RMH with cob stratification chamber bench - how to actually build the shape.

HI All,

I've been reading back and forth for a good while and want to go with a stratification chamber/bell in the traditional RMH style.

I'm struggling to work out the best way to build it using cob.  With regard structure I can't find any good information on building a bench with cob that won't collapse during the building process.

Anyone able to point me in the right direction?

John  

Hi John;
The trick is to use a locally available material for the walls and roof.
For example, concrete blocks, large square stones, or clay bricks. Some folks use metal; you can even use wood as your box, away from the core (J-Tube).
Then there is the Walker, half-barrel bench.
Any of these materials can be finished with cob if desired.

For your core, firebricks are the most common.  Clay bricks can be used, but they will not last long term.
Or, you can use fireclay mixed with perlite and cast a core yourself.

John,

if you want to build the bench from the cob then you will have to work in increments, let's say 10 cm high per day or just cast some cob bricks and then use them as normal bricks. Not in all locations though, as cob bricks would be not the best choice to span openings unless some angle bar was used for support.

The rate at which you can build up cob depends on its composition and wetness. More straw makes it stiffer, as does less water. More straw also makes it more insulating and less massive, so there is a tradeoff. I have found that a mix stiff enough to stand but not so stiff as to have a poor bond with a preceding dry course can rise 8"/20cm a day. For best structure and airtightness it is best to keep building so the previous course has not dried past solid stiffness when new cob is added.