Cob is a building material that is composed of soil, sand, straw and water. It is formed into walls by hand and allowed to dry, creating an extremely hard and durable structure. Because cob is different in character and technique from a more typical wood framed wall, the method of attaching the roof to a cob building is quite different as well. This technique must be done while the cob wall is being built to make it integral with the structure.
Cob cottages may have different types of roof treatment, including thatching and earth, but they all are supported by a rafter system underneath. An alternative method of construction is to build the frame of the cottage first, including the rafters and roof, then fill in the wall areas with cob, making the cob non-load-bearing.
Set anchor bolts into the top face of the cob wall as you are applying the cob. Cob is applied in wet, sticky handfuls and hardens as it dries. The anchor bolts must be set as the cob is applied, with the cob packed tightly around them, and 2 1/2 inches of the bolt protruding vertically above the top of the wall. Set the anchor bolts every 24 inches along the top of the wall, centered between the inside and outside faces.
Drill 7/16-diameter holes into 2-by-4s, spaced 24 inches apart so they correspond to the anchor bolts. Set the 2-by-4s on top of the walls. Place washers and nuts onto the anchor bolts to secure the 2-by-4s. Don't tighten the nuts until the cob wall has had at least a week to cure. If you try to tighten them when the cob still is wet you will dislodge the anchor bolts. Check that the 2-by-4s are level with a 4-foot level.
Place the bird's mouths of the rafters on the 2-by-4 lumber, which is used as the top sill. The bird's mouth is the angular cut at the bottom of a rafter that rests on the top plate of a wall. Secure the rafters to the top plate by screwing or nailing two nails or screws from each side of the rafter down through the rafter and into the top plate.