Case Study: Hard external render to a domestic dwelling circa 1800

Discussion document.

Hard external cement render applied to a dwelling circa 1800 constructed of solid random rubble stone in lime mortar can prove to be problematic.

It is not uncommon for cracking to occur on the external face with the result that moisture enters the substrate through the cracks and cannot readily evaporate, because this type of external wall finish and when painted with an external stone paint is virtually waterproof, with the result that the stone substrate maintains a high moisture content. This phenomenon can be proved by the condition of the external paint. If the paint remains in very good condition it means that it is indeed waterproof and cannot be pushed off the face of the wall from the inside.

This type of building benefits from external walls having the ability to breathe. In extreme cases with exposed external walls facing prevailing weather, other forms of protection must be considered i.e. Hung tiles or natural slate or similar. Further consideration may be given to a membrane system, fixed on the external wall which has the benefit of ventilation top and bottom and finished with lime/cement render.

If internal dampproofing is required, where the outside ground level is much higher than the internal floors, consideration can be directed towards a cavity drain system (membrane). If this type of system is employed on the internal face of the external walls, together with a chemical DPC at least 150 mm above the outside ground level, it is essential that the external face of the walls is allowed to breathe naturally.

In extreme cases, if the inside and outside of the wall is provided with waterproof hard cement render and the wall floor joint is sealed, the result can often be that moisture will rise to at least the level of the first-floor joist, putting the joist ends at risk of rotting

It is therefore essential to install a chemical injected damp proof course and remove the external render to prevent bridging and to allow the wall to breathe. It is generally accepted that all hard cement render applied to the external walls should be removed and re-rendered with a softer lime-based render. In this case, great care must be taken during the application to ensure that the lime-based render is not allowed to dry too quickly during the application and during curing.

Care should also be taken to ensure that the area of external wall finish below the DPC line is not covered with a render coat and is left exposed to allow the wall to continuously breathe.

The brick or stone finish can be cleaned and brushed to remove all loose material and repointed with a lime based mortar.