COBBuilding Construction

ByLakshunha

KanyaAnusha

What is Cob?

• The word cob comes from an Old English root meaning "a lump or rounded mass."

• It's a traditional building technique using earth mixed with water, straw and often sand.

• It dries to a hardness similar to lean concrete and is used like adobe to create self-supporting, load-bearing walls.

COB

What is Cob made up of?• Cob is a very old method of building

with earth and straw or other fibers. It is quite similar to adobe in that the basic mix of clay and sand is the same, but it usually has a higher percentage of long straw fibers mixed in. Instead of creating uniform blocks to build with, cob is normally applied by hand in large gobs (or cobs) which can be tossed from one person to another during the building process.

Cob is normally applied with hands.

How is cob different from adobe?

• The most basic difference is that adobe is formed into rectangular bricks which are dried in the sun before being used, while cob is built wet.

• Cob therefore lends itself to organic shapes: curved walls, arches and vaults.

• Building with cob is a sensory and aesthetic experience like sculpting with clay.

• ADOBE • COB

Water Proof• Cob is very resistant to weathering

and can withstand long periods of rain without weakening.

• Too much exposure is best avoided by building roof overhangs and gutters to protect the walls and constructing a high impervious foundation.

• In windy areas a lime-sand plaster is traditionally used to protect exterior cob walls from wind driven rain.

Fire Proof• Cob does not catch fire. • However, still be aware of

your ceiling or roof as it might not be flame resistant like the body of the cob home.

Acoustic Privacy• Earthen walls have very good

sound-absorbing properties that keep it quiet inside, both from exterior noises and noise generated in the same building.

Termite Proof• Since cob is a mixture of soil,

aggregate sand, and straw it is not attractive to termites.• To add to that, cob homes

have not been known to become burrowing grounds for insects or animals either.

Healthy• Unlike conventional homes which are constructed with

synthetic, industrial-formed materials, cob homes are built almost entirely out of natural, clean materials.

• Industrial homes are full of indoor air pollutants and off-gassing of chemicals contained in the building materials. Cob homes do not have this problem. In fact, cob actually “breathes” through its tiny pores and keeps air fresh and clear. If you suffer from indoor allergies then living in a cob house could significantly improve your quality of life by eliminating the toxins that may cause them

Energy Efficient

• With a cob house, you will not have to heat your home with nuclear electricity, Persian Gulf oil, or strip mining in Wyoming!

• Cob is a “thermal mass” that absorbs sunlight and warms the building over the course of the day. This is called passive solar heating, and it will keep the inside of the building warm in the winter and cool in the summer

Owner Built

• Cob houses can usually be built by the owner. It is very rewarding to be able to build your home with your own two hands.

• You don’t need a degree in architecture or to be a structural engineer to build a cob home.

Construction Technique• It's very environmentally-friendly - all done by hand. In fact,

you can have the walls take on any shape you wish, something you'll find in many adventurous modern cob homes.

• The only thing you really have to remember is to lay the material in courses, and let each one dry thoroughly before laying the next, and make sure they taper slightly as they rise.

• You can embed windows and doors as you build, or simply cut holes later.

• Cob is excellent for load bearing, meaning you can easily make a two-storey house, and it has very good insulation for both heat and noise.

• During the day it absorbs heat outside, so it's cool inside, but at night radiates that heat into the interior.

The Traditional Building Technique

• The traditional material for English cob was soil (clay-based) mixed with water and straw, sometimes with crushed flint or sand added.

•  People shovelled or stamped the mixture together, after which a cob fork was used to ladle it onto a stone foundation, before workmen on the walls trod it into place.

• It was quite possible to lay a course or "lift" of cob between 150mm and 900mm high (but usually averaging 450 mm) in a single day. 

• After it had dried - which could take up to a fortnight - the next lift would be added.

• The walls would be trimmed to plumb and straight as they rose and made between 500mm and 900mm thick.

• The builders would either leave openings for windows and doors, adding stone lintels as they went, or carve them out later. It was a community effort, with men working one day a week to build a house in a season.

Modern Construction• The biggest development has

been Oregon cob, where people mix the material into mud loaves, then add them individually to the wall before treading them in. • This method means houses

can have walls that are stronger and thinner (generally 300-500mm thick on load bearing walls, as little as 100mm on others).

Foundation• You need to start with a foundation that's wider than

your wall will be - 300mm wider is recommended - and deep enough for the load.

• Usually you'll build a cob house on a stone plinth or a concrete base, raised off the ground about 600 mm - for obvious reasons it can't come in contact with the ground. There are any number of foundation options you can use, like a rubble trench, earth bags, or even rammed tyres.

• You'll also need a good roof overhang to protect the cob, at least 200mm. If you find any vertical cracks, use cob or even clay tiles to fix it before moisture can penetrate.

Types of Foundation• RubbleTrench- For proper drainage, it’s

important to build a rubble trench below the perimeter of your cob building. It’s located beneath the stemwall and is filled with drain rock (small stones or round gravel).

• Once the trench is dug, it is good to cover the bottom with a few inches of drain rock and then lay a 4-inch perforated polyethylene drainpipe along the bottom to improve the drainage runoff. The bottom of the trench should also be sloped.

StemwallStone-You can use any type of stone that you like. Stones give the house a natural and homey charm to a cob house. However, the stemwall may be one of the most expensive parts of building a cob home due to the cost of some stones.

StemwallUrbanite-If you want to save money constructing your stemwall then you may want to consider using urbanite instead of stone. Urbanite is recycled concrete that comes from old sidewalks, buildings, etc. It is very versatile, free, and lasts practically forever.

StemwallFired Bricks-Another option is to use fired bricks. They can be any type. The price can vary, but you may be able to find old ones to save on money.

StemwallConcrete Blocks-You can use solid concrete blocks or cinder blocks. Cinder blocks work, but I would not recommend them due to their fragile sides. That’s just my personal preference though.

StemwallPoured Concrete-You can also create forms and pour concrete for your stemwall. This just seems like a lot of extra work to me though. Creating forms and filling them all with cement. It is more technical and doesn’t look as nice or natural as stones.

Cob under construction