So here we have it! A rock stemwall with a stabilized earth mortar for our earthen home!
Stemwalls are a time-told building technique that have helped historic cob houses throughout the world last for hundreds of years. Their main purpose is to raise the earth part of the building off the ground, protecting it from heavy rains, run-off and damage from back-splash from the roof during rains. This has allowed cob structures to survive for centuries and still be liveable! Far better than some modern day American homes that have a 50 year lifespan and 30 year mortgage!
Traditionally stemwalls were often built by dry-stacking, a very skilled method where the builder would chisel away at the rock to get ‘the perfect fit’, allowing the rocks in the stemwall to fit snugly together like pieces of a puzzle. We briefly considered dry-stacking, but for that you really do need skills. So we opted for the next best option – using a stabilized earth mortar. Stabilized earth simply means an earth mixture with a small percentage of building (not agricultural) lime or cement. We opted for a ratio of 1:3:6 (1 bucket of Portland cement to 3 buckets of clay to 6 buckets of sand). Add water until the mix is right. It shouldn’t be watery or runny. It should be more like a paste that you smear between the rocks. We were lucky.. With a termite mound right next to the building site, we had a perfect source of clay for the mix!
Also important is that the top of the stemwall is level. This was done quickly using a very simple tool – a tube with water in it! We took a thin transparent tube (one of those thin, clear tubes used as siphons) and filled it nearly full with water. We then went to one part of the stemwall and held one end of the tube against the stemwall, adjusting it so that the level of the water within the tube was the same level as the height of the stemwall. The other person holding up the other end of the tube to the stemwall can then see if his part of the stemwall is level with the other persons.If the water level is below the top of the stemwall, then that part of the stemwall is higher. If the water level is above the top of the stemwall, then that part of the stemwall is lower. And if the top of the stemwall is aligned with the water level in the tube them bingo! The stemwall is even. Mark out the even level with a string to make it clear. Also good is to add some ink or food coloring to make the water in the tube more visible. This is a very simple and very accurate way to get a level stemwall!
Dimensions: Our stemwall is appx. 1 1/2 feet high x and 1 foot wide. (45cm x 30cm)