So what are the disadvantages of using cob?
Well, the fact that it is mixed wet means that, in a thick wall, it will take some months (depending on site conditions usually 6-9 months) after building, for the walls to finish shrinking. Most of the shrinkage will occur in the height of the wall. This means in practice waiting around a week of good weather between each lift (a lift being two feet (600mm) or so in height) before it is sufficiently dry and strong to take the next lift. Therefore, three months is ample time, allowing for a typical English summer to build a two-story building. Although the roof structure can go on as soon as a week after the wall plate height is reached, it’s best to wait about six months before fitting windows and door frames. Otherwise there is a danger of the reveals either side of the opening shrinking and then the lintel crushing the window frame. In practice, this means it is usually fifteen months from start to finishing a cob house.
Over the last twenty-five years, I have built over 30 buildings of one sort or another – and all of them except two rather complex split-level part-retaining walled buildings and Dingle Dell itself, have been finished well within this time frame. Although this is longer than a typical new estate house, it’s not actually especially long for any bespoke one-off design building. It’s also worth noting if any green oak is used in the structure for lintels or roof structure then this will continue to shrink for much longer than the cob (about one year per inch thickness of timber). This means it is a good idea to delay the final rendering for a further year if possible. The extra weathering on the cob surface during this year also serves to create a better mechanical key for the render.
Another disadvantage is that usually site space is at a premium these days and the simple fact that to pass the latest “part-L” insulation performance requirements for building regulations, unless insulated the exterior walls will probably need to be three feet (900mm) thick! This means the whole building will take up a significantly larger footprint and also need a larger roof area all around for the same internal space. However, for me, as long as the site is not really tiny, this is a small price to pay.