I’ve just done a few pre-winter repairs of the outside walls in spots where surface render has lost its grip on the clay daub beneath. To my surprise, chalk lime plaster bought in 2014 is still usable.
It appeared to have solidified under an inch or so of water. But after scraping at the surface it quite quickly began to recover its original consistency. Lime putty when stored properly lasts many years – I still have an old tub of it in good condition – and this plaster is simply putty, chalk and fibre.
It is not entirely traditional: it uses synthetic fibres and they last indefinitely in the tub, unlike haired plaster, where the hair begins to disintegrate after a month or two.
The old plaster is not ideal, because it takes hard work with a trowel to get back to the right consistency. But having forgotten to order a new tub, it’s good enough to protect small areas of exposed clay from rain and woodpeckers for a while. Next, I must order a new tub.
Chalk-lime plaster used externally has proved extremely effective over the years in temporarily patching areas of old wall that we haven’t fully repaired yet. A thin layer behaves almost like a sticking plaster over the cracks where old render is beginning to come away from the wall. Once cured, it has considerable strength and resists tearing.
There’s one quite large patch where the old render has almost entirely lost its grip on the clay – tapping reveals a hollow behind – but plaster over the cracks has kept it in place for the last few years.
It’s admittedly a bodge, but we wanted to delay full repairs as long as possible while effort and money went into other priorities. In due course the whole of one gable and about half of one wall still await a full overhaul.
Both will be a challenge. In the days when our house was part of a farm, a lot of running repairs on the clay were done with cement render. A bit less than a third of the walls is still cement render. The rest is our past repairs and some areas of wall that have been untouched for many years.
The latter are not even rendered, but are clay daub covered with many coats of limewash over a long period. The limewash has built into a thick layer that, until we examined it closely, we thought was in fact a render. So our farm building’s exterior walls may once have been rough clay daub given a protective limewash.
saving The Old Brewhouse 
Is there a recommended period of time whereby you should render all walls anew, or is that an impossible task and you re-do wh
at you feel is necessary? How long can you get away with merely patching up, before you decide that it is better to render it all again?
No, it should last centuries if small defects are quickly repaired and the surface regularly limewashed. The farmer who owned the building didn’t look after it very well and some large areas were rendered with cement, which makes damp build up inside on the timbers, so has to be removed. That’s when large areas of re-rendering are needed but once done it should last. The current repairs are minor damage, possibly caused by vibrations from piling for the extension a few years ago.
Thank you! That all makes sense.