Lime Mortar and a Cement Mixer

Tuesday this week saw the commencement of plastering.  To facilitate this protracted process, I decided to invest in my own cement mixer.  This was a financial decision based on the probable number of days that one would have to be hired at about £27/week, not to mention the effort of taking to and from the hire shop when not in use.  Boots, wheelbarrow, mixer... where will it end?                                         
As a listed property, there is no doubt about it: we have to use lime mortar to plaster, just as I had to replace rotten ceilings with oak laths, NOT plasterboard.  Another important principle is that you must not strip all plaster from complete walls regardless - only really bad plaster has been taken down, and as much as possible of the original plaster has been retained.  Due to the leaking roof, we had a fair amount of poor plaster, not only in two whole ceilings but also on solid walls which had been subject to continual dampness for years.  We have left several small patches of popped plaster where they were surrounded by good plaster; this benefits you for several reasons:

1. Applying lime plaster is not cheap as the application is a three part process which is quite labour intensive.
2. Removing the old plaster is a VERY messy job!
3. The lime mortar itself is not particularly cheap unless you slake your own lime and mix it all on site.
4. Replacement laths are not cheap either - I have already used almost 1000 feet of lath (at 27 pence per foot plus VAT).
5. Getting rid of the old plaster is also not cheap - and we have so far removed between three and four tons of it!

(You might ask why one does such a renovation?  Well, in my view, either you do it properly or else you buy a new house - it is not fair to our heritage to buy a traditionally built house and then fail to respect its method of construction.)

Anyway, progress this week has been good.  In three days, both entire bedroom (lath) ceilings, both sides of the lath wall between the two bedrooms and a stone/brick wall have been scratch coated, as have two large repairs on the landing ceiling, and some other parts of a wall (right).  This means that you can no longer see the rafters from the bedrooms, and so the roof space is now physically separated from the first floor, for the first time in several years, I suspect. 
If you think this doesn't sound much, then you try making this soft mixture attach to the roughly 1" x 0.2" oak laths without dropping too much on your own head.  Actually, the process is made easier by adding hair to the lime mortar to strengthen it - our mix uses straight horse-hair.  I admit that some of ours had been bought (ready-mixed) before Christmas and the original hair was probably weakening as it dissolved in the alkali, so to be sure we added extra hair.  This decomposition was certainly happening as it was starting to smell quite bad!

Just to be clear about the general process for plastering: a fairly thick layer of haired lime mortar (normal ratio 3 sharp sand to 1 of lime putty) is applied to a prepared wall or ceiling - this is known as the "scratch coat".  Then when that has "gone off", another thick-ish coat (without hair) is applied, called the "float coat" because a float is used to get it smooth, ready for one or two thin coats of lime plaster (which is 2:1 very fine sand mixed and lime putty) to give a smooth finish.  There are variations, of course, but that's the general gist of it.  It is a very labour-intensive process, spread over a considerable time, so you can see why modern plaster (gypsum-based) is so attractive to builders.  However, lime mortar is more carbon-friendly, as it absorbs carbon dioxide from the atmosphere as part of its setting process (as it reverts to calcium carbonate), and it is also easier for a DIY-er to use as it does not set so quickly but stays workable for hours (or even days in low temperatures, as  I have found out!).