This ancient building technique use simple ingredients from the earth to make a waterproof, permanent structure that can be as big or as small as you’d like. With the right amounts of straw, water, sand and clay, you can create the ultimate building material-cob! Once it’s been stuccoed (with cob) it can be painted and waterproofed. Learn the secrets of building with cob using the following tips and advice for working with this ancient build material.
Build a Foundation
The first step is to build a foundation on which to set the cob bricks. A good stone or concrete foundation is preferred. Steel reinforcement bars placed in the footer can lend additional support to the cob walls.
Prep the Mixture
Cob is prepared by mixing one part straw, one part clay and four parts sand. Both the clay, straw and soil should be soaked in water overnight prior to mixing. You want the soil and clay to be the consistency of pudding once they are ready to mix.
Mix the Cob
The best (and funnest) way to mix cob is with your bare feet. Get a large plastic tarp out and lay it out onto the ground. Spread a five gallon bucket of sand out, then clay and then top it with more sand. Mix the clay and sand together. As you work, add in the other two buckets of sand. Last but not least, add a five gallon bucket full of straw to the mix and work it together so that all of the ingredients are thoroughly mixed and evenly distributed.
Build your Bricks
You’re not really building bricks per say-just grabbing a good compacted handful of cob and bringing it to the foundation. Stack individual clumps or “bricks” together to create the structure. Weave the pieces together as best as you can while working your way around the structure rather than up it. Add about one foot of material at a time as you work around the foundation and up the wall.
Once your structure is complete, you can cover the rough walls with a few coats of plaster or stucco. To make a basic stucco mix, you’ll need to mix three parts of sand with one part clay and one part of finely chopped straw fibers. Mix it with enough water to create a pancake batter like consistency.
Apply stucco to the walls in three successive coats, allowing each coat to dry overnight before applying the next. Apply coats of mud to the wall using a stucco trowel in smooth consistent lines until a solid finish coat remains. Paint or seal the structure to complete your cob build, but only after the structure has had a chance to dry for at least one week.
Keeping your cob structure maintained is as easy a keeping it painted. If any loose pieces of cob come free, simply take more freshly mixed wet cob mix and add it to the wall. Once it’s dry, repaint it the same color for a complete matching patch job.