Sept 30th, 2018: Cob House

[xoxoxoBruce]

Cob houses can be really cool because it’s basically mud and you can shape it like pottery.
Depending on who does the sculpting, carving, it can look like anything you want.
Mine would probably look like a third grader's art project, but I know monster could do a cool one.

Earth is probably still the world's commonest building material. The word cob comes from an old English root meaning a lump or rounded mass. Cob building uses hands and feet to form lumps of earth mixed with sand and straw, a sensory and aesthetic experience similar to sculpting with clay. Cob is easy to learn and inexpensive to build. Because there are no forms, ramming, cement or rectilinear bricks, cob lends itself to organic shapes: curved walls, arches and niches.

Earth homes are cool in summer, warm in winter. Cob's resistance to rain and cold makes it ideally suited to cold climates like the Pacific Northwest, and to desert conditions. Cob has been used for millennia even in the harsh climates of coastal Britain, at the latitude of the Aleutians.

Wait a minute, resistance to rain? I have a yard full of clay. Three drops of rain turns it to grease and with all the rain this year it could trap dinosaurs.

If you want to build with cob then you must test the quality of your clay. Cob also depends on the coarseness of the sand. This is how to do a snowball test. Combine your clay soil and sand in different proportions: 3:1, 2:1, 3:2, 1:1, 2:3, 1:2 and 1:3. For each, mix the sand and clay thoroughly and add just enough water to make the grains stick together when you squeeze a double handful very tightly.

Make compact balls of the mixes about 6cm (2.36 in) diameter. Then while holding a ball between thumb and index finger of one hand squeeze the ball with the thumb and index finger of the other hand at right angles to the first.

A ball made dry enough from the ideal mix should not distort by more than 0.5cm (0.2in) and be hard enough not to break. Then hold each ball 1m above soft ground e.g. a grass lawn, and drop the ball. If the ball breaks it is too dry or contains too much sand. If the ball deforms it contains too much clay or too much water. The ideal mix will maintain its shape on impact

When you have discovered the correct clay, sand and water mix use a tarp to mix large batches of clay and sand. Mix these dry in the tarp until you can't see patches of clay or sand then add water little by little treading and rolling the mix in the tarp. Once lumps are broken up begin to add the straw. Tread the straw in until it is covered with the clay/sand/water mix and then turn in the tarp. Repeat adding more straw and turn in the tarp from different corners making sure the centre is also turned. Continue adding straw until the mixture feels like a tough substance rather than a loose (squishy) mud. The cob in the tarp will now turn as a single mass. Adding more straw now is very difficult so this is when the mixture is ready

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[glatt]

I would be curious to see a recipe for adobe. I don't think they used straw with adobe and it was used in the desert with little rain.

[Diaphone Jim]

Straw of many kinds is an essential binding ingredient of adobe.

[Gravdigr]

What keeps stuff (grass, weeds, flowers, marijuana) from growing on it?

Don't bugs of every description lurve dirt?

[Griff]

Cob may be a little different, but I think rammed earth construction usually has a binder mixed in it, a little cement. The soil itself is screened mixed and tamped making it a bad growing medium. I've got a book called Alternative Housebuilding by Mike McClintock on my shelf that gets into the process.

[Carruthers]

Until this thread, I'd never given much thought to the wychert houses in the area around Haddenham to the west of Aylesbury.

Wiki:

Wychert or witchert (with a number of variant spellings existing and meaning "white earth") is a natural blend of white chalk and clay which is mixed with straw to make walls and buildings, usually then thatched or topped with red clay tiles.
This historic method of building construction is localised to Haddenham and the surrounding local area in Buckinghamshire.
One of the largest known wychert structures is Haddenham Methodist Church.

The method of building with wychert is similar to that of a cob building.
To maintain the rigid nature of wychert it must not become too dry for risk of crumbling, nor too wet for risk of turning to a slime.
Keeping wychert well ventilated and not subject to excess condensation is therefore highly recommended.
Any render applied to a wychert wall must therefore be of a breathable material — rendering wychert walls with a lime based render is therefore common practice.

Link

There's a short academic paper (pdf) from the Texas A&M University College of Architecture linked in the above Wiki article via Wayback Machine:

The Witchert Buildings of Buckinghamshire, England:
Learning Sustainable Construction from our Ancestors

Wychert appears to be a nightmare to maintain and I wonder if it is possible to raise a mortgage on such a house.

Restoration work in progress:



M H Restoration

Earth Buildings and Their Repair

[xoxoxoBruce]

Originally Posted by Gravdigr

Don't bugs of every description lurve dirt?

From Carruther's link...

The deterioration that can be seen in figure 3 was caused not by water but by the activities of Masonry Bees who have nested in the walls. Insects and rodents make no distinction between earth in the ground or in walls, and their activities have caused considerable damage to many buildings in the area.

[Gravdigr]

I takes m'victrees where I finds 'em.