Lime Rendering & Plastering

Cottages to Cathedrals
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CORRECT EXECUTION OF PLASTERING IN GOOD CONSERVATION WORK

An example:

Christchurch Cathedral, Waterford

 

This job demonstrates a good example of good conservation plastering vs. unsympathetic execution of the work.  Two different contractors work on separate sections of the same building.  OldBuilders Company is an experienced conservation contractor while the other works were carried out by a good contractor but one unfamiliar with good conservation practice.

A few years previously some lime work was done to the south wall, whilst that plastering job was technically carried out adequately it was a classic demonstration of an inexperienced contractors insensitivity and misunderstanding of the dynamics of old buildings.  During the application the strait edge was in full use with the result that a flat perfect wall of plaster became the primary goal rather than working with the existing contours of the building.  This resulted in the covering of the bevelled edges of the quoins stones and protruding plaster past the edges of stone cornices and string courses

 

 

Lime by its nature needs to be built up in layers to give it time to cure, this and the nature of the tools available to plasterers of old, along with the imperfections inherent in rubble stone walls gave the old plaster walls the character, the gentle subtle curves and undulation typical of the time.  In new works today it�s most important to replicate this by utilising similar methods, but without exaggerated contrived bumps so popular with the standard tradesman unfamiliar with conservation work.  As a result the cut stonework is well defined; the bevelled edges clearly visible, the plasterwork imperceptibly worked to give the finished wall a traditional look and feel.

 

 

Right well executeed lime plastering job following the contours of the wall and leaving the bevelled edges of the quoins stones correctly visible.

 

left, a technically correct lime plaster but executed by a non conservation experienced plasterer, the emphasis on a flat surface guided by a straight edger, this has resulted in the covering of the bevelled edges of the quoin stone.

 

                                                                       




Pointed cut stone quoins

left a standard strap pointing covering the stonework. 


Right
the correct pointing method, fills the joint only and leaves the stone bevelled edge fully visible.
     

EXAMPLES OF LIME PLASTER FINISHES

External Finish, Wetdash (roughcast or harling)

This would be the most common vernacular finish, consists of small aggregate 3-6mm, rounded stones naturally occurring like those from a river bed or glacial gravel

deposits.  This is also the most efficient with respect to the performance of lime with its ability to wick away moister from the wall as it provides a greater surface area for evaporation.

 

Often there is confusion with this type of finish as people are familiar with what appears on most old buildings to be a smooth undulating finish with no evidence of the aggregate.  This is due to years of frequent lime washing which has formed a multi layered film over the render.

With experienced tradesmen familiar with conservation lime plastering, with the correct application of the plaster in multiple layers, then hand trowelled without the use of strait edger's creates a naturally but gently undulating surface that reflects a little of the underlying stonework. Over emphasis on levelling the plaster surface results in modern looking finish of a concrete block building, it also shows problems where there are features such as cut stone quoin or string courses on the wall when the strait surface protrudes over the stonework, hiding details such as bevelled corners etc.

 Likewise the attempt to deliberately make surface uneven by creating bumps, lumps and a bad finish results in a contrived finish that despite the practitioners efforts to make it look "oldie worldy" actually bears no resemblance to any real historical finishes, only to that of a the London Irish pub or Disney world. In fact the correct "imperfect" finish is largely only visible close up, looking along the surface of the wall, its effect on the building is considerable, but from a distance but not necessarily noticeable uneven to the average passer-by.

Another common mistake is window reveals, where the accepted method is the creation of a "sticky out bit", patent reveal.  This is again a strictly modern detail and has no place in conservation finishes.  Plaster formed Plinths would be another inappropriate detail common in modern buildings but not in old ones.  See below centre and right.

     

  External finish, sponge


This is a more smooth finish whilst still following the contours of the wall.

More common with large formal buildings and in cities.

Left shown with "Ashlar" lines, replicating a fine courced cut stone, known as Ashlar stone work. 


On the right, an old stone cottage with spomge finish.


Here to the right is an example of sponge finish on a modern building.
     
Internal finish, skim coat,

trowelled lime skim coat finish

This finish is suitable for most internal work except in perhaps the country cottage of small farmhouse, its appearence would be similer to a modern gypsom skim coat.
     
Internal finish, sponge

This is a rougher finish, usually in a 3 or 5mm sand, whilst still following the contours of the wall, rounded corners to window and door reveals. More common and suitable for small country cottages.


Example below of Fat Lime rendering procedure on rubble stone wall

 

 
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