What Is Cob Plaster?

A: What you’re referring to is an earthen plaster, rather than cob. Cob is a structural mixture of clay, sand, and straw, used to build walls. The same ingredients can be used to make an earthen plaster.

How do you make a cob of plaster?

Natural plasters and finishes can be applied to cob walls or even to drywall board!
Now for the plaster recipe:

1. 1 part sand.
2. 1 part kaolin clay (you may need to add a bit more than the sand)
3. 10-25% wheat paste (you will have to experiment with your unique mix for best results)
4. Water (amount varies)

What is cob made of?

Cob, cobb, or clom (in Wales) is a natural building material made from subsoil, water, fibrous organic material (typically straw), and sometimes lime. The contents of subsoil vary, and if it does not contain the right mixture, it can be modified with sand or clay.

What is cob used in building?

In short, cob is essentially clay, sand, and (usually) straw mixed together to be used as a building material. In some ways, it is similar to bricks, but it is also advantageous to the builder because most of the building materials are available on any given site, or they can be purchased easily and cheaply.

Can you plaster over cob?

Cob and earth walls often have a friable surface which needs to be prepared before render or plaster can be applied. The wall surface should be dampened down on the day before application. A harling coat which is designed to stablise the friable cob is then applied to the wall by casting on from a HARLING TROWEL.

What is the mix for cob?

Cob is a mixture of clay, sand, straw, and water. Clay is the binder; sand gives the mixture strength; straw provides tensile strength (think “rebar”); and water helps activate the clay to hold the mixture together while building.

Are cob walls waterproof?

Cob is very resistant to weathering. Because of its porous nature, it withstands long periods of rain without weakening. However, too much exposure is best avoided by the “boots and cap” strategy: wide roof eaves to protect the walls and an impervious foundation.

Is cob as strong as concrete?

Cob has unique properties which make it as strong and more durable than concrete (see ‘How is Cob so strong’) the oldest Cob building is over 10,000 years old. Cob is resistant to fire, termites and mould.

Why are they called cob?

“Cob” is an old name some people gave to spiders. Generally we call them cobwebs when “old” spiderwebs collect dust and debris in them and no longer have a spider active in them.

Does cob absorb water?

Corncob is an enviro-friendly and biodegradable product that can be used to absorb a variety of fluids, including water, oil, grease, sludge, and coolant, making it ideal for routine shop spills or large industrial absorbency needs such as oil fracking.

How thick should cob walls be?

How thick should cob walls be? The thinner that cob walls are the less load bearing capacity and thermal mass that they will have. I recommend the width of cob walls be between 18 to 24 inches as a standard. This gives plenty of load bearing capacity for a one story building and plenty of thermal mass.

Can you paint cob walls?

Yes! Cob can be painted. You can create all kinds of natural paints and pigments to paint your cob walls.

Are cob walls warm?

Cob has several other important characteristics, firstly walls are usually very thick, typically 2-3 feet and this, as well as being very good insulation against heat and cold is a massive heat store. So there is a fly-wheel effect on thermal movement – in other words, cool nights and warm days are evened out.

Can you waterproof cob?

A: Linseed oil and lime plaster are both sometimes used to seal cob against weather.

How do you damp proof a cob wall?

There are a number of strategies that may be employed to help control levels of damp within them.

1. Using Air Flow to Control Damp.
2. Perimeter Drainage to Control Damp.
3. Maintaining Gutters and Downpipes to Control Damp.
4. Using the Heating to Control Damp.
5. Maintaining Chimneys to Control Damp.
6. Breathable Finishes to Control Damp.

How do I stop my cob from cracking?

Another possible solution is to increase the amount of long, strong straw in the mix. This will also reduce or eliminate structural cracking as the cob dries.

Can you add cement to cob?

Cement mortar mixerswill mix cob if you can handle the fumes and noise. As with manual mixing, the clayey soil and sand is mixed first, then the straw added. The cob has to be mixed wetter than with other types of mixing to allow the cement mixer to turn without too much stress.

Do you need sand for cob?

There are some rare areas where you will find soil that naturally contains the right amount of aggregate-to-clay ratio to make cob, but this is not usually the case. One of the main guidelines for choosing sand for your cob mixture is to use a rough and coarse sand that has many different particle sizes included in it.

Can you use a cement mixer for cob?

A concrete mixer will only tumble the materials round and round. This works for mixing very wet material such as concrete and some earthen plasters. A concrete mixer will not mix cob.

Do cob houses get moldy?

There are no mold issues with it at all. So essentially, as long as the cob gets a chance to air out and dry out there’s no real chance for that mold to grow on it.

What are the disadvantages of a cob house?

Disadvantages:

• Labour intensive.
• Relatively slow to build.
• Obtaining a mortgage from lending institutions is difficult.
• Cob walls cannot be laid in wet or freezing weather.
• Insects, notably termites and small rodents can burrow into the walls and weaken them.