Stacking Functions: Trusses as Concrete Forms

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As the house progresses steadily, it’s pretty amazing to realize that every step we’re taking is bringing us one step closer to realizing our dreams. It’s all of the little details which add up to become a house.

Yesterday and today we have been working steadily on building the parallel chord trusses, which will double as concrete forms.

Here is an example of the parallel chord truss we are building. This example is actually using pallet wood, but we are using dimensional pine lumber from our nearby sawmill. Image from Mother Earth News.

We’re at the beginning of getting our systems down and work site set up. We’ve chosen a spot with great morning-midday shade, but the afternoon can be brutal. Most days we are getting high humidity and 80-85 degree sunny weather. This means the sweat is dripping and the sawdust is sticking to our legs!

I’ve set up making the cuts for the diagonal pieces (“webs” in the diagram above) and 6 foot lengths and Ini is putting them together. Each truss takes 12 diagonal lengths and so I’ve taken to organizing my cuts into buckets for easy delivery.

Meanwhile, Ini has been hammering the trusses together. (Note my choked up hammer grip due to wrist fatigue, he says…)

This method of truss building is simple, albeit labor intensive and also uses small dimensional lumber (rough cut 1x4s in this case). As noted in the above diagram, this method can make great use of pallet wood, a ubiquitous offshoot of our consumptive culture. Furthermore, the style of truss will allow us to easily span 16 feet and maintain a cathedral ceiling while providing a 12″ deep cavity which will be filled with blown in cellulose for an R value of 36. Add some dead air space and radiant barrier and we should be achieving closer to R 50.

The simple geometry involved in truss building relies on the inherent strength of triangles. By using this simple principle, we can create a unit that is much stronger than the sum of its parts and efficiently utilize wood.

As always we are seeking to maximize outputs and minimize input. This is at the core of Permaculture. In this case we are stacking functions of using these trusses as concrete forms when we pour our stem wall. We’ll add plywood and then oil it so the concrete doesn’t stick. Once the concrete is set, we will remove formwork and install as roof assembly. It might seem weird that we’re building a roof before even starting on the wall, but there is a method to our madness…

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