Dig the footings to the required depth.
Build form work for the concrete pour 150 mm above ground level around the outside of the footings. You will need to come up a minimum of 100 mm off the ground level (same depth as slab floor).
Prepare the slab foundations as a normal slab, with moisture barriers, plumbing outlets, electrical and telephone conduits and of course the reinforcing mesh as specified.
You may like to consider using a chicken-wire nailer for rendering when using cement renders; earthen renders do not require netting. Build this in place to attach the bottom of the netting to the footing. This is an important detail, because if the plaster seals well to the footing there will be no place for the entry of insects into the bale wall from the outside. One way of doing this is to use a 50 mm x 25 mm hardwood nailer that runs around the top edges of the footing.
First, install screws into the 50 mm x 25 mm nailer every 300 mm or so, with the heads poking out for the concrete to grab, and imbed the nailers solidly into the foundation. Then tack the nailer assembly to the top inside edge of the form boards by screwing from the outside of the form board and into the nailer. These tacking screws are just to hold it into place until the concrete sets, and must remain accessible so they can be removed before removing the form boards.
We prefer to use a bottom-plate system for many reasons. We use two 4 x 2s (100 mm x 50 mm) on their edge with noggins every 450 mm. The photos on this page will give you a good idea how to use a bottom-plate ladder system. The main benefits of our system is that you have somewhere to nail the netting as well as somewhere to take the high- tensile wire through that goes up through the top- plate assembly. We use the gripples to tension the high-tensile wire, and we put this wire through every 450 mm. We then take the wire up through the top plates.
If you are pouring a concrete slab at the same time, install a 200mu plastic vapour barrier that completely covers the base of the slab and footing trench. Then put the reo into place. Take care not to put holes in the plastic membrane. If you do, make sure to tape them up.
Hardware required for strawbales at the time of the concrete pour will depend on the method of strawbale construction; the common method is to pin the bottom bales to the foundation. You will require two 12 mm rebar pins, 450–600 mm long, per bale (cut these before the pour). These should stick out 150–300 mm from the footing. (Our testing at the Building Research Centre of the University of New South Wales, Sydney has shown that, apart from these starter bars, we do not need re-bar through the bales.)
However, depending on your engineer's requirements for the above detail, starter bars, threaded rod and rebar in the bales can now be completely eliminated. We have found that the high-tensile wire and gripples have plenty of shear strength, and our testing at the University of New South Wales confirmed our feelings.
If you are building a load-bearing strawbale house, then the high-tensile-wire-and-gripple system is more than adequate. The top plate assembly should mirror the bottom plate assembly, with the exception that we make the top plate ladder assembly 100 mm narrower than the bottom plate. This allows the high-tensile wire to act as a triangle in the pre-compression. This gives more strength to the wall.
