A stitch in time… Makes good sense and saves money
The deterioration of pointing, render and plaster often reflects lack of maintenance in other parts of the building, such as defective rainwater goods and excessive vegetation growth. Such defects must be dealt with before repairing pointing, render or plaster, or the problems are likely to recur. Abutments with other building components, such as timber or roof coverings, are particularly vulnerable weak points, and the maintenance inspection should make a special point of examining these areas.
Pointing and render
Ground level inspection of pointing and render should form part of the annual maintenance inspection, with more detailed inspection (for example close up inspection of high level pointing) normally being carried out every five years.
The purpose of pointing and rendering is to help prevent water from penetrating into the masonry, so missing, cracked or crumbling mortar should be noted. Also look for areas of blistering, powdering or white crystal growth.
Pointing and render usually deteriorate fastest near ground level (especially next to roads treated with salt in the winter), where they may be exposed to damp, and on copings chimneys and parapets where they are exposed to driving rain. Provided erosion is fairly uniform and even, patch re-pointing of the failed joints or patching of the render with a matching lime mortar will usually be sufficient. Only those areas that have failed and are vulnerable to water penetration should be replaced. Although the repaired areas may stand out to begin with, provided the mortar is a good match for the original, it will soon weather down. Mortar should not necessarily be replaced simply because it is soft – lime mortar is much softer physically than cement mortar, but provided it fills the joint solidly it is likely to perform well. Cement mortars or strong hydraulic lime mortars should never be used for patching weak, historic lime mortar because it can trap moisture in the masonry
Localised green staining, vegetation growth, or severe erosion are usually indicative of high levels of moisture, often arising as a result of defects elsewhere, such as open joints in copings, leaking downpipes or inadequate drainage. In these cases, such defects must be remedied before re-pointing or repairing render.
Cracks extending from joint to joint through the pointing, or long directional cracks in render may reflect structural movement which should be investigated before undertaking repair. Once any structural problems have been dealt with, cracks should be cleaned out and filled using a lime mortar to match the surrounding material.
Where patches of render are bulging and sound hollow when tapped, it may be possible to consolidate them to prevent further detachment, but this is a specialist job for a conservator, not least because, if done badly, it can exacerbate the problem and even cause collapse of large areas of render. In most cases, areas of limited detachment that do not appear to be worsening are probably best left well alone.
Wholesale failure of recent pointing or render is likely to relate to the materials used or the quality of workmanship (including protection during curing of the mortar), and specialist advice regarding the cause of the problem should be sought.
Areas of heavy soiling may need cleaning from time to time. Lime mortars and renders are particularly vulnerable to damage by masonry cleaning, and only gentle methods and skilled operatives should be used. Trials should always be carried out to establish the most appropriate method of cleaning.
Where masonry or render are limewashed, this will need to be re-applied from time to time, in order to maintain the protection that it provides. Limewash should never be patched or over-painted with other types of paint: no other coatings match the performance or aesthetics of limewash. Colourless water repellent treatments should also be avoided, as they often end up causing more problems than they solve. Problems with water penetration can nearly always be solved by more appropriate means, and you should seek specialist advice.
All internal lime and gypsum plaster is at risk from water penetration through walls, floors, roofs and via chimney stacks, or from leaking water tanks, service pipes and radiators, and these should be checked annually. Any staining should be noted and the cause investigated immediately. If ignored, water penetration can cause serious damage to the timber laths supporting plaster ceilings and partitions, occasionally leading to collapse. Gypsum plaster is soluble so is particularly at risk from water penetration.
Where accessible, attic spaces should be checked and any large accumulations of debris resting on the back of the ceiling carefully removed with a vacuum cleaner to avoid undue loading of the ceiling. Care must be taken to avoid damaging the nibs holding the plaster onto the laths. If there are no floor boards in the attic, it will be necessary to use crawling boards to spread your weight and avoid excessive flexing of the ceiling joists.
Traditional lime plasters are applied to timber laths which are themselves supported by studs, joists and beams, so any movement in these elements can transmit stress to the plaster, resulting in cracking and possibly detachment. New cracks in ceilings should not be ignored, particularly if there is displacement of one side relative to another; they are often the first signs of serious instability, and further investigation should be undertaken as soon as possible. Further monitoring and investigation may be needed to determine cause of movement.
Any surface damage due to abrasion or knocks should be made good using matching lime plaster, or, for very small patches or cracks, using an interior-grade proprietary filler. The edges of such repairs should be finished flush with the plaster surface and not feathered over it; when painted they will be practically invisible. Exterior-grade fillers should not be used as they are usually too strong and hard for repairing soft lime plaster, and will often cause cracking in the lime plaster around them.
As with external render, low level plaster may be subject to condensation and damp. Any white salts that appear on the plaster surface should be regularly vacuumed or dry brushed to remove them. If the plaster eventually becomes soft and crumbly, it can be removed and the area patched using matching lime plaster, applied in the same number of coats as the original.
Traditional interior decorations for lime plaster include limewash, soft distemper, oil- or casein-bound distemper and lead-based paint. A piece of soft white bread, rolled into a ball, is effective for removing some marks from painted plaster surfaces. Soft distemper will soften in water, and cannot be washed or wiped with water without removing some of the paint. Limewash and oil- or casein-bound distemper are more resistant than soft distemper and are described as wipeable, rather than washable. Localised marks and stains can be gently cleaned using water and detergent. Lead-based paints are fully washable, but they should not be vigorously scrubbed, as this could loosen fragments of toxic paint.