Questions & Answers

Q1. I have read that there is a problem in using cement on Clay Lump buildings and that it is likely to cause long term structural failure. Is this true?

 A1. The use of cement renders on earth structures can create the potential for structural failures.

Cement and Earth react at different rates to changes in temperature, causing cement renders to crack and delaminate from the clay lump blocks, if water is then allowed to penetrate the render it will not be able to evaporate through the cement and can accumulate in the base of the walls. If this happens the straw in the blocks will rot away, reducing their strength, eventually the blocks will reach a point of saturation and will fail.

 This process is easily masked in buildings that have been rendered inside and out.


Q2. Is it ok to have an Injected DPC in a clay lump structure?

A2. It may not be necessary to install a DPC in a Clay Lump Structure if it is properly constructed and maintained.

The reasons for any damp problems should be ascertained and understood before any remedial action is undertaken.

The majority of damp problems are caused by poor maintenance or inadequate drainage.              


Q3. I want some remedial work carried out on my Clay Lump Shed but I want it done by someone who is knowledgeable with regard to earth structures. How do I get in touch with such people? Can you recommend anyone?

A3. please refer to services offered by EARTHA members 


Q4. Where can I buy some Clay Lump Blocks from?

  A4. please refer to services offered by EARTHA members 


Q5. Are there any listed Clay Lump Buildings?

A5. Yes but not very many. Clay lumps houses were built after the date when surviving houses are normally listed.

 here is a growing awareness that they are a valuable part of the regions vernacular tradition and the authorities (through the work done by EARTHA) are more receptive to the preservation of earth buildings.


 Q6. Is it possible to get planning permission for a new Clay Lump structure?

A6. Yes planning permission has been granted for other forms of earth buildings and there is no reason why permission would be refused because the walls are of clay-lump or any other form of construction. More pertinent is that there is more understanding amongst Building Control Officers that clay-lump can be accepted under the building regulations because there are thousands of dwellings made of clay.

 There is a lot of very useful information in Larry Keefe's new book "Earth Building  Methods and Material" Laurence Keefe  Taylor and Francis ISBN 0-415-32322-3 £30.00p.


Q7. I want to build a Clay Lump shed, will I need permission?

A7. The Building Regulations do not apply to single storey buildings with a floor area of less than 30 m², with no sleeping accommodation provided it is situated 1 m from the boundary of the property or if less than 1 m is constructed of substantially non-combustible materials (this would include clay lump or other earth techniques). However, always check with the local planning authority as even small buildings can sometimes require planning permission.


Q8. We are starting the conversion of a clay lump barn into what will become our home. At present it is rendered with haired lime mortar and coated with  (probably) tar. The render is not in good condition and over the years has been patched, sometimes with cement render. It will probably have to be completely replaced. We want the final finish to be black, as it is at present. We are proposing to render with haired lime mortar and finish with a micro-porous black paint. Would this be a good way to do the job, and if it is, what brand of micro-porous paint would you recommend?  If not, what is your recommended method of weatherproofing clay lump?

 A8. It would be a shame if all the existing render is removed. If you can keep one wall as it is it would be a good thing. Two walls would be better, these are usually the north and the east walls which get weathered least. It will be an economy and a conservation. The cement render should be removed.

Although it is advisable to retain as much of the existing structure as possible, you may be required to insulate the walls to comply with current Building Regulations if the building is being converted for habitable use or changed from an unheated building into one in which heat will be required. As a general rule, insulation should be applied to the outside face of the walls. See technical paper on insulating earth walls.

The best and least expensive micro porous paint is tar which is available from:

R & K Jones

15 Bennet Street

Downham Market

PE 38 9EE

01 366 387135


Brushing Tar is usually used as the substitute for the gas tar which was used in the past. The brushing tar will eventually form larger islands of tar than the crocodiling of the tar that survives on your building. The tar allows the water that has got in to evaporate out through the cracks. It also reduces heat loss from the building because it provides heat gain.

You will notice that your lime render has pieces of rounded chalk in it. This is because it was slaked from quicklime. The best way to make your render is to buy quicklime in bags and to slake in sand as described in the attachment. This is the least expensive way of buying lime as it increases to more than two times in volume. 

If you do not want the bother then you should order lime and sand from the ready mix companies and have it delivered in a skip. I work with a man who does a lot of rendering and he says it is very good. You add the correct amount of hair to each mixer load

The best and only paint is made from lime putty which is sold in 25 kg tubs at most builders merchants. (See Technical Papers)

Cautionary Note: slaking quicklime can be an extremely hazardous operation and should not be undertaken unless experienced in this operation and suitably equipped.

Norwich Ochre which I think is the same as Bury St Edmunds Ochre is made by adding iron sulphate which is sold as a moss killer for lawns. When you add the iron sulphate  the limewash turns dove grey and when it is put on the wall it changes to a colour between palest cafe au lait to ginger biscuit depending on how much is put in.

You may like to consider some earth alternative like reject chalk which is compacted with a vibrating plate for the foundations and the oversite slab instead of cement concrete. If you plan a new roof or floor you might like to use coppice poles in chestnut which can be round, halved or quartered. Chestnut is extremely durable and strong and the chestnut paling industry has been in distress since red netting was adopted to go round building sites.


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