The secret of lime wash revealedComment on this story
Roofs, especially old roofs, become slippery after heavy rain and can take days to dry out, but we expect contractors to jump up and fix the problem. What will you do if somebody slips or falls and you have not entered into a health and safety contract? You can be held responsible under the act if somebody is injured on your property.
Also, a roof can be fixed only one way, and that is into the prevailing weather. So logically in Cape Town our roofs are fixed to handle bad weather coming in from the north-west, so if it happens to rain heavily from the south-east, expect leaks.
I appeal to contractors to be honest with their clients. In today’s tough economic times, both partners are often breadwinners, so usually one has to take a day off work to remain at home to be there when you arrive. If you have made an appointment, keep it; if you have promised to start work on Monday, make sure you are there, or at least let the client know in advance if you have a problem.
I am project managing a large contract at the moment and twice the tiler failed to keep his appointment. All you need to do in Cape Town to be a successful contractor is ensure you arrive at your first appointment on time and then deliver. Sixty percent of our contractors would not last a month in Joburg.
I saw a sign at my favourite butcher’s the other day: “The customer is always right, sometimes confused, misinformed, rude, stubborn, changeable and even downright stupid, BUT NEVER WRONG.”
Questions and answers
I want to go back to Roger’s question about his damp mud-brick house.
Mike advises: Our first house in Lakeside was pre-1900 with sun-baked brick walls about 40cm thick finished with lime plaster. Great insulation and you hung a picture by whacking a 10cm nail straight into the wall – the bricks were too soft to use a wall plug.
There were huge damp problems and the plaster was cracked and falling off when we moved in. On one wall we even had plants growing on the outside with roots through into the inside. Rising damp was no problem as below the floor level, the wall was of rock and I’m guessing they put in a waterproof layer by painting bitumen on to the top of the rock – there was thick black stuff sticking out at this level.
After much advice we took all exterior plaster off (dead easy, a lot fell off on its own). Then we repaired the areas where the bricks fell out with the plaster. We treated the whole wall with a bonding agent. We plastered with a cement plaster with a waterproofing agent, increasing the cement ratio in the hope it would act as a bit of a structural skin.
This sorted out the damp for the period we still had the house. Thirty-plus years later, the wall is still standing with the plaster intact, and the owners do not seem to have had to do anything more.
Vincent, an architect, says: The technology applicable to these walls is different to that used in modern construction, and needs to be approached differently.
First, these walls never had damp proofing. Sometimes the foundations may have a layer of slate or flat granite stones that performs this function. However, if bridged with plaster this barrier is bypassed, making it useless.
Cement mortars or plasters must never be used on such walls, as this prevents the normal movement of moisture within these walls, causing them to disintegrate.
For the same reason, modern acrylic and enamel paints can’t be used on such walls. It is best to use the traditional lime wash. If properly made, it should last two or three years between applications. The sea air might have something to do with the poor life of your correspondent’s application, but the Post Huijs at Muizenberg seems to last quite well and this is also exposed to severe wind and is right next to the sea.
Modern waterproofing systems, must never be used.
I got the following recipe for lime wash from Gawie Fagan (still considered the master of restoration in the Cape):
Application of lime wash:
Prepare for lime wash by wetting previously prepared walls for three consecutive days and immediately prior to lime washing.
Apply three coats lime wash with a block brush, at least one day between coats.
Keep lime wash damp for at least three days after application with light spray.
Manufacture of lime wash:
This formula was found to be effective at the Castle:
Place 5kg unslaked lime into a 25l container and fill to half level with water, stir and heat to boil.
Add 340ml coarse salt. Add tallow or boiled linseed oil as follows: first coat – 340ml; second coat – 380ml; third coat – 415ml. Boil mixture and stir well. Fill up container with water when slaking is completed.
Colour to lime wash:
Mix inorganic pigments, ochre and red oxide, in separate containers at a ratio of 1kg to 5l of water.
Add colour paste to the lime mix while boiling. Paint sample on previously prepared wall.
If your correspondent has access to a lot of mussel shells, he can try crushing these and baking this rough powder in a kiln, as the probable original application. This is a laborious process, and he will have to do his own research.
Another thing to note with any cob, raw brick, adobe or rammed earth building is that one “has to keep the feet dry”.
Install an apron right around the building, at least a metre wide that falls away from the building to keep water away from the base of the building.
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