To avoid damp getting into cob walls, it is necessary to build a plinth wall at least 450mm above ground level on the foundations, on top of which the cob walls will be built.1 To save money, the bulk of the plinth walls have been built in brick. S looked for second-hand bricks. Most ‘modern’ second-hand brick can’t be reused because it tends to be mortared with Portland cement. The binding strength of Portland cement is stronger than the material of the brick itself, so it’s difficult-to-impossible to clear the mortar off a brick without breaking it. You can find older second-hand bricks, but because it’s labour intensive to clean them off properly (even with older mortars) and tend to be used by people who need to match existing bricks when repairing or extending an existing building, they are expensive.
What S did find is brick seconds from a brick manufacturer in Glasgow. They are cosmetic seconds – bricks that aren’t the right colour or are slightly burned — but structurally sound. Normally they would be broken up for use in aggregate and concrete so as well as using a product that would otherwise be degraded, they are a lot less expensive than ‘normal’ bricks. A win-win!
For aesthetic reasons, we then faced the brick with stone. Again, finding second-hand stones was difficult and in the end all we could find was some rubble stone. The problem with this is that while the stone may be relatively cheap, the labour required to shape and fit it is enormous compared with nicely cut and faced new stone. The stone we ended up buying is a sandstone from Yorkshire, where most new UK stone seems to come from. [Update, 10-10-2013 20:01] My mother (who along with my father helped us pick out the stone) reminded me that the stones we used were off-cuts from ashlar. Ashlar is the ‘finest’ type of stone you can use – each block of equal size and cut very flat on all sides that come in contact with other stones. However, the off-cuts are not a waste product – they are (and always have been) commonly used to build stone walls that have a less regular look. Fortunately, this was the style we wanted the wall built in, and the off-cuts are less expensive than buying ashlar.
This stage of the build ended up delaying us by three weeks as the stonemason who was going to do the job overran on a previous job, and we eventually had to book an alternate team of masons. But once they started, it was quite something to see the walls rising.
Technically a plinth wall might not be necessary. Some old cob houses don’t have them, and they are clearly situated in such a way that ground water isn’t an issue and hasn’t ‘melted’ the walls over the years. What we don’t know is how many old cob houses without plinth walks did fall down over the years. It is certainly highly advisable to build a plinth wall to keep the cob from sitting directly on the ground. ↩