Introduction: Building a Cob Wood Shed

To build a cob wood shed you will a supply of clay rich soil and some form of fiber reinforcement.
       The soil I dig up from my back yard and for fiber I use either shredded wood fibers that I get free from a local tree service or I buy a bail of straw and use my lawn mower to chop the straw. These are typical wood fibers.

Step 1: Make Test Brick

This is long straw being converted to short straw.

The next step is to mix a handful of straw and some clay and water to make a test brick which you will test for strength. Make a 4"x4"x4" brick and dry it completely and see if it will support your weight then try to scratch it with your fingernail or break it up with your hands. If it supports your weight and does not crumble when scrapped or twisted by hand you have a suitable building material.

There are several ways to mix mud and fiber. I prefer to do it with a rototiller by simply tilling the soil adding water and fiber and tilling to form a thick mud. You can also do this with your feet, or you can put the mud on a small tarp and mix the soil, water and fiber with your feet and roll the mud by pulling the edge of the tarp to put the dry soil on top of wet mud.

Step 2: Build Foundation

When you get near the top of the wall you will need some form of anchor to ensure that the roof does not blow off the wall. I use a dead man anchor embedded one or two feet from the top of the wall.

These pictures also show a two types of dead man anchors which can be used to tie the roof beams to the mud walls. In the first I pre-drilled holes in oak strips to prevent splitting them, when the dead man anchors are completely imbedded, I ran weather proof screws through the strips into T-members at the bottom of the strips and then into roofing beams at the top of the strips.

The second picture shows an alternative form of dead man anchor. To make it, wrap a galvanized steel wire around a piece of wood and embed this into the wall as you build. Note the wire should have at least 400-600 pounds of tensile strength. This means that it will take at least 800 pounds of lift to lift the beam off your wall. The U-shaped nails are used to attach the wires to the roof beams.

Step 3: Building Walls

As you build use a level to ensure that the sides and edges of the walls remain vertical.

When you get near the top of the wall you will need some form of anchor to ensure that the roof does not blow off the wall. I use a dead man anchor embedded one or two feet from the top of the wall.

When I get to the top of the wall I simply wrap the wires around the beams and attach them with u-shaped nails.  In these pictures you can see the two horizontal beams. They have been secured to the dead man anchors and the anchors and beams have been imbedded into the walls. In the second picture I am drilling holes in the beams Gergo is securing bamboo rafters to the upper beam and Desta is imbedding the upper beam in mud.

Step 4: Attaching Rafters

The next step is attaching rafters. I used 7 foot sections of bamboo for my rafters because they were free and easy to cut. I drilled holes in the roof beams, fed wires through the holes, wrapped the wires around the rafters and twisted the wires to make the connection.

If you do not have bamboo thin wood saplings will work or you can buy 2"x2" studs, but I much prefer to go for free stuff. These rafters are six inches on center. I want a very stiff roof to prevent flexing and cracking of the finish layer.

I covered the bamboo rafters with bamboo mats, seen resting on the ground and about to be lifted into place. To split the bamboo for weaving you can use a bamboo splitter from the Hida tool company as seen below. Alternatively you can anchor a burlap sheet above each of the L-shaped and T-shaped columns and to each of the rafters. Then support the finish layer on the burlap.

If you choose to use burlap to support the roof you might want to provide rafters 3 inches apart.

Looking at the fourth picture below the burlap would be pushed down between two rafters to the top of the wall Then the rafters would be stitched together as shown but through the burlap fabric. Then you would fill the pocket formed by the burlap with mud. To the left of this figure the end of the burlap sheet would be wrapped around the end rafter and stitched to the rafter. Then the end rafter would be secured to the beams just like the other rafters.

The burlap support which crosses all rafters would be tied to the rafters by wire ties.

Step 5: Plaster Roof

The roof is then covered with a thick layer of paper and mud plaster. Here again it might be good to make a test brick to find the best mixture of paper and mud. I also add sand to make the finish water resistant and abrasion resistant. My preferred mix being 1:1:1 soil, paper, sand by volume, but this depends on how much clay, silt and sand is in you soil. The layer of plaster should be strong enough when dry to support your weight between rafters with out cracking. Adding lime will retard mould while drying.

If you are expecting rain cover during the rain events and uncover to dry.

To make the plaster I put newspapers in a pit and soaked them for a few days then mixed them with a tiller until smooth. Again, you can simply walk back and forth on the paper and stir or mix them with a dirt fork until smooth.

The dirt fork pictured below is almost indispensable for breaking up packed clay and for moving the finished plaster or cob from the ditch to a wheelbarrow for transport to the structure.

A ladder leaned up on the roof makes a good scaffold. The roof structure should be steep to shed water fast and smooth so that there is no place for water to collect on the roof and set.

One of the nice things about paper and clay is that you can build up the roof in several layers. In the final picture you can see that I formed a drip flange on the top, bottom and sides of the roof by applying multiple layers of paper/clay plaster.

Step 6: Finishing Touches

Once the roof is plastered and completely dry, apply a mix of 50-50 boiled linseed oil and solvent to render the roof water proof. It may take several coats. Allow each coat a couple of days to dry.

       I got the roof beams and cinderblocks free. The linseed oil and solvent for waterproofing cost less than $100.00. That is the final cost.