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?
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
It should not be necessary to have a DPC in a Clay Lump
Structure if it is properly constructed and maintained.
reasons for any damp problems should be ascertained and
understood before any remedial action is undertaken.
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
Unfortunately there is a skills shortage and
consequently we are unable to recommend anyone at
present but you could attend one of our workshops and
gain sufficient knowledge to have a go yourself!
Q4. Where can I buy some Clay Lump Blocks from?
We recommend you contact Eartha in the first instance
because people telephone when they have lumps to sell.
Eartha can sometimes provide new lumps.
Malcolm Carrington makes new lumps. Telephone 01 487
20 High Street,
Cambs. PE17 2JS
Q5. Are there any listed Clay Lump Buildings?
Yes but not very many. Clay lumps houses were built
after the date when surviving houses are normally
There 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?
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
Q7. I want to build a Clay Lump shed, how high can I
safely go using a single block thick wall?
Foundations are permitted under the Building Regulations
as rubble foundations on the highest bearing subsoil
except in shrinkable clay. Ordinary masonry plinth walls
of any construction: bricks, flints, conc blocks or a
mixture of all. A damp proof course is a good thing
because if there is not one and it becomes necessary
itís a nuisance to retro fit.
Under the Building Regulations the height of the walls
should be no more than 16 times the thickness, so the
shed should not be much more than 12 feet high. Gables
are a weak point and are better done in something other
than clay-lump such as wattle and daub or weather
boarding. In such a wall the lintels can be green oak
(fence posts) side by side and one inch high for one
foot of span plus one inch with nine inch bearing each
There should be a good overhang at the eaves and the
There is nothing in the building regulations that states
there has to be a damp proof course except under
Crushed chalk is produced at very low cost for
agriculture and small loads will be delivered for very
little money. Spread the chalk and compact it with a
hired vibrating plate whacker until it is perfectly
Paint the outside of the walls whether rendered or not,
with lime wash per the instructions.
Add sulphate of iron which is sold as a moss killer and
the lime wash will go dove grey and paint it on the wall
and it will turn to Norwich Ochre colour like magic.
The reason that battle ships are grey is that they were
traditionally painted with lead oxide, yellow lead aka
lithage which is yellow but while the paint was being
applied, the salty marine atmosphere changed the colour
to battleship grey.
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.
The best and least expensive micro porous paint is tar
which is available from
R & K Jones
15 Bennet Street
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
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
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
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.
Revert when in doubt.
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