Today I received the following question about building with straw bales on a steep slope and I thought it was worth sharing my response with you all. Here’s the question, my response follows:
“Well the time for using all this information is almost upon us. We are looking at a property at the moment to build on; however, it is quite a steep property. Do you have any ideas or photos of straw bale houses built on steep sites with out doing major earth work? Thanks in advance.
Regards Chris and Jenny”
Keep in mind that straw bale houses do not have to be built on slab foundations. In fact, straw bale walls work very well on top of framed floor systems, but be sure to include their weight in any engineering of the floor system. There are a number of ways to work on steep slope sites. Perhaps the best use of space is to incorporate a daylight basement into the design. This allows the owner to utilize the space under the house that would otherwise be wasted.
The construction of the foundation to support three stories as opposed to two in a slope situation does not add that much cost and, again, you get the finished space out of it. In fact, it could be used as the first story and then only one story could be added above it so the foundation would not have to support three floors. You can see in the photo below that some additional foundation work is required to hold back the earth on the uphill slope; however, if the slope is very steep, the depth of the retaining wall will be limited.
Photo Credit: Sierra Log Homes
Another option for a steep slope building site is to utilize a pole structure concept. In other words, if you do not want to enclose the space below the house as a daylight basement and want to avoid as much additional foundation work as possible, you can build the house on a stilt like structure. This foundation limits the amount of necessary concrete and can support a home of adequate size above it. The size of the home will dictate the size of the concrete footers and poles as well as the bracing in between them.
This style of construction is often used in flood prone areas along the coasts and, of course, in steep terrain. The sketch below will give you an idea of how these foundations work. The sketch is courtesy of the Food and Agriculture Organization of the United States. The height of the poles will also have an impact on how much cross bracing is required. This type of foundation will have to be engineered professionally as will the daylight basement. There is simply no way around foundation engineering when working on slopes.
Another option is to create a hybrid system that uses both the pole foundation and daylight basement technology. Portions of the home can be enclosed with the daylight foundation concept, while the remainder of the home is supported by poles. This is a good idea when you want to use some of the space beneath the house, but not all of it. By employing both options, you can get the space you want while saving money on the foundation by using the poles.
No matter how many stories you choose to build or what system you choose to employ, building on a slope is more expensive than building on a flat lot. You will have to get some good numbers from a local concrete contractor for the cost of any foundation in this situation before you purchase the land or design the house. It is crucial that you know what costs you will encounter before you commit to the lot. As mentioned above, you will also need to hire an engineer for the structure. Happy Baling.