Much of the technical advice given by the SPAB over our telephone enquiry line concerns the 'undoing' of well-intentioned but ill-conceived work carried out to historic buildings from the mid-20th Century. Such work has frequently involved the replacement of lime renders on external walls with highly unsuitable modern cement coverings, the source of a potential time bomb over the next couple of decades. Douglas Kent, SPAB Technical Secretary, explains.
Q. Why may a modern cement render be inappropriate for an old building?
A. Because it is incompatible with the construction of most old buildings and can cause or accelerate serious decay. Modern buildings generally depend on an impervious outer layer and cavities to keep out moisture. By contrast, old buildings tend to rely on their permeable nature ('breathability') to allow water absorbed by the fabric to evaporate back out.
The use of an impervious Portland cement render in place of a traditional lime-based covering restricts evaporation. Hairline cracks form due to the mortar being more rigid than the wall. These then draw in water that becomes trapped in the fabric. Timber-framed and earth constructed buildings in particular can suffer major structural damage if moisture builds up behind a cement render.
Q. How do I tell whether an inappropriate cement render has been used?
A. If the building pre-dates about 1800, the original render is likely to have been of ordinary (non-hydraulic) lime or natural hydraulic lime that has a weak chemical set.
Lime-based renders provide a different aesthetic effect to cementitious ones. Although a range of finishes exists with each, the latter has a more uniform appearance, and corners and details are sharper and more defined.
Weathering characteristics also differ. Cement renders often fail in patches and detach from the wall, whereas lime renders gradually erode back in a more even manner.
Q. Does this mean all the cement render on my pre-19th century house should be removed?
A. Not necessarily. Although remedial action should ideally involve removal, this may cause further damage to the fabric. A small area should therefore be removed as a trial.
Where the render adheres well, it is probably best to leave it to age naturally. Sometimes a compromise may be possible, perhaps just removing the render at the base of walls or hollow-sounding patches where localised deterioration has occurred.
Q. What is the best method of removal?
A. Cement renders can sometimes be removed by working over the surface with a hammer, aided, where necessary, with a chisel. Great care must be exercised. If large sheets are levered off, soft underlying materials can be seriously damaged.
Various power tools can also be suitable but only in highly experienced hands.
Q. Should the render be replaced once removed?
A. Yes, it is generally a mistake not to replace render. There is a good chance that the building was rendered originally. Even if it was not, the rendering may have been applied at a later date as necessary protection against the weather.
When a cement render has been removed, re-rendering should be delayed for a short period to allow drying out if the underlying fabric is saturated. Additionally, any areas of decayed backing must be made sound before the new render is applied to prevent its early failure.
Q. What should I replace the inappropriate cement render with?
A. Generally a soft and porous lime render without the addition of cement. It is important that the render is applied by someone familiar with lime-based materials. The SPAB may be able to advise on suitable contractors or courses.
While a non-hydraulic render may be appropriate in certain circumstances, in many cases a mix with a faster and harder set may be desirable. This can be achieved by adding pozzolans (materials such as dust from soft and low-fired bricks) or using a natural hydraulic lime without additives (strong hydraulic limes, however, should be avoided). Mixes with a hydraulic set may be used for patch repairing walls where appropriate cement renders can only be partially removed. Epoxy resins should be avoided.
Q. What should I use to paint or treat the render?
A. A traditional limewash will normally be most appropriate. The high water permeability will allow the walls to 'breathe'.
As with cementitious renders, modern paint systems usually have a lower permeability and there is the risk of moisture becoming trapped. For a similar reason, the application of colourless water repellent solutions is also inadvisable.
Q. Will any further remedial action be necessary?
A. Possibly. A wall previously covered with a cement render for some time is likely to have a high salt content. A new lime render may act like a poultice and initially draw the salts out. Areas of excessive salt loading can cause localised premature decay (particularly at the base of the wall). Some remedial re-rendering after a couple of years might be required.
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An Introduction to Building Limes, SPAB Information Sheet 9, by Michael Wingate (nd)
Proprietary Colourless Water-repellent Surface Treatments on Historic Masonry, SPAB Statement 2 (1995)
'Success with Lime Renders' in Context, 59, pp16-18, by Tim Ratcliffe and Jeff Orton (1988)
'Mistaken Protection' in SPAB News, 20 (2), Oxley, R
© Douglas Kent / SPAB