Can I Convert a Shed to a Straw Bale House?

Bret asks:

I have purchased a property which has a shed in the position that we want to build our house. The shed has a concrete floor and steel frame. Can I remove the steel cladding and rails from the shed and replace them with straw bales to convert the shed into a straw bale house.


Brian
It must first be noted that you will require a building permit to do the work as it is a change of the use of the building, and requires structural alterations. Given this, I would say that it is highly likely that the shed can be converted into your dream house, and not with too many obstacles, however there are few questions that must be answered.

Firstly, does the concrete floor of the shed have plastic beneath it? If not it you are likely to have problems with rotting floor coverings or lifting tiles, apart from the fact that it will not comply with the requirements of the building regulations. Secondly, is there sufficient height in the shed walls to comply with the building regulations? If it has 2.4 meters at the outside walls you are fine. If it has a sloping ceiling with a minimum of 2.1 meters at the outside walls you may still be OK although this will need to be confirmed.

Regarding the existing concrete floor. In the majority of these situations you will have a 100mm thick concrete floor that goes all the way to the outside of the building. It will most likely have posts set in concrete about every three meters apart. The concrete supporting the uprights of the shed will have been poured into holes, thereby giving them the required support. Depending on the age of the shed, the council may have details of the shed and its foundations. They may even have details of a soil report, however it is unlikely. If not, it will be necessary to have a soil report done on the property and to have an engineer check to see if the existing concrete that is to support the straw bale wall is sufficient, however this too is unlikely. Assuming that the existing floor requires additional support, this can be done by digging out the foundation below the existing floor, or by cutting the floor back to enable the installation of an edge beam around the building. If you choose to cut the floor back, it will be essential to tie the remaining concrete to the new edge beam. Ramset fasteners have masonry anchors for this specific purpose. They are fitted into the cut edge of the remaining concrete, and have reinforcing rods extending out of anchor. The reinforcing rods extend into the area of the new concrete, thereby fixing the two sections of concrete together. Your engineer will be able to advise you on the appropriate size fixings etc. Ensure that the appropriate termite treatments are incorporated into the design. Once the concrete is upgraded, you would fix a 90mm deep timber structure known as bottom boxing to the floor which will support the straw bales and keep them off the floor. This will provide a barrier between the underside of the straw and water from any accidental spill that might come from overflowing laundry troughs an alike. Refer fig1. The bottom boxing also provides one of the necessary components for the later compression of the straw bale walls. It can be fitted to the concrete floor using 75mm masonry nails, however ensure that the compression strapping is installed beneath the bottom boxing prior to fixing it to the concrete. Refer fig 1. If you require more details on the construction of the bottom boxing I have architectural drawing available that can be lodged with your application for the building permit.

Fig. 1

Alternatively, if the depth and width of the concrete supporting the posts is sufficient to carry the load, it could be possible to bridge the area between the posts of the existing structure with timber or steel. Depending on the depth of material required to bridge the area, this might cause some complications with rendering, as you will have to conceal this material, however this would not be too difficult to overcome. If this option were chosen, the structure spanning between the posts would also perform the task of bottom boxing. The top of the beams would be covered with structural ply to support the bales, and bulk insulation inserted into the cavity between the beams to control heat loss. I trust that this answers your question, and I look forward to many more. Remember, if you have a question, chances are 100 others have the same question, so send them in.

Brian Hodge has over thirty years experience in the building industry. Brian’s book “Building Your Straw Bale Home” (published by CSIRO publishing) and 12 DVD home-study workshop “You Can Build a Straw Bale House” have received rave-reviews both in Australia and abroad. For more information call 0428 246 868 (In Australia) or click here to contact us.