Dashing Information – Rectifying Render Problems

Rectifying problems with render applications

The long term effectiveness of any treatment requires the correct identification of the defect and the removal of the source of that defect be it bad design, workmanship or conditions. There are three courses of action that are appropriate:

Patching

Patching involves cutting out the defective areas to a sound substrate to produce a suitable key and re-rendering in two coats with a matching dash or textured finish.

Re-rendering

Providing the defective rendering can be entirely removed without damaging the substraight re-rendering can be an effective long-term solution.

Overcoating

Overcoating is technically the most problematic remedial process. It is not possible to over coat a building with additional render and assume that it will perform unless careful thought is given to its specification, materials and application. The quality of the surface preparation is very important. The main issues relating to this process is that the new render is likely to overstress the original, causing shear problems particularly when curing and drying.

Overcoating can overcome technical issues. One option would be to use a mechanically fixed metal lath. This would require the preparing the existing surface thoroughly by cleaning with a biocide to remove any moss. Hammer testing and repair any defective areas. Surface cleaning and roughening, preferably by grit blasting. An application of a polymer modified stipple coat with a heavy peaked texture. This would only apply if there is inadequate surface roughness to make the lath stand off the surface and give a mechanical fix. The metal lath would require fixing to all areas in accordance with the manufactures instructions.

By using this method the walls are keyed for the new render by the lath which holds the new coat in position. Problems with shear or delaminating are overcome and should not affect the new rendering. The use of lath allows the new render to perform more or less independently.

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