Learning How to Make and Build With Cob

A few months before we started on our cob building Henry - we went to a natural building workshop an hour from where we live. One of the things we learnt at this workshop was how to make cob.

Cob really is not rocket science and with a little practice and experience - anybody can do it.

Cob is a mixture of 3 materials...

  1. Clay
  2. Sharp Sand or crushed rock (River sand is sharp, beach sand is smooth)
  3. Straw

The key is to learn how to identify clay soil (silt and clay can be quite similar) and then to build test bricks to see what your strongest mixture is.

Some people get lucky and their soil is almost the perfect cob mixture. They just need to add a little straw, mix it up - and they have cob. For most people though, there will be a little playing around with the mixture required. For example... Our soil was very high in it's clay content and so we needed to add a lot of crushed rock. We got this from the local quarry and it had a 'largest size stone' of 5mm. What this means as far as I am aware is they sift it through a 5mm 'sieve' to get the tiniest little crushed pieces out. This stuff is perfect for cob.

 

How to test your cob for strength?

We trailed a few different mixtures by building 'test bricks'...

testing cob mix

To make a test brick, you just mix up a few different cob mixtures with different ratios of clay/sand/straw - then you clearly mark which one is which and make bricks out of them...

Leave them to fully dry in the sun but out of the rain (this is MUCH easier to do in summer, or at least not in winter as we did it!). Once they are dry, you test them for strength. You can do this by holding them with a corner facing the ground, one meter above a flat concrete floor and dropping them...

If your brick crumbles, you know you have a weak mix. If it doesn't have much damage - you've got a goodie.

In the Earth Building Standard of New Zealand I believe that if a piece 10cm long or less breaks off, it is considered a pass and a mix that is suitable for building. Yes, that really is an official and acceptable method of testing - so wonderfully simple! There are also shrinkage tests and erosion tests you can do - but we didn't do that on our little cob building.

Our final ratio of materials was:

3 parts crushed rock (5mm and less) to 1 part clay. We then added about a 1 bucket of straw. It's not critical to measure the straw very accurately - you just get a feel of when you have enough.

 

How to mix cob

For the first 1/5th of our building we mixed the cob by foot - old school style. It was lots of fun and we played Bob Marley music while we did it. We kept laughing and calling ourselves "Hippie Central" even though we're not really hippies at all... Although admittedly, I do not know the definition of a hippie!

To mix cob by foot, you...

  1. Get a big tarpaulin (or as we discovered a big square of old carpet is easier to come by, and cheaper as people often throw it out!)
  2. Lay tarpaulin or carpet on the floor and put sand and clay on it (plus some water)
  3. Stomp it together with your feet
  4. Turn the mix from time to time to make sure you're mixing it well
  5. Add water as is needed to make mixing manageable
  6. Once it's looking nice and mixed, you can add the straw and stomp it in too

Here are a few pictures or mixing cob to explain it in a visual way...

cobbing

Turning the mixture

cobbing

cobbing

Well mixed cob

mixing cob

 

And once you have your nicely mixed cob - you take it in manageable sized blobs and smoosh it on the wall. Traditionally you make the mixture into "Cobs" small balls of cob about the size of a small loaf of bread - and then pass it to somebody who is doing the cobbing. This makes it easier on the back, but if you're high on cob, like Tom and I were - you can take it in as big a load as you can handle and stick it straight on the wall. We nicknamed these large loads of cob "Shloggles".

Make sure you 'knit' the new cob you've just applied to the wall to the cob around it. You do this by sticking your thumbs into the mixture all over it. You can also use pieces of wood for this, for example, the end of an old broom. Ideally you will not see where the new 'shloggle' or cob has been applies as it will just blend into the last one.

Mixing cob by foot is fun, but it is hard work. It's ideally suited to building with communities - where there are lots of people to chip in and help. If you are just two lonely cobbers in the bush, like me and Tom were... your thoughts may eventually drift to mechanical help...

 

Mixing Cob With A Concrete Mixer

When we drove past a concrete mixer for sale at a second hand hardware store... we pulled over and purchased it!

The concrete mixer didn't make cobbing fast, but it did speed it up. We worked out it made it twice as quick... or should I say 1/2 as slow ;)

To mix our cob in the concrete mixer, we would put in all our clay, sand and any water that was needed - and allow it to mix. Then we would put it on our tarpaulin or carpet - and mix in the straw by foot.

Some people hate the inclusion of this type of machinery in cob building, and I agree in an ideal world, doing it by foot with a community would be awesome. However in order to help make cob a more mainstream building method, I think using some mechanical help can be a positive thing.

As we do more cob building in the future I will explore other ways of mixing cob on a large scale... So keep an eye on our website and see what discoveries I make. I think I'm going to give it a try with a bob cat as our strawbale home will have a large internal cob wall for thermal mass.

 

Next up in our cobbing adventure - installing the windows (Eeeek so exciting!!)