Creating Round and Curved Straw Bale Walls

Round buildings have long been a part of the construction world, and creating round details in conventional homes has been a hassle for about as long. Straw bale walls, on the other hand, lend themselves nicely to the creation of round and/or curved shapes.

Because a bale is essentially a bundle of loose straw tied into a rectangular shape, it can be bent into curves while retaining its structural strength. The key is not to over-tighten the bale when curving it.

In other words, you will need to make sure that the diameter of your curve is not too tight. If you try to make a bale fit around too tight of a curve, the inner string will pop free, and the bale’s structural integrity will be lost.

Framing Considerations

The hardest part about building round walls in straw bale construction is still the conventional part: the framing and the inclusion of windows and doors. Toe-ups and other framing materials are rigid and don’t offer the same level of flexibility that bales do. Get as close to the desired shape as possible with the rigid framing materials, and then use the bales and plaster to smooth out the differences. There are several approaches to consider to create the base for your curves in the framing (click the arrow to see more details):

  • Use small sections of 4×4 toe ups fastened together along the curve with miter joints at each union. This works, but it means a lot more anchor bolts if you are placing your toe ups directly against the slab foundation because code requires a minimum of two anchor bolts per piece of wood. In this way, bolts that would otherwise be placed 4′-6′ apart would be placed only 12″ apart on small sections of toe ups.

  • Use a flexible steel channel for the toe ups as shown in the image. This option allows you to fasten the channel to the foundation and then span the toe up material in the channel such that you minimize the number of anchor bolts as what was discussed in the above option.

  • Use larger pieces of wood and cut them on a band saw or with a saber saw to match the curve. This allows for anchor bolt minimization, but tends to waste wood.

  • Use a board steamer to bend the toe-ups to match the curve. A great option if you have access to a steamer, but not so great if you don’t.

Incorporating Windows

When windows are added to the equation, it gets harder.  Once again, you are dealing with a rigid building material and trying to conform it to a bend.

Because you cannot bend a window or door, you have only a couple of options. You will need to either place a flat window or door into the curve and provide protection for any areas of the bale ledge that result or pay the extra expense of buying curved openings. To be sure, curved windows and doors are not cheap; however, they are an available option if your budget allows them.

For the first option, you will have to create extra sill protection to stop the walls from taking on water because the center of your windows will be recessed into the bales by the simple fact that they are straight and the walls are curved. With proper attention to flashing details and the creation of a sill, this does not present a problem and can enhance the look of the building.

Round Walls & Load-Bearing Wall Systems

Round walls can be an excellent application for load-bearing construction. Although load-bearing straw bale construction (the bales themselves act as the structural support) is outside this course's scope, it is important to note that curved walls in load-bearing construction have their nuances.

As mentioned above, the most challenging part of building round walls with in-fill construction is incorporating rigid framing members. This holds for load-bearing members, too, but for different framing elements.

While load-bearing straw bale construction simplifies some aspects of curved walls, the primary challenge is constructing the continuous top beam or box beam. Using traditional wooden framing can make maintaining structural integrity around the curve challenging. 

An alternative is to cast a curved concrete beam at the top of the wall. This provides the required structural support while allowing the curve to be more easily maintained. However, the concrete beam must be designed with proper thermal breaks to prevent it from acting as a thermal bridge and undermining the insulation value of the straw bales.

Careful detailing is required to ensure the concrete beam does not create a pathway for heat loss. A curved concrete beam can be an effective solution, but it requires specialized engineering and construction to address both the structural and thermal performance needs.

In summary, while load-bearing straw bale construction simplifies some curved wall challenges, the top beam detailing remains a crucial consideration that requires thoughtful design and execution, particularly to mitigate thermal bridging.

Tomorrow we will cover construction options for when a straw bale wall section cannot be built in a running bond pattern.

Happy Baling!

Timbo & Team StrawBale.com