When building a load bearing straw bale wall, you need to support the wall from falling over into the house or out of the house. The direction in concern is called the out of plane direction and is perpendicular to the wall. A wall should be supported in a ratio of 13:1 (length to thickness of wall). For a standard 3 string bale, this is every 25′. The question often arises about how to support a wall where an intersecting wall is not desired, such as in a living room or kitchen. Below we discuss building buttress walls to support out of plane wall loads.
One way to accomplish this is to use buttress walls. A buttress wall is a wall that does not go full height of the main wall. Instead, it steps up from the bottom to the top with the base being its widest point and the top being the same width as the main wall. The bales in this buttress wall should be interlaced with the bales of the main wall for maximum benefit.
The best part of a buttress wall is that you can support your main wall while still keeping an open feel in the house. The buttress can be used to hold plants or display other items that make sense in the room. The walls can also be filled in so they do not step, but are a diagonal slope if that look is preferred. There are many things one can do with a buttress wall to both support your main walls and improve the design of your house.
Anything over a ratio of 13:1, that’s 25′ for a standard 3 string bale as pointed out above, should be supported by an intersecting wall. A buttress can be designed so that the bottom course is roughly a bale long, the second course is 3/4 bale long, the third a 1/2 bale and so on. The width of that wall (length on the buttress but it looks like width in comparison to the main wall) is determined by the height of the main wall. Figure how many courses you have with the top course ending up flush with the main wall and then work out by 1/4 bale length at a course from there. That will determine how long the bottom section needs to be.
Buttress walls have been used for years in masonry and other forms of construction. They are a time tested tool that can be used to improve the strength and design of your straw bale house. Have fun with them. Make them a part of your dream, not a bummer of a wall you don’t want but need to keep your building safe.