Build a Structurally Sound Cob Wall

Reader Contribution by Tom Keeling and Fazenda Tomati
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Cob wall on stone foundation.

Building with cob is pretty straight forward. Anyone in the family can learn the basic techniques and with a bit of guidance, you can build structures together as a group. (Check out Cob-Building Basics.)

When building a cob wall, you are going to need a solid foundation stem wall, usually made using natural stone or cement. After this foundational stem wall is in place, you can start building right on top with your cob.

Prepare for Building a Cob Wall

But first, before sticking the cob on the foundation, you are going to need a bucket of clay slip. This is made with fine, filtered clay and water. When it is a nice pastey consistency, you can use it to paint on to the stone foundation. It doesn’t have to be an exact consistency — just whatever feels nice to paint with and sticks to the wall well. The slip is used almost like a glue between the cob to the wall.

The ideal surface of the top of the foundation wall is jagged and rough so that the cob gets a really secure and strong key to lock in to. This will not only hold your cob wall to the foundations, but will also serve to really strengthen your top layer of rock. Do not build down over the side of the rock as this thin piece of cob will be very weak and will eventually break.

At the edges of your wall, make sure to keep an eye on your vertical and horizontal lines. Your horizontal line should be straight, and not necessarily following the lines of the stones as they may be more uneven. Your cob should protrude a little from the stone wall and then go straight up. The protrusion is to allow any rain running off the wall to drop in front of the house rather than running in to the foundation wall.

Constructing a Cob Wall



Mixing Cob In A Wheelbarrow

After you have covered the section you are working on with a 5- to 20-centimeter (2- to 8-inch) layer of cob, you can start building up. You can take massive handfuls of cob and spread them in to your wall — just really make sure you merge the layers in well. You can use a stick with a blunt end if you like, or use your fingers and thumbs. It is really important to integrate the layers. Otherwise, when the cob dries, it will start to seperate into the lumps you have made.

Keep going up. Pretty soon, you will need benches and scaffolding to keep going as you can really hurt your shoulders working too high up.

You can move your cob around in wheelbarrows and buckets, but a nice quick method is to make balls and throw them along a line, with the last person placing them on the wall.

After you have finished a raise — and I recommend that you not build up more than 60 centimeters (2 feet) in a day — you can cover your wall to protect it from the rain, or in some cases, stop water from evaporating too quickly and drying the wall. You want the top to remain soft when you start again the next day. If you miss a few days and the wall is dry when you come back, then just spray it with water and mash it up a bit with a stick to get a better connection.

Incorporating Wooden Beams, Window Frames, and Electric



Cob Wall With Wooden Posts

As you build up, you can add in logs with nails sticking out of the sides. Make sure the flat surface of the log is flush with the finished wall — maybe a sticking out a bit at this point to account for finish layers. These logs are called “dead men” and are really strong spaces to attach shelving, pictures, or door and window frames. It is worth planning out where these might go before you start building. (More on dead men in a future article.)

If you built your window frames in advance, you will also want to keep an eye on the height of your cob in order to set your windows in at the right time. Some people make simple wooden box frames to place whilst they build. The real window is installed at a later date.

If you plan to embedd your electric tubes in to the wall, then you also need to remember what height your plugs will be and remember to put the tubes in at around this height. Leave some conduit exposed.

Your walls will probably start to bulge at some point, so you can either bash them back into place, or cut them with a modified wood saw: Take an old wood saw and cut 1 centimeter (0.5 inch) teeth into it, then you can put the cob back on top of the wall after cutting it from the sides. Just make sure to cut when the wall is still a bit wet.

Finishing Touches

After building, whilst the cob is still wet, you can stab lots of holes in the sides of the walls with a 2.5-centimeter (1 inch) thick stick to make sure that the centre dries well.

When you get to roof height, I would recommend building slightly farther than you need. This will account for shrinkage of the wall during drying but it also allows you to chisel out the exact level you need rather than adding little bits of cob to get a bit higher. These little bits will be difficult to make strong, especially after the wall has dried.

The best thing to do is get out there and try it! The most important thing to remember to create a strong wall is to integrate the layers as you build and make sure you build up vertically where required.

Tom Keeling is based in Portugal and has traveled throughout Brazil and Eastern Europe learning about natural building and farming. He’s working on a two-story stone barn renovation using clay and wood, and including a shower and toilet block built using rammed earth and adobe bricks. Connect with Tom at Fazenda Tomati and on Facebook and Instagram.

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