How to Build a Cob Roof

Wet the sand until it reaches a soft, sticky consistency without being runny. Pile the sand into a rough dome form on top of the oven foundation. Cover this form with plastic sheeting.

Create the base cob by combining about 25 percent clay with 75 percent coarse sand. Mix these two components on a tarpaulin with your hands and feet until the sand is thoroughly spread through the clay. Add water as needed to create a sticky mixture that does not fall apart when formed into a ball. The cob should not be so wet as to drip or run. Roll the base cob into balls of about 3 inches in diameter and set them aside.

Place the balls of base cob in a layer on top of the plastic-covered form. Start by laying a ring of cob around the bottom of the dome. Add another layer immediately above it on the form, working your way upward until the dome has completely been covered in balls of cob and none of the sand is showing through.. Make sure to keep this layer at a uniform thickness as you work. Rub a wet hand over the surface of the cob to smooth it.

Make the insulating cob by combining a batch of base cob with large amounts of straw. Use as much chopped straw as you can mix into the cob. The insulating batch should still be sticky and should not fall apart when formed into balls or lumps. Apply this material in handfuls to the outside of the base cob, creating an insulating layer several inches thick. Try to keep a consistent thickness all over the oven, to prevent later cracking. Smooth the exterior of the cob oven roof with water.

Allow the oven to dry for one to two days, then excavate the door. Use a machete or other large, sharp knife to cut a hole in one side of the oven dome. Reach inside the oven and grasp the plastic sheet. Pull the sheet out of the oven. The cob roof should remain stable.

Enlarge the hole in the side of the oven to the width of a cookie sheet or pizza peel. The doorway should be about 60 percent of the height of the dome. If the cob appears to crack as you cut, wet the surface to improve its flexibility. Once you have finished creating the hole, dig the remainder of the sand form out of the oven. Allow the cob oven and its roof to dry completely before attempting to use the oven for baking.