| Cob is a very simple (and efficient) form of construction that has probably been around forever in some form or other. Structures are formed of earth (normally of a high clay content), with some straw and sufficient water to make the mixture readily workable. The exact ingredients are prone to change along with the many different names given to the various techniques of earth building within the UK, depending on where you are in the Country and who you are talking to, yet they all follow the same, very simple principle which has often been tried and tested over many generations. Once built they are then often covered with some form of protection, normally a render coat. *
Choice of materials Preparation Dubbing out: Suction control: |
| The initial dampening should be followed by as many further applications as appropriate, at least until there is a run off of excess water down the wall in the form of moisture beads. This pre wetting should continue right up to the application of render. There are no hard or fast rules as to the exact amount of dampening or the number of applications that will be required: too much is likely to result in a loss of wall material, with a reduction in bond for the render, too little will result in very rapid absorption of the mortar’s water resulting in de-bonding and cracking of the mortar. Either way, failure to address this issue from the outset is highly likely to result in a failure. Common sense is very much the policy for the dampening operation.
Reinforcement: Over demanding structural cracks, where movement is likely, should be repaired before any rendering is attempted. An appropriate reinforcing net material in the render base coat placed over the repaired cracks can be used to good effect. This approach is very subjective with differing criteria or factors relevant to each situation and further advice should be sought from the appropriate source if it is a likely structural problem, or an unfamiliar technique. Thickness of Render: |
| Second Coat (Undercoat/scratch coat) 2 : 5 / Lime (NHL2) : sharp sand
The scratch coat should not be applied for at least 3 – 4 days (or more, depending on atmospheric conditions) after completion of the first coat. Once again pre wetting is very important prior to application. If using a laying on trowel apply using firm and even pressure. The coat thickness should be even and once applied should not be overworked. In simple terms “lay it up and leave it alone.” Thickness not exceeding 5/8″. Keying is best achieved by providing a crisscross pattern of a 2″ On completion provide adequate protective and curing measures. Any initial shrinkage taking place in the drying out phase can be dealt with by dampening the affected area and rubbing back using a plasterers wood float. This will need to be carried out within the first 24 hours. Pressing the float home evenly and firmly in a close circular motion. Re-key as necessary. Proper protection, the addition of reinforcing along with regular humidifying (using a fog mist spray), will all greatly reduce the amount of shrinkage likely to take place. Top Coat 2 : 5 / Lime (NHL2) : sharp sand or EcoMortar (premixed) Overworking mortars results in free lime and fines being pulled to the surface thus affecting the properties and visual appearance of the work. Protect and cure for 7 – 10 days, longer if the weather dictates. The choice of sand in the top coat is important, dependent on the finish required. A roughcast finish will require a grittier sand, smoother finishes require well graded fine sharp sands, silt and clay free (see also General Guidelines: Sands for NHL mortars.) As for all NHL 2 renders, do not work in temperatures above 85oF or below 45oF and never when frost is forecast during the curing period. Our document “General Guidelines – NHL Renders” contains further notes on protection and good working practice.
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