This is the first post in a series explaining how we built our very first earthbag dome at Milkwood Farm. Earthbag is a natural building technique that uses bags of earth to build structures that have incredible tensile strength and a very low energy footprint…
The first step of the build was choosing the site. We chose a spot 2/3 of the way up our hill, above our tinyhouse, as we’re planning to use this dome as guest accommodation. The site faces due north so that, when the dome’s door is open in the cooler months, sunlight will flood into the space. Now for some foundations!
There’s a couple of ways to approach earthbag foundations, but we went for a french drain beneath the foundation course to ensure good drainage. This involved digging a trench (and as usual we hit lots of boulders – ahrg!) in the shape of the foundation course of bags. Then we created a french drain using slotted pipe and gravel.
Digging the foundations also involved defining the middle of the dome, in order to erect a center pole to be used during construction. It’s all about distance from the center, when you’re ensuring a consistent circumference for each course of bags! On this pole slides the movable compass, which is the all-important tool for creating a dome shape. More on that once we get to building the walls.
Next step was a waterproofing layer between the drain and the foundation course of bags. We used heavy-duty plastic. Because sometimes, plastic is the best solution. Strange, but true.
Finally it was time to do some earthbagging! First of all we all sat down while Neil and Stella of Guiding Star, our earthbag building experts of the moment, explained how it would all work:
We would work in teams of two, each with a specific job. Each day, our jobs would rotate, and we would teach another team how to do the task we had been doing. Simple, effective knowledge transfer. And now time to get bagging.
The bags were long and tricky to fill until you got the hang of it, which over 6 days we all most certainly did. The first four foundation courses of bags needed to be made of stabilized earth, so we added a little cement to the earth and gravel mix.
Once the first ‘course’ of bags was laid down with the help of many excited hands, it was time to ‘tamp’. Tamping is the process of squashing the bag flat. Tamping gets all the air pockets out and turns that bag course into a rock-solid sausage which can take tonnes and tonnes of weight.
There would be much tamping over the course of this week.
The next step before laying another course of bags is the barbed wire. The barbed wire is cut and attached to each course of bags to improve the overall tensile strength of the dome. With barbed wire between each course, the bags cannot slip sideways at all.
And once the structure is completed, all that tensile strength makes it so strong that 20 people can climb on top (we checked).
The barbed wire is hooked into the bags of the course on which it is laid, with more barbs sticking up. When the next course of bags go on, the barbs will attach to this new course, locking the two courses together.
Foundation course laid. Three more of these, and it will be time to start the walls! Stay tuned…