In this second article in a series on low cost, sustainable strawbale building methods, John Glassford from Huff ‘n Puff Constructions, looks at the original and the cheapest method of building in strawbale. LOADBEARING strawbale walls were first built in Nebraska around the 1880s. As a mark of respect to these pioneers I like the idea of retaining this name to describe loadbearing strawbale walls. The Nebraska method uses strawbales to bear the weight of the roof directly, that is, the walls are loadbearing. The bales are laid in a running bond much as bricks are laid, however without the mortar. Although there are examples of running bond strawbale homes with mortar used between each bale, this is both unsustainable and unnecessary. Loadbearing walls require well-made bales that have a minimum calculated dry density of seven pounds per cubic foot. That means tight strings! This means that for the average Australian bale we need a dry weight of 36 pounds or 16 kg. Never use any bales that are wet, and by wet we mean over ten per cent moisture content. Most bales that we have tested have been well below ten per cent. Common rules for loadbearing bale walls
Pre-compression
The grippling method This is our preferred method of pre-compression because it requires few tools and uses manual labour. We use 2.5 gauge or 2.5 mm high tensile wire to tie the strawbale walls from the bottom plate up to, and over the top plate assemblies. We use one tie down every 450 mm or two per bale. The medium gripples are placed on opposite sides so we can get even pre-compression so you don’t end up with a lop-sided top plate. We can normally, with good strong bales, achieve a pre-compression of 50 mm.
The fibrehouse system This method of pre-compression is available through Linda Chapman and Bob Platts from Ontario in Canada. Briefly, the method uses an air bag that is inflated on top of the top plates and with a tie down system this can compress the bales up to five inches or 125 mm over a wall height of 2.5 metres. We feel it is too strong and slow to use. The backhoe method This requires the use of a backhoe bucket on top of the wall plate system. It works well if you have plenty of space around the building to operate a backhoe. You will need to have the following sub-assemblies ready when you raise the walls: Top plates
Alternative top plate assemblies
The search continues for the best method for top plates. We have found that as long as the walls are pre-compressed evenly then most of the above methods work well. Our preference is for the ladder system with plywood (right) . We would like to use natural fibreboards instead of plywood, and would appreciate any help and feed back from anyone who knows of where we can purchase chemical-free fibre board. Window and door frames
The most important design issue for consideration in making window and door frames in loadbearing strawbale walls is to work out the most effective method of absorbing the total loads (dead and live), from the roof structure to the foundations. The more window and door openings there are then the more critical the design of these frames becomes. In other words the frames themselves will need to be structural and be able to absorb the loads generated. We recommend that you again err on the conservative side and make frames that will be strong and easy to make. A rule of thumb is to use large recycled timbers such as railway sleepers. Once you have determined the sizes of your window and door openings you simply make the box. The width of the frames can be 250 mm. This allows us to shape the walls into beautiful subtle round curves around the window and door openings. In loadbearing walls it is important to keep windows and doors at least 1.2 metres from any corners to maintain the structural integrity of the building. Where a lot of glass is required for passive solar design (such as on northern walls) then a hybrid system will be needed. It is also a good idea to have the windows and doors set to the middle of the walls to obtain even compression of the walls. Miscellaneous hardware Termite protection
Although there are very few termites that like straw compared to many that like wood, it is still necessary to protect the building from termites. We therefore suggest that you use 26 gauge sheet metal galvanised iron lengthways along the perimeter of the footing, laid on top of the moisture barrier. This sheet metal strip should start in the middle of the footing. This allows you to use the edge as a render stop. You must also protect other areas such as plumbing in the usual way. Corners It is a good idea to keep the corners square and neat so as the top plates fit neatly. One way of doing this is to use lengths of timber or plywood as a guide with props to the outside. You will also need to tie the bales on the corners to each other using baler twine. This helps in keeping the corners straight and true. Custom bales It is also necessary to make a number of half bales to match up corners, windows and door frames in the running bond. We use two bale needles, which are easy to make and use. As long as you keep the tension on the bales then the half bales remain as solid as the large bales. Your plan will determine the number of half bales you will need to make. The persuader As the wall goes up you will have great fun straightening up the wall with the persuader. Make a large wooden mallet say 300 mm long x 150 mm wide and 100 mm deep with a good strong handle. Belt the beejeebers out of the wall to “adjust any anomalies that may occur” — this will depend on how straight you want the finish to be. Tarps Have on site enough good tarpaulins to cover the bale stack before building and during building. You will need to keep all the bales dry until the roof takes over the protection. Make sure that if you leave the site for any length of time to tie down the tarps well. Best to get the bales delivered as close as possible to the day that you intend to raise the walls. Wall raising Use well-made strawbales from the closest waste resource to you: for example, if you are in a wheat growing area use wheat. There are many types of cereal straw as well as sugar cane residue, spear grass and other baleable straws. Do not use hay: “Hay is for horses and straw is for houses”. DIY workshop Invite your friends, or run a workshop and see how quickly your walls go up. It is imperative at this stage to ban smoking, oxy welding, open fires and naked flames. The last thing that you want is to lose your house before you render. Also very important is to clean the site up as you go: do not leave any loose straw around at any time. If you have a garden, put the loose straw in bags and use it for mulch, no waste here! REMEMBER: ONCE YOU HAVE RENDERED THE WALLS YOU HAVE A MINIMUM TWO HOUR FIRE RATED WALL. Most important: HAVE FUN! |
