The walls of Nebraska style strawbale houses are termed load bearing because they support the weight of the roof. Strawbale walls take some time to settle after the roof is installed, which can cause cracks in stucco which is applied before the wall has fully settled. Load bearing strawbale walls are therefore pre-compressed before the stucco is applied. This procedure also satisfies another very important design consideration: the prevention of the roof from being dislodged from the walls by high winds. Thus the method used to pre-compress the strawbale walls also functions as the roof tiedown system.
Before talking about tie-down methods however, it is necessary to address the component of the strawbale structure upon which the roof rests, and to which it is tied: the roof plate.
The roof plate functions to distribute the load of the roof on the bale wall, and various designs are utilized. They all have in common a horizontal piece (which is as wide, or nearly as wide, as the bales used) and a vertical piece or pieces, which gives the box beam strength against bowing down in the middle. One common method (for 23", 3 string bales) uses a 24" wide plywood sheet (a 4 by 8' sheet cut in half lengthwise) as the horizontal piece, and two 2 by 6's or 2 by 8's stood on edge along the long edges of the plywood sheet and nailed to the sheet. Another method, termed a ladder type roof plate, uses lengths of 2 by 6 or 2 by 8 lumber connected by short cross pieces of the same lumber. Whatever method is used, the functions of spreading the load of the roof must be satisfied.
Another type of roof plate, rarely used, is composed of poured concrete. This method is different enough from other types of roof plates that it is given another name: a bond beam, and it serves to tie the top course of bales together as the bottom of the poured beam bonds to the top layer of bales. The bond beam has been used in cases where the top layer of bales is inconsistent in height (elevation).
Several types of tie-down (pre-compression) systems are used. Two of these methods use steel rod attached to steel fittings embedded in the concrete foundation. Of these two, one uses short lengths of threaded rods running up through the courses of bales to the roof plate, which means the bales must be impaled on the rods as the bales are stacked. Washers and nuts are threaded onto the rods above the roof plate, and the plate drawn down against the bales by tightening the nuts.
The other steel rod method places the embedded foundation fittings and the rods on either side of the bales. This method makes leveling the roof plates easier, and avoids the difficulty of impaling bales on rods which speeds bale stacking and is easier on workers. The rods are only installed after all the bales have been stacked and the roof plate has been laid in position.
A third method uses a product called a Gripple which, unfortunatly, I know little about at this point in time. When I learn more, I'll add that info.
A fourth method uses 2" by 2" steel mesh, nailing the mesh at the bottom of rthe wall to a wooden wall plate which is bolted to the concrete foundation, and nailing the mesh to the roof plate at the top of the wall. This method requires a force to be applied to the roof plate to compress the wall before the mesh is nailed in place.
Yet another method embeds eye-bolts in the foundation on either side of the wall, and then threads 1/2" flat poly strapping through the eye-bolts, over the top of the roof plate, and back down to the eyebolt on the other side of the wall. The poly strap method uses small metal buckles which allows tension to be placed on the strapping, with the overall advantages of minimal tool needs and maximum speed and ease.