Water Repellent Surface Treatments

SPAB Statement 2: Proprietary Colourless Water Repellent Surface Treatments on Historic Masonry
The Society has become aware of the increasing use of water repellent treatments on historic masonry. The issue has been considered by the Society’s Technical Panel who advise:
  1. That although numerous proprietary water repellent preparations have been developed for use, all are relatively modern and their long-term effects cannot be predicted accurately.
  2. The use of such treatments reduces the rate of evaporation from porous building materials and can lead to increased dampness due to trapped moisture when applied to historic building materials which rely on a moisture equilibrium being reached via ‘breathability’ throughout a wall.
  3. Water repellency is likely to be lost in a period of a few years – re-treatment is possible but the Society believes that there will be increasing reduction in vapour permeability. There is also a risk of areas of different permeability.
  4. It is usually impossible to avoid the build-up of water pressure at the interface between the natural material and the treated layer; or salts already present in the construction being mobilised by changes in moisture movements. As a result there is a significant danger of this outermost area being lost due to salt crystallisation.
  5. Any water repellent treatment to masonry requires the removal of micro-organisms and any very friable material. The removal of surface deposits may adversely affect the character of the buildings. Severe staining may occur if moulds or other micro-organisms become entrapped.
  6. That stonework may undergo a change in physical appearance following treatment with water repellents. Usually the surface has a sheen, appears darker and weathers out patchily.
  7. The use of water repellent treatments can lead to a sheet of moisture on the surface during which rain, if wind driven, is more likely to find weak places in the construction and lead to water penetration.
Treatment of this type involves significant intervention and in view of the potential hazards and unknown long-term effect, their general use should be avoided. Historic buildings should not be used for experimentation.
Buildings constructed of porous materials should be repaired in such a manner that they work in the way originally intended. Proper attention needs to be given to protective weatherings, rainwater disposal and the use of finishes, renders or pointing materials which are softer than the material from which the wall is constructed.
The defect leading to the perceived need to apply a water repellent must be correctly identified and, if possible, overcome by an alternative approach – perhaps rectifying faulty detailing or the incorrect pointing, render or finish of a building. Carefully consider, and if necessary avoid, treatments which may damage the surface of the building (e.g. some cleaning techniques).
There are no panaceas for remedying defects in old buildings – care should be taken not to add new problems by the ill-considered application of proprietary water repellent treatments.

Further Reading
The Need for Old Buildings to Breathe, SPAB Information Sheet 4 by Philip Hughes