Fa ades that had undergone stonecleaning have been found to have a significantly higher incidence of stone decay and repair compared to uncleaned fa ades. On average, the amount of decay and repair on cleaned fa ades was approximately twice that found on uncleaned fa ades. The biggest difference between cleaned and uncleaned fa ades was in the amount of repair that had been carried out, with substantially more repair on cleaned fa ades. This is thought to indicate a relatively higher level on intervention on these fa ades. Rates of decay for individual sandstone types were found to range from slower than before cleaning, up to rates that were approximately 10 times as rapid, in some cases even higher.
Results indicate that stone decay is most accelerated in the years immediately following cleaning.
The effects of stonecleaning on decay rates appear to approach the normal background levels after about 20 years. Many practitioners have changed their methods of cleaning in the past decade in response to planning department guidelines, information and experience with respect to the potential damage which may be caused to fa ades. Harsher forms of chemical cleaning and higher pressure abrasive methods are increasingly avoided. The overwhelming majority of practitioners have observed some detrimental effects of stonecleaning including erosion, increased biological growth, discoloration, spalling and efflorescences. Those avoiding stonecleaning often cited how earlier witness to the effects of stonecleaning has led them to believe that the risks outweigh the benefits.