Cob building: introduction

What is it?

Cob is an ancient earth building technique using a combination of clayey subsoil, sand, straw and water. These materials are mixed together either manually, by stomping or dancing on the mix on tarpaulins, or with machinery. The mix is then formed into lumps or cobs and compressed together to form the walls of a building. The mix can also be compressed in moveable wooden forms into wet blocks, which are integrated into the wall. This is known as ‘box cob’. The cob building process is rather like building a giant clay pot.

Traditional thatched cob cottages in the South-West of England

Traditional thatched cob cottages in the South-West of England.

Cob has been used for centuries by many cultures worldwide. Yemen is particularly famous for its cob buildings – with 14-storey examples that have existed in seismically-active areas for hundreds of years. In the UK, there are many thousands of old, still occupied cob buildings – especially in the South-West, Scotland, parts of East Anglia and Wales – some over 400 or even 500 years old. The Industrial Revolution marked a move to the use of more standardised products and materials, and thus more local, natural building materials were abandoned. There has been a recent resurgence in cob building however, especially along the west coast of the US, due to over-logging and the need to reduce the amount of timber used in builds – but also because of its novelty, and the popularity of its durable and sustainable nature.

Cob is a mixture of clayey soil, sand, water and straw

Cob is a mixture of clayey soil, sand, water and straw.

Adobe or mud-brick building uses a cob mix to make bricks that are dried in the sun before being used for building. Adobe is a traditonal building method in the south-west USA and Mexico.

What are the benefits?

Environmental benefits:

Cob is arguably the most environmentally-friendly building material there is, because:

  • around a third of the world’s land mass contains soil suitable for cob building, which means that the material can usually be found on or near to the building site – the costs and emissions associated with the processing and transport of bulk materials is reduced or eliminated
  • reclaimed materials and products can easily be incorporated into the design – cob can be moulded around them
  • typically uses 60% less timber than a stud-frame house
  • the flexibility and fluidity of a cob build allows for curvilinear (yes, it is a word) shapes which enclose space more efficiently, reducing the size of building required for a specific use
  • cob is biodegradable – abandon cob walls and they will become incorporated back into nature without a trace very quickly
  • cob has very low embodied energy, is non-toxic, and can be recycled
  • as cob is breathable and flexible, it works well with other natural materials such as timber, stone, straw bales, slate, lime etc, as part of a natural home
Building a cob wall mud pie style

Building a cob wall mud pie style.

Other benefits:

  • your building material is free (although you will need a digger to get it out of the ground and dumped next to your proposed building); however, a cob building will only be cheaper than a conventional building if you do it yourself, as it’s very labour intensive
  • high thermal mass and good humidity regulation results in stable temperatures and good air quality
  • airtightness in houses is costly to achieve, and has contributed to the huge rise in cases of asthma in children over the last 50 years; a cob house is more eco-friendly than an airtight one – keep it small, build with natural, local materials and heat it with passive solar and wood
  • it’s a fun and creative building method – the nature of the material and the sculptural / mud-pie style of building is inclusive, and can bring together people with diverse backgrounds and abilities who can work together as part of a team
  • cob has compressive and tensile strength, so your building will be very solid – but it’s flexible, and so won’t crack if there is any movement
  • it’s easy to repair – just patch it with more cob
  • cob buildings are beautiful and unique – they are ‘of the earth’ and have a feel that a bricks and mortar building can never achieve
A modern cob home with a living roof, in the US

A modern cob home with a living roof, in the US.

What can I do?

You can go on a practical course to introduce yourself to the material and the techniques to see if it is the right choice for you; and you can buy a book to get a bit more background information. Then, off you go. Here are a few things to think about.

Planning and building regulations

Planning permission is sought in the same way as for more conventional housing. Problems may be encountered around location of the building, access, design and finishes, not specifically with the building material itself. More information can be found here. The government are supposed to be promoting sustainable development, which could work in your favour.

plastering detail on a cob earth oven

Plastering detail on a cob earth oven.

Building Regulations compliance shouldn’t be a problem provided the officer is given sufficient detail to make an accurate assessment. Don’t be vague – give them as much detail as you can; impress them. New cob structures with full building regs approval include the RIBA-award-winning ‘Cobtun’ house and Kevin McCabe’s house at Keppel Gate in Devon. See here for more information about building regs and planning permission related to cob.

Location

When considering a cob build, ensure access to water, and suitable clayey subsoil on site or in the local vicinity. It is also important to consider access onto your site due to the heavy nature of the materials, especially in relation to the precise location of your building – you don’t want to be wheelbarrowing all your materials uphill. If you are in a flood plain, then maybe cob isn’t the best option, and maybe you could consider something more appropriate, such as round wood timber frame.

The Yemeni city of Shibam - the world's first skyscrapers, made from unfired clay

The Yemeni city of Shibam – the world’s first skyscrapers, made from unfired clay.

The build

Cob building is labour-intensive and seasonally-dependent (Apr-Nov for the cob part of the build), so start with a small design that can be added to at a later date. This will give you more confidence with the material, and allow you to learn from your mistakes before moving on to a larger structure.

Cob always needs a good ‘hat and boots’ – i.e. solid, dry foundations, and a roof with a good overhang (at least 30cm). Foundations can be a rubble and gravel-filled trench with a perforated land drain to a soakaway to remove excess water.

Kevin McCabe's cob house in Devon

Kevin McCabe’s cob house in Devon.

Cob walls can take driving rain, as long as they have a chance to dry out. The outer walls can have a layer of lime render for protection, or an earth plaster that is re-applied every few years. Earth plaster is a finer version of the cob – sieved earth, finely-chopped straw, plastering sand and flour paste (which acts as a plasticiser and improves flexibility). Cement renders must never be used, as they destroy breathability and flexibility.

Dancing to mix earth

Dancing to mix earth, sand and and straw on a tarp.

Cob does not have good insulation properties, but to compensate, it has good thermal mass. Foundations can be insulated with pumice or vermiculite mixed with clay slip and rammed into cavities in the foundations. Floors can also be insulated with vermiculite, and ceilings can be insulated with recycled newspaper or sheep’s wool insulation.

 

Thanks to Annabel Fawcus of Earthed for information and Linda Royles of Cob in the Community for the main photograph


We'd love to hear your comments, tips and advice on this topic, and if you post a query, we'll try to get a specialist in our network to answer it for you.