We constructed a similar ring beam / box beam (roof plate) to go above the bales & hold the rafters.
Roof plates perform many different functions; they evenly distribute the load of the roof, they provide a rigid structure to evenly compress the bales, they provide a more uniform structure to attach the roof to, they act as a lintel over windows & doors and you can wrap ratchet straps around them to aid in the compression process.
We constructed the roof plate at ground level, for ease of access and to avoid working at height for as long as possible. Because this roof plate was going to have to be lifted up, we constructed it in four parts so it was lighter to lift.
The lift was done in 2 parts using a team of six people. The first part of the lift was about 4 foot. This meant that everyone could lift from the ground into props.
We then got on our floor beam to carry out the second part of the lift. Raising the roof plate sections to the marked height on our window posts (6 uncompressed bales high + a bit to allow us to get the final course of bales in). At this point temporary posts where screwed into the window posts to hold the roof plate in position. Temporary bracing was also added to stop racking and any unwanted movement that may have occurred.
After the 4 pieces where up, we where then able to glue and screw them into one box beam / roof plate. We then added the mezzanine beam, infilled the roof plate with sheep’s wool insulation, added the top layer of OSB to the box and glue & screw into position.
Temporary bracing was also needed to stop the whole building shifting, this was done with diagonal bracing, as well as bracing at the corners. This bracing at the corners will come in helpful later when we are going to be straw bales, as it will help keep the building square.