CARING FOR YOUR COB PROPERTY
Left: Mixing cob with a fork while adding in the straw.
Right: Smoothed cob surface of the new wall
Most cob walls tend to be built on stone plinths and rising damp can normally occur when the external land drainage is inadequate or where a build-up of soil alongside or above the plinth causes excessive moisture penetration. This can normally be rectified by field drains being installed. Penetrating damp can be rectified by a properly applied lime render rather than troublesome sand and cement. Careful consideration should also be taken over the selection of paint and decorative finishes. Modern paints containing acrylic (liquid plastic) can significantly affect the ability of moisture to pass out of the structure through diffusion. All traditional buildings should be finished with a lime wash or silicate paint externally, or other types of natural, breathable paint internally, such as clay paints, chalk paints and breathable emulsions.
STRUCTURAL MOVEMENT The site of leaning or cracks in cob walls can be initially alarming. However, whilst structural faults may appear in cob walls, they are often not as serious as they may appear. It is always advisable to check the verticality of the cob walls, both internal and external as apparent ‘leaning’ of the wall may be deceiving. This may be the result of out-of-plumb building or ‘slumping’ during the construction of the wall.
Attempts should never be made to realign leaning sections of cob walls by the use of hydraulic jacking or levering devices. Walls should always be stabilised or consolidated as they stand by using stabilising bars on the buttress. In severe cases, where an unacceptable outwards lean has developed or where the wall is showing signs of severe cracking, careful dismantling and rebuilding of the wall may be the only viable solution.
PLASTERS AND FINISHES The use of sympathetic plasters and renders are essential for the optimum performance and longevity of all traditional properties but particularly so with those built from cob. Earth materials such as clay plasters were used extensively on internal surfaces of historic buildings prior to the 19th century. Their disappearance was partly an effect of the separation of agriculture from construction as a result of the industrial revolution. The disappearance of clay plasters was not because the material was considered primitive or inadequate in any way, but because of the changing economic conditions of that time.
Renders made from lime should be the choice for external coatings to maximise protection
60 Listed Heritage Magazine September/October 2018
and durability whilst regulating moisture efficiently. Fine lime plasters have also been used extensively for internal coatings dating back thousands of years, and are currently more popular than clay coatings which have seen a bit of a revival in recent years.
Both lime and clay plasters have very specific qualities which are extremely well suited to historic building conditions. Unlike cement and gypsum, clay is also reworkable, provided it is not contaminated (particularly by salts). Clay and lime plasters have a degree of flexibility and so will accommodate small movements with cracking in the same way as conventional plasters.
Traditionally cob buildings would have been rendered and plastered before decorating with a form of limewash, which can still be used today. It is now widely proven and accepted that low-permeability paints and decorative finishes can significantly improve the performance and durability of cob walls. A better knowledge of these types of buildings and improvements to traditional paint formulas mean that there are many more appropriate paints readily available on the market than the old fashioned paints which many believe they are limited to.
SUMMARY Cob houses offer a range of benefits from necessary, functional strength and thermal
mass, to the positive impact on our own health. Cob houses are free of man-made pollutants which can often aggravate sufferers of conditions such as asthma. Cob ‘breathes’ through tiny pores, and the hygroscopic ability to manage water vapour in the air keeps the inside of the house fresh and balanced in its humidity.
Walls built with cob are sound when functioning correctly (contrary to what is commonly believed) and have been known to last longer than many wooden structures. They offer fantastic acoustic insulation because of the naturally thick walls obstructing the flow of noises between rooms of the house, as well as restricting outside noise.
Thermal mass in cob houses is another distinct benefit. The walls can absorb the sun’s heat in the summer to help keep the building cool, and in the winter help to keep the home warm by storing the heat within. All this without the need for environmentally unfriendly heating and cooling methods which of course helps to keep the running cost of your house down!
For further information contact: Mike Wye on 01409 281644
sales@mikewye.co.uk www.mikewye.co.uk
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