Some parts of the external structure are more vulnerable than others, and provide useful early symptoms. Rapid deterioration occurs where water can collect on surfaces (such as window sills), or becomes entrapped within joints or by bad design detailing. End-grain timber is especially vulnerable to water uptake: the rate can be several hundred times greater than that of lateral wood surfaces.
Sloping or horizontal surfaces collect water and suffer greater erosion and pollution damage than vertical ones. Several pollution and marine atmospheres all deteriorate painted surfaces more rapidly. Elevations facing south or west are affected by sunlight and wind most, both of which accelerate the ageing process.
Overpainting existing coatings rather than removing them can produce a range of problems—incompatibility, poor base surface quality or integrity, cleanliness or extent of preparation. Different types of paint may cause differential stresses, and the additional weight of the new coating may
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overstress an old, friable base. Particular problems arise between old and new coatings of oil and water-based formulae.
The potential of the modern environment to damage coatings and building structure alike has imposed a greater criticality on paintwork. Durability is no longer just a function of the product, the preparation of the substrate and its application. Although there is now sufficient understanding to design paints on a chemical basis, the problems with the variability in substrate conditions and the local environment remain. Once deterioration to paintwork starts it can progress very rapidly. Frequent inspection for damage or general deterioration is advisable.
Fig. 6.12. Peeling paintwork on a rendered wall. The leakage from the downpipe may have contributed to the paintwork failure.
(K.Bright.)
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Revision notes
¢ Defects have many possible causes. May occur in combination.
¢ Defects occur within the paint and in the background.
¢ Common causes include:
– damp background, high porous backgrounds, dirty background;
– dimensionally or chemically unstable background;
– incompatibility between background and paint;
– permeability of coating;
– loss of adhesion;
– poor paint quality, uneven application.
¢ Common symptoms include (probable causes in brackets):
– flaking (loss of adhesion);
– bubbling/blistering (mixing/application fault or moisture/resin entrapment);
– peeling (differential movement);
– cissing (patchy adhesion);
– checks/crazing (paint quality);
– chalking (eroding of paint surface);
– bittiness/pimpling (incorporation of dirt);
– blooming (chemical response to environment);
– wrinkling (uneven paint application);
– colour variability;
– balling/saponification (poor drying out/alkaline reaction).
¢ Dark-coloured coatings can produce greater problems.
¢ Areas susceptible to water collection will generally exhibit faster paintwork deterioration. South or west-facing elevations are more vulnerable because of high exposure to sunlight and wind.
¢
Discussion topics
• Describe the range of visual defects that paintwork can exhibit.
• Discuss the causes of such defects, and the possible remedies.
Further reading
BRE (1977) Painting Walls. Part 2: Failures and Remedies, Digest 198, Building Research Establishment (February)
BRE (1989) External Masonry Painting, Defect Action Sheet Building Research Establishment (July).
BRE (1990) Painting Exterior Wood, Digest 354, Building Research Establishment (September).
BRE (1995) Maintaining Exterior Wood Finishes, Good Building Guide 22, Building Research Establishment (August).
Hinks, A.J. and Cook, G.K. (1989) Defects in paintwork. Building Today, 21 September.
Snelling, J.E. (1996) Painting and Decorating Defects: Cause and Cure, E. and F.N. Spon, London.
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6.6 External cementitious rendering
Learning objectives
You should understand that:
• rendering defects occur within the coating itself and in the background;
• background defects may be a cause of consequence of faults in rendering;
• faults in rendering can mirror underlying defects and exacerbate them;
• there are general forms of defect in rendering.
Introduction
Cement rendering can be an effective and durable external finish to buildings, but can suffer from a range of defects associated with its design and workmanship. There is an increased incidence of reported rendering defects, which could be related to a range of reasons beyond the defects attributable to construction design and operations, including the general awareness of the problem, the extended liability for construction work and the growth in the rendered building stock. The common forms of render defect are cracking and/or discoloration of the surface.
Where the render coat has developed defects as a result of faults in the underlying wall, this may create a wall which is doubly defective.
Alternatively, a fault in the protective render coating can lead to accelerated secondary failure in the wall. This complicates the diagnostic process, and in these instances it is particularly important to eliminate the root cause of any defects before attempting remedial works.
There may be defects in the material, for instance dissolved salts and minerals in the water, which can directly affect durability. When working under winter conditions, any retarding effect of the dissolved salts could leave the render vulnerable to early frost attack. It is unusual, however, for render mortar to be attacked unless there is a high salt content. Hence vulnerable locations are around or below the DPC.
The Technology of Building Defects. Dr John Hinks and Dr Geoff Cook.
Published in 1997 by E & FN Spon, 2–6 Boundary Row, London SE1 6HN, UK. ISBN 0 419 19770 2
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Defects may also arise in the background or as a result of incompatibilities between the render and the background. The mechanical bond between the two may have failed, owing to chemical or physical incompatibility. Movement (both temperature- and moisture-induced) as well as the irreversible inherent shrinkage that takes place in cementitious materials may combine to stress the interface bond. Renders which have a very high cement content will tend to exhibit abnormal shrinkage problems. Faults in the line and level of the background (which may be seen through some of the thinner-coated rendering systems) can produce attachment problems.
Loss of adhesion may also be due to renderings being applied to saturated walls or an incompatibility between the render and the background. The cause of this may lie with the render specification or the background stability. Frost and cold weather can also cause weakening of the rendered coat. Dashing
Fig. 6.13. The banded cracking and detachment of the render is most marked where the gable parapet is exposed at the eaves.
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may fall off the render coat owing to the undercoat having a high suction. The tensile forces of dashing attachment can be significant, and weak backgrounds may not be suitable.
Loss of strength, powdering of the surface and general failure of the render may be due to the application of ready-mixed mortar beyond the recommended retardation period.