So I find cob houses to be super interesting and something I really want to try. I do a fair amount of construction work in college (I am a college freshman but I'm collecting information for my hopeful future) and will be learning welding next year. I am also pretty skilled in ceramics work and drafting, so I feel like a cob house would be a truly excellent experience. We've got clay soil and cheap clay and sand nearby, as well as plenty of straw.
However, I was wondering if it would be feasible to use a different plant or make my own straw. We have a very serious phragmite (water reeds) invasion on our local rivers. If I were to harvest and trim the stalks and get them dried out, would this be a good add to the cob mix? They're such a pest but I want to be able to put them to use. They're pretty thick and strong, and I'm sure I will find a use for them somehow (it'd be a waste not to), but I can't think of any off the top of my head.
I also have big fields of native grasses and plants around my house (my property so I can take them), would it be possible to harvest, dry, and use these in cob? I doubt they'd be as tough as straw, but some of them are pretty strong.
I could buy straw, and probably will, but I feel like it might be nice to add some of my own handmade dried grasses/reeds to it.
Hi Sparrow, welcome to permies. Surprised no one got this one before me, but the phragmite would be an excellent fiber source for cob as would any other grasses that are dried out. Straw is normally just readily available and will save a ton of time since you don't need to harvest. It is really always surprising how much straw you need for good quality cob.
You don't have any info about yourself posted. What region/state are you located in so that we know if cob is an appropriate technology for your area. Cob is well suited for warmer climates, but not as efficient in cold weather locations. You should build a small cob oven or wall with your own materials and see how you like harvesting and processing before you commit to a larger project.
I'd think phragmites would work fine for cob; like Daniel mentioned I think straw's use is ubiquitous only because it's more commercially available. Warm-season grasses like big and little bluestem, prairie dropseed, and Indian grass should work fine too- the species are different, but the material (cellulose) is structurally the same.
Maybe experiment, build a couple bricks of different materials, a couple 3x5 walls and see what works best for your situation. I don't think you'd have a problem substituting one fiber source for another, though.
A major reason straw is used instead of hay (which is generally cheaper in places I know of) is that straw is just fiber without nutritious leaf or seed material, so it presents less to support microbes inside a wall. I think you would need to harvest the reeds after they die off for best results. Also, they might be too coarse for good cob unless processed to make them straw-sized. I do agree that reeds might serve best as thatch instead of cob reinforcement.
Pine needles often occur and in thick mats under the tree and they are the perfect size. I would go with something that doesn't require processing like pine needles.
Last year I coated part of a seasonally submerged wall with clay (clay has some fine sand in it) and pine needles and to my surprise it has not peeled off. I might try and coat some more ponds with the mix this year.
We have an accidental automatic cob mixer. The pond receives clay deposit mixed with fine sand and the pines above are always adding the straw. I just have to wait for the water to dry up and then add a little more pine needles to it.
Adding hydrated lime to it makes it more brittle but harder. I think that the extra hardness is ok as long as I use lots of pine needles. Can anyone tell me if I'm wrong has anyone tried lime cob.
Location: Upstate NY, zone 5
posted 1 year ago
Pine needles are a good size, but I think they are considerably weaker and more brittle than straw when they dry out. I expect they would be better than nothing.
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