Our build is going to be a load bearing structure. This means that as well as providing insulation, our straw bales are also going to be structural and what will essentially be holding up the roof!
Load bearing has a reduced amount of carpentry work involved and the carpentry is simpler. This makes it more suitable to the self-builder. It’s also getting the straw to do two jobs, therefore your getting your moneys worth!
The ring beam is made out of durable timber (in our case larch), with 18mm OSB3 (exterior grade OSB, we used ‘Smartply’ as it has no added formaldehyde) top and bottom, glued and screwed together. Infilled with sheep’s wool insulation and with full depth noggins at 1200 centres, and at either side of a window/door post.
When building our ring beam we used a chop saw, this not only speed up the process, it also ensured our cuts where square and it all fitted together nicely.
Of course we had to have joints in our box beam between the timber and OSB3 top & bottom. Where this occurred we made sure our joints where staggered by 600mm. This helped to ensure we weren’t creating weak points within the box beam. We also made sure to stagger our joints at the corners for the same reason.
Where timbers are joined, the joint has a secondary timber spanning 600mm (300mm either side of the joint) on the inside of the box beam This can be seen above.
It’s generally agreed that the straw needs to be 300-450mm higher than the ground. This is due to splash back, moisture etc. On a conventional house the DPC would be at 150mm for the same reason. On our build we achieved this with a combination of the pillar foundations and the box beam. We were actually higher than necessary. This was to give us an easier space to work in when under the building etc.
For our window/door posts we chose to use two 100x50mm (4x2) screwed together. The reason being its cheaper and stronger than 100x100mm posts!
If you are trying to save costs it is possible to use a less durable timber on the inside face of the box beam. However this only really applied to the box beams higher up e.g. wall plate and roof plate. As you want the one sat on your foundations to be very durable!