbale walls were constructed
The strawbale wall only makes up about a third of the total wall area of the roundhouse. The rest comprises door spaces and walls made from cordwood and bottles held together in a cob matrix, with straw stuffed into the centre of the wall between the logs or bottles.
The walls aren't loadbearing - the weight of the roof is taken by a wooden 'henge' which is then filled in with strawbales etc. You can see the assembled henge frame and the rafters for the reciprocal roof below:
Don't use metal staples etc. to key the render onto the wall, as the lime will corrode them.
Make sure you are wearing old clothes and sturdy rubber gloves and wear safety specs - you don't want this stuff in your eyes! Cover any cuts as lime will interfere with healing. We kept some clean water handy, in case any eyes or skin needed to be washed. Spread old tarpaulins on the floor to protect the ground underneath and to catch falling render, which can be added into the next mix.
First, moisten the wall slightly to help the render stick. Then, take handfuls of the render and push it into the straw walls with a kind of massaging, circular action to get it right in. The first layer will have bits of straw sticking out, but they can be covered by subsequent layers or trimmed later. After about a week the next layer can be applied.
While drying, protect the render from hot direct sun and, more importantly, from frost. In hot climates, hanging hessian in front of the render and then keeping it moist may help avoid problems caused by over- rapid drying.
Finally, paint over a couple of washes of lime putty mixed half and half with water to make a 'slip'. If you don't want the finish to look dazzling white, you can add a bit of acrylic paint. I used some yellow ochre from a small tube of artist's acrylic and it gave a very nice yellowish tone to the final finish.