Anyone who explored how our past generations built is bound to have discovered mud walls, be it of Didi Contractor up north at Dharamshala or Laurie Baker in deep-south Kerala. We owe much to such pioneers who re-created our confidence in eco-friendly approaches. However, introduction of newer ideas has eroded societal belief in traditional systems, including the cob wall, which is among the greenest material with not even cement as an additive.
Soil mix with approximately 20% clay, 20% silt and rest sand, a proportion that can vary slightly, is ideal for cob. Chopped straw, rice husk or such non-decomposing fibrous binders are added to control cracking, and lime is added to make it termite proof. Once this smooth mix with no lumps is added with water, it shapes well as a ball or elongated egg. Testing it is important, checking for consistency by how it cracks while dropping, breaks while being pulled or feeling its stickiness to hand.
These handmade lumps are placed side by side, gaps filled with soil, pebbles, or burnt brick pieces. The surface can be levelled by hand and compacted by hammering. Each layer is checked for consistency in width, say 2’ or at least 1’ 6”, and allowed to dry before next layer is placed. The side of the wall needs to be smoothened before it dries too much using wooden levelling bar with sharpened edge or even hacksaw blade.
Protection
Exposed cob is not advisable anywhere, especially lower part of wall vulnerable to erosion by rain, where we may use stone. Since the soil mix is unstabilised, walls facing lashing rain would also need protection. Besides, the wall surface tends to crack, more initially and slowly later on through one full cycle of all seasons, i.e. up to a year.
Given these reasons, cob walls need crack filling with same mix, later plastering with mud stabilised with cement and quarry dust. The final coat can be with lime or left as mud wash with oxides for colour effect.
To locate openings, we need to fix temporary shuttering, to be removed once wall is shaped. Stone or wooden lintels suit the cob most, though thin precast RCC lintels can be inserted above very wide openings, to be finished with mud later. Electrical conduits can be fixed externally on the wall, though they can be embedded by making grooves like in normal construction.
Being a loamy material, cob walls can take any shape, including curves, sculptural forms and furniture profiles. Feasibility of cob depends much upon space available for wall thickness, local climate, availability of the ideal soil mix, local precedence, speed of construction expected and of course owner’s acceptance.
In India, we do not document our works adequately and provide knowledge to others. Given this, the documentation at Sacred Groves is really remarkable, besides which workshops by Thannal and website postings by many consulting architects today provide basic information if one desires to revive cob walls.
(The writer can be contacted at varanashi@gmail.com)
