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.
19 Responses
I too own property with a steep slope and have been wondering if I can still build a bale building on it. It seems like using the pole foundation like you describe would not really support the weight of a heavy straw bale structure above it. How can I be sure that it will be safe in an earthquake prone area? Thanks.
The key is to make sure the posts, beams, and bracing are engineered by a professional. These structures can be very strong if properly designed and implemented.
You say “There is simply no way around foundation engineering when working on slopes.” But in the load-bearing video, don’t you build on piers because the ground slopes? Does that mean engineering may not be necessary if you build really small, or if the slope isn’t too severe?
TC,
That is correct. If the building is small and/or the slope is not steep, you can usually build a foundation to code with no engineering. When you get into two stories and larger structures, the loads are much bigger and so the foundation must be capable of handling the weight and the pressures of the slope.
Andrew, I want to build a very small studio (224sq ft, one story@12″ high) on a slight grade. I wanted to do this on a pole-built system, using four uprights connected by a crossbeam “platform” floor with a daylight basement. I was told placing straw baless on this platform would cause the dew point to rise into the bales and make them rot- is this true?
And as a second question, do you think this project is doable for $3500?
Shan,
I do not see there being any problem whatsoever with the raised floor system. You can add some cheap insurance by using the roofing felt and gravel or insulation underneath the toe ups to keep any migrating moisture away from the bales. If, however, you are creating a conditioned space in the daylight basement, then there should be no additional moisture to worry about. If the basement will be unfinished and unconditioned, you will need to provide adequate ventilation to keep air movement high and thus moisture build up low. The price seems reasonable, but will depend on material costs in your area and the labor costs you deal with. If you plan to do the work yourself, then you should be fine; however I doubt you will be able to complete the structure with paid labor for the price you name.
Can bales be used below ground?
I do not recommend that bales be used below grade. I believe the risk of decomposition is far too high.
When you build a wood basement, you can spray a water/insect coating over the wood. I hoped a similar method could be used. When using bails below grade/ground level.
Hello
I am looking at building a small 400 sq ft straw bale building. I live in British Columbia where the code for the frost line is 4 feet below grade. I am looking to save some money on the foundation and I am looking to use sauna tubes with a sub floor built on top then the timber frame and straw bales on top. Have you used or heard of anybody using this technique before? The other idea I have is to use a frost protected shallow foundation slab on grade.
Ian Kirschner
After watching your loadbearing wall dvd, I got the idea to use our local natural no-rot black locust tree as piers for a 24 x 24 sb cabin.
What would be a good width to use that is both wide enough for structure and also for visual appeal – seeing the wood. If I use a wide enough log, can I avoid digging down to the frost line? If not, how far down should the post go before it touches the cement in the hole?
This is a pretty technical question in terms of structural support and is best answered by an engineer which I am not. The size of the beam will depend on the loads it is carrying and the span of the area over which it is carrying them. In terms of the depth, you will need to go below frost line for sure so there is no heaving of the posts. The depth of the concrete will also depend on things such as the soil type, water table depth, and so on. Sorry I can’t give you better answers, but I don’t want to mislead you by generalizing my response either.
hi, can i use 15 lb. roofing paper for my foundation flashing instead of aluminum? Would it do as good of a job of preventing water seepage or would i just be wasting my time? rhanks
aaron
Hi Aaron. If you plan to replace the flashing with felt, then I would suggest you use a 30# felt. 15# will rip far too easily.
I am a student wanting to use straw bale construction for my architecture project. The site is in Kentucky and on a steep slope. I would like to use it for a great sound proofing solution as well as making this project LEED – Platinum! is this a smart building choice or should i consider something else. I thought about just applying the bales to the east and west wall because that is where the noise from the city and the highway enters. Also, do you know if I applied bales to the south wall how much that wall would heat up from the sun and if/how i could use that heat and transfer it to heat the inside of the building? Thank you!
Hi Amy. Straw bale would be great for the soundproofing and insulation. You can use it on the South wall as well, but it won’t transfer heat into your house. SB is not a thermal mass but a thermal insulator. In line with that, it does not absorb or transfer heat, it insulates from it. Be sure that the humidity is not too high. In high humidity climates that don’t have a dry season, bales may not be the best option for you.
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When you’re talking about SB, are you always taking about SB pasted with cement or without? Could you deal with the damp issues by using a waterproofing over the cement & SB?
I would not recommend using a cement based plaster at all. I recommend lime plaster or earthen plaster only. You don’t want to add waterproofing either because moisture will get trapped in the walls then. It is better to use the lime or earthen plaster to allow the walls to self regulate.