Cob and Lime work
Please use this page to hopefully find the answers to your questions. Below you will find a series of questions and answers that i think are the most relevant to your needs and will be updated all the time
1) What is a Cob
A Cob is a structure or wall built usually from local materials such as chalk, clay or mud, some mixed with straw. In most cases in Hampshire chalk was used. Many farm buildings and dwellings were built in this method, farm labourers on mass carrying out this work in between farm duties. Some dating back to the 14th and 15th centuries. These structures having walls between one and two feet thick (300-600mm). Even though a Cob can stand up to severe weather on its own most of them have a lime mortar render coated in lime wash. Lime being used so the cob can still move and breathe.
2) Where can you find cobs
Cob structures and walls can be found world wide, but in England mainly in Devon, Hampshire and Wiltshire, they are also found in Wales
3) how are cobs built
A cob is built from shuttered compacted natural materials, built up in layers and very important roofed, obviously in cases of dwellings but on garden walls too, this is commonly know as its Hat or Cap.
4) why are cobs built
Cobs are built most commonly in areas where the raw materials are found in abundance, the cost of materials is vastly reduced, the thermal values are very high, the co2 ratings are much better than modern building thus making it much more environmentally friendly. A well built thatched chalk cob is cool in the summer and warm in the winter due to the thermal values of the materials used and because it is a breathing structure.
5) Why use lime and sand render
The reasons lime and sand is used to render chalk walls are :
a) because lime and sand still allows the cob to breathe thus regulating the moisture content of the wall. The only way you can make sure your cob can breathe properly is to make sure the proper materials are used in the rendering stage and just as important is the decorating stage, the use of wrong painting medium can also damage your cob, a paint should not reduce the breathing factor of your cob more than 5%, modern masonry paints reduce the breathability by 60 % and more per coat applied.
b) lime mortar is a much softer material and moves with the cob when it expands and contracts. Lime mortar is about a 1/5 the strength of sand and cement based mortars that can destroy a cob wall. A cement and sand mix is much too hard for cob walls, when a cob expands the render moves outwards but when the cob contracts the mortar stays put thus causing a hollow void between the cob and render. Usually when this occurs the render will pull the surface of the cob away.
In the 1970s and 80s there was a very bad habit of facing the cob with either chicken wire or sheets of expanding metal lathe fixed with big unbrella nails and then rendered with sand and cement, all this did was that when the cob expanded and contracted the render that came away from the cob was in one massive rock hard sheet, in many cases very dangerous if it were to fall on someone.
(see photos for damage caused through neglect)
The use of cement in the render mix drastically reduces the ability for the cob to breathe thus causing the cob to become unstable.
6) How important is the type of paint used on cob walls
One of the biggest long term problems with decorated cob walls is that they have been painted using the wrong type of paint. The use of modern masonary paints prevent the cob from being able to breathe, you may not think this is a severe problem but a well built cob wall self regulates the amount of moisture content within the wall, too little and it becomes dry and crumbly, too much and you will have big problems with damp and become unstable.
Remember modern masonary paints reduce the breathability of your cob by at least 60 - 85% per coat applied. Thus 3 coats at only a 60% reduction would mean you have reduced your cobs ability to breathe by 92.6%, redecorate after a few years and you will totally have sealed it.
Limewash reduces it on an average of about 3%. There is another excellent paint made especially for this type of work and that is from a company called Keim. It is rather pricey but has many advantages over Limewash.
If you have any questions that you think should be on this page please get in touch and i will gladly add them
This page is under construction and being added to every day, even though online i hope to have it completed over the next few weeks or so until this page is completed please visit the Home page