Commonly, those that require cob blocks will buy them from a stockist. Most conservation building material outlets stock cob blocks and they carry an average retail price of £3 each. Excluded from this unit cost is VAT and delivery charges. The overall cost can mount up significantly if a large quantity of cob blocks are needed.
Sourcing the right material
Traditionally, all cob buildings were constructed with material that was readily available from site and/or nearby. Therefore, to achieve maximum savings of total costs to your project, raw material would ideally be situated close-by. This isn’t pivotal, as raw material can be transported in, however a consistent source of clayey soils will save £££ if locally available. So, what are we looking for when it comes to identifying a suitable, clayey soil? Clay content is the crucial ingredient.
Most clay seams can be found relatively close to the surface. By scraping away the top soils, clayey soils can be identified by their lighter coloured appearance and heavy density. Clay on its own though is not going to produce effective cob blocks. We also require an aggregate content. Gravels, sands and silts play an important role when making you own cob blocks.
Do I have to find all these materials and mix them together?
No. In most clay deposits throughout the UK, aggregates are present as part of the natural geology. The above chart describes a good combination ratio to achieve successful cob block making. However, these figures are fully flexible……more sands less clay is OK, more aggregate less silts is OK. It doesn’t have to be a science….but we do recommend experimentation though……make some trial cob blocks before committing to full scale production.
What else do I need to add to make my own cob blocks?
Having established suitable clayey soils to make your own cob blocks, straw is added. Here in the west country we use barley straw within the cob mixture. This helps to bind all the raw materials together. For hydraulically pressed earth blocks, straw can actually be excluded and just raw, clayey soil used...but that is another post…..we’ll concentrate on making cob blocks for this blog. So, straw is combined with the clayey soil
Mixing cob by compaction is the most effective method of achieving a workable, manageable and successful material to make your own cob blocks. Traditionally, this was undertaken by farm hands, building labourers and/or livestock. Nowadays, we use whatever is locally available to make cob blocks…ie a digger!
Can I use the cob to make cob blocks straight away?
This will depend on how much moisture content there is in the cob mix. Generally, newly mixed cob is too wet to make cob blocks right away. We suggest it is left to mature for a couple of days before using it to make your own cob blocks. Maturing the cob encourages excessive moisture to disperse and evaporate, leaving a firm yet still pliable material to make your own cob blocks.
Cob block making machines
Cob block making machines are now readily available to buy or hire enabling anyone to make their own cob blocks. Our cob block making machines have been specifically designed for ease of use, fast-paced production and most importantly….they can be taken to the raw material source thus eliminating costly transportation.
Our cob block making machines are principally hydraulically driven and can be powered by electricity or connected to hydraulic take-offs from tractors, diggers etc. With the basic model of cob block making machine, a hydraulic power-pack is easily connected and these are available from your local hire centres.
