I’ve been asked a lot about building straw bale houses using the ‘load bearing’ method rather than the ‘post and beam’ one. Here’s some thoughts on that:
Straw Bale Construction Load Bearing Method
First, let’s define load bearing: That means, simply, the straw bale walls are stacked up on top of the foundation to the roof height, some kind of a header (usually wood) is placed on top of the straw bales with attachments of one sort or another to the foundation, and then the roof is put on top of that header. Therefore, the straw bales bear the direct weight or ‘load’ of the roof–in technical terms, that’s called a ‘dead load’–and if there’s any snow resting on the roof in winter the bales have to bear that weight too–such temporary loads are called ‘live load’. Not least, the straw bales will need to be able to withstand horizontal pressure–technically called ‘shear’–which wind produces against the walls of a building.
Straw Bale Post and Beam
Next, the definition of post and beam: That means there will be a frame to the building which could be out of wood (2×4′s or 6×6 posts or the like), steel beams, concrete blocks, railroad ties, telephone poles; you name it. However the frame is built, and with whatever material is used, it’s that material which will take the ‘dead’ load, the ‘live’ load and the ‘shear’ load of the roof and wind. The straw bales will be fitted in-between the frame, so all they do is hold up their own weight (though it turns out that bales in both load bearing and post and beam situations are remarkably good when it comes to resisting wind ‘shear’).
The first straw bale houses were load bearing, and the US’s oldest existing straw bale house (built circa 1896) is load bearing, but these days there are many more post and beam ones. A number of states–California, Arizona, Colorado, Texas, Utah, etc.–have incorporated load bearing into their building codes. New Mexico, where I primarily work, has not yet done so but has issued loadbearing permits on a case by case basis; I’ve built five such houses in this state
Advantages of loadbearing:
-Obviously, you use a lot less wood when building the house. -One doesn’t need to fit the bales around the post and beam structure. -It can be constructed faster than post and beam. -One doesn’t need the carpentry skills of post and beam construction. -It could be considered the ‘greener’ option.
Disadvantages of loadbearing:
-One has to be careful to not put too many doors or windows into the wall, or there won’t be enough strength in the straw bales to hold up the roof. -One needs to brace the walls as they go up, otherwise there’ll be nothing preventing them from falling over until the top plate or header is put on. -No matter how consistent and tight the bales are, the top plate will in all likelihood be a bit lower at one end than the other end. -Doors and window openings need to be treated carefully, or the weight of the roof will tend to push down against the tops of them. -One is limited in the size of the building. In other words, if you have a large house (or a two-storey house) then the roof is going to weigh so much that the bales can’t take its pressure.
Advantages of post and beam:
-One can design multiple doors and windows and two (or three) storeys and all kinds of other things without worrying about whether the house will stand. -It’s easier to produce a square and level house, since the post and beam structure is, well, square and level. -There are plenty of attachment points on the inside of the house for cabinets, shelving, countertops and so on. -Building officials and inspectors have an easier time accepting the post and beam concept, which makes for a smoother building application process. -One can put the roof on before bringing the bales on site, thus ensuring they will stay dry.
Disadvantages of post and beam:
-Construction is somewhat more expensive than the loadbearing option. -Erecting the post and beam frame requires carpentry skills. -One has to fit the bales around the post and beam structure. -One needs to brace the house very well, until the bales are installed and resist wind shear.
So what is the best method of building straw bale houses, I’m often asked: loadbearing or post and beam? My answer is that I personally have a preference for loadbearing, due to its ease and simplicity—but in many cases that’s not practical and post and beam will work better. Sometimes, in instances like two of the photographs above where most of one wall is doors and windows with too few straw bales to support the roof, a combination of the two methods could work well. Whichever method one chooses, building with caution and consideration for all elements of the design is a must!
- The Original Nebraska Straw Bale Buildings
- The Last Straw History of Straw Bale Resurgence
- Load Bearing Wall – Wikipedia