It should be noted that the replastering is as important as the insertion of the damp-proof course (DPC) and care must be exercised when carrying out the works.
WHY REPLASTER?
High levels of contaminant hygroscopic salts can build up in both the old plaster/underlying masonry over many years of active rising dampness. Even though the DPC is effective the hygroscopic nature of the salts may cause moisture to be attracted to the wall so causing further dampness, decorative spoiling, and giving the impression that the DPC has not been successful.
Removal of the old contaminated plasterwork will remove the contamination at the surface but underlying contamination may pass into new works unless it is designed to be resistant to the passage of residual moisture and these contaminant salts.
The design function of the new plasterwork must therefore be twofold:
- It must prevent the passage of residual moisture reaching the decorative surface during the drying process which can take some considerable time as well as control the limitations of the injection system itself.
- It must prevent the passage of hygroscopic salts from the underlying masonry to the new decorative surface to prevent further spoiling.
It is important that the following specification is strictly adhered to. Please ensure that the plasterer understands its importance.
Preparatory Work
- Timber skirtings, architraves, etc., should be removed as outlined in the survey report/specification.
- Remove plaster back to masonry to the height outlined in the survey report/specification, but this should not be less than a height of 1 metre, or 500 mm above the maximum level of the visual rising dampness and/or salt contaminated plaster.
- Rake out all mortar joints to a depth of 15 mm (1/2″) – this is important in order to help resist the natural shrinkage of new cement renders.
- Remove any timber fixing grounds that are present in the masonry.
Important Points to Note:
- Where walls are known to be excessively contaminated with hygroscopic salts (e.g. old barns, old kitchens, chimney flues, stables) then consideration should be given to tanking the walls prior to replastering as added protection the decorative surface.
- Where masonry is unstable, this must be made good prior to the application of the renderings. Where it is not possible to obtain a proper bond between the wall fabric and rendering, as in the case of cob walling for example, the rendering must be applied direct to the wall face but over expanded metal lath, previously fixed to the wall surface.
- Renders and plasterwork should be cut short of finished solid floor level or at suspended timber floor- board level. This will prevent any damp which may be present within the solid floor from being transferred into the soft setting coat, or any subfloor condensation passing into the new work.
- Gypsum plasters, lightweight premix plasters must not be used to bond metal angles to corners. Ideally, use plastic angles or, better still, form them.
- It should be remembered that the walls will take a considerable time to dry out and it is possible that sufficient moisture could be absorbed by the new joinery to cause fungal decay.
- It is important that the re-plastering specification is strictly adhered to and not varied in any way. No other additives must be added to the mix.
We cannot stress the importance of plastering strictly in accordance with the specification and it is essential that the plasterer realises the implications should this specification not be strictly adhered to. Experience has shown that whilst the DPC may be fully effective and no action has been taken to prevent the migration of hygroscopic salts reaching the new plasterwork, then problems do exist, often resulting in a situation where the property visually appears to be no drier than prior to the work being carried out. It is therefore essential to make sure all recommendations, as stated in the contractors report and recommended by Mr Salt Damp are strictly adhered to.
