Biological growths on sandstone buildings: Control & treatment

The following is an extract from Technical Advice Note 10.
"Biological growths on sandstone buildings: Control & treatment".
If quoted, it should be referenced as:

Cameron, S., D.C.M. Urquhart and M.E. Young, 1997.
Historic Scotland Technical Advice Note 10.
"Biological growths on sandstone buildings: Control & treatment."
ISBN 1 900168 40 5.

Copies of the Technical Advice Note are available from:

Preface

As the Governments' agency responsible for safeguarding Scotland's built heritage, Historic Scotland maintains some 330 monuments in the care of the Secretary of State for Scotland. In the past, the control of biological activity and plant growths on the historic masonry of these sites was routinely carried out using a variety of "approved" treatments. However, there was some disquiet expressed that these applications may have been damaging the physical fabric to which they were applied. At the same time, there was a developing Health and Safety awareness in the general use of such chemicals. As a result, some 10 years ago, the treatments were stopped, pending a programme of detailed investigation into the need for them, and their effects.

In a related area, there was a growing concern over outbreaks of algal growths which were occurring on many of Scotland's stone buildings cleaned in the extensive "facelift" programme of environmental improvements carried out since the 1960s. This "clean to green" phenomenon inevitably provoked the use of other untested masonry biocides and chemical applications.

Responding to the emerging evidence of physical loss and damage to sandstone by some stonecleaning methods, Historic Scotland and Scottish Enterprise initiated research with the Masonry Conservation Research Group (MCRG) at The Robert Gordon University, Aberdeen, in 1989. The research findings were published in 1992 as Stonecleaning in Scotland, followed in 1994 by Stonecleaning: A Guide for Practitioners. This work led to a number of additional questions being raised, among them that of the "greening" of buildings by surface biological growths, and what could be done to effect control of this without creating the risk of further damage.

In August 1991, Historic Scotland commissioned the MCRG to undertake a programme of further research. The aim of this was to provide a better understanding of the mechanisms of biological growth on sandstone structures, their influence on stone decay, and the efficacy of various biocide treatments which might be applied. Building on earlier associated studies, the resulting research report, Biological Growths, Biocide Treatment, Soiling and Decay of Sandstone Buildings and Monuments in Scotland, was presented and published in 1995. The findings of that detailed study form the basis of this publication.

This Technical Advice Note has also been informed by RGU's related PhD research work by Maureen Young, and their Engineering and Physical Sciences Research Council funded project, carried out at Historic Scotland's Hermitage Castle, by Dr Melanie Jones and Dr Rachael Wakefield.

The 10th in the Historic Scotland TAN series, this publication is not intended to be a prescriptive document. Rather, it aims to better inform practitioners as to the range of technical issues which should be considered when faced with a building covered by surface growth, in order that individual specifications can be devised to respond more effectively to the particular factors involved.

It was written by Sonja Cameron, Dennis Urquhart, Rachael Wakefield and Maureen Young, all of The Robert Gordon University, MCRG, Aberdeen, with support from Robin Kent and Una Lee, Senior Conservation Architects, TCRE Division, Historic Scotland.

Particular thanks are due to the Macaulay Land Use Research Institute for producing and providing the scanning electron micrographs, and to all other associated individuals and organisations, who participated in, and contributed to, the research work.

Ingval Maxwell
Director
Technical Conservation, Research and Education Division
December 1997

Contents

1. Introduction

2. Types of biological growths and their recognition

2.1 Algae and cyanobacteria
2.2 Bacteria
2.3 Fungi
2.4 Lichens
2.5 Mosses
2.6 Liverworts
2.7 Succession of colonisation
2.8 Identification of common biological growths on stone

3. The nature of sandstone

4. Factors influencing biological growths on buildings and monuments

4.1 Climate and geographical location
4.2 Orientation of elevation
4.3 Stone characteristics
4.4 Effects of stonecleaning
4.5 Existing growths
4.6 Nutrients
4.7 Architectural form and detail
4.8 Mortar joints
4.9 Building maintenance

5. Deterioration of sandstone

5.1 Formation of patinas
5.2 Biological growths and physical weathering
5.3 Biological growths and chemical weathering

6. Biocides

6.1 Generic types
6.2 Mode of action of biocides
6.3 Selection of biocide
6.4 Biocide efficacy and life span
6.5 Biocide effects on sandstones
6.6 Recolonisation after treatment

7. Application of biocides

7.1 Planning the work
7.2 Preparation of substrate
7.3 Application

8. Alternative methods of control of biological growths

8.1 Building design and maintenance
8.2 Copper strips
8.3 Washing and brushing
8.4 Water repellents and consolidants
8.5 Ultraviolet light

9. Health and safety issues

9.1 Hazard information and risk assessment
9.2 Precautionary measures
9.3 Legislation
9.4 Work at heights
9.5 Statutory consents

10. Glossary

11. Bibliography

12. Appendices

Introduction

Biological growths such as algae, bacteria, fungi, lichens and mosses are common on the exterior of buildings especially in rural areas. They will colonise stonework wherever conditions of moisture, light, temperature and nutrition are suitable. Dark soiling on a surface is often equated with particulate soiling when closer inspection would reveal soiling of a biological nature. The issue of biological growth on buildings is to a large extent a matter of perception and aesthetics. It is important to bear in mind that their presence on stone is not necessarily harmful and, depending on circumstances, it may not be necessary to remove them. In the past, growths, in particular lichens, have sometimes been encouraged on buildings by a number of methods including applying a wash of cow dung and water, human urine or skimmed milk. A patchwork of differently coloured lichens over a stone surface may be considered aesthetically pleasing and indicate a relatively clean atmosphere. Due to modern air pollution, established lichens on older buildings may be increasingly rare examples of threatened species.

However, many people do not consider a covering of green algae or moss to be aesthetically pleasing, and occasionally the presence of biological growths may be thought undesirable. They can obscure and cause deterioration of inscriptions and carvings. Some organisms have sticky surfaces which can trap dust particles from the atmosphere, increasing the rate of soiling of the building surface and aiding the establishment of higher plants. These in their turn may increase water retention and block gutters and downpipes, leading to further defects.

In some cases, damage to stone may be caused or initiated by the presence of biological growths. In other cases, a biological growth may act as a protective layer shielding the stone from other factors which cause decay, such as wind and rainwater. Biological growth is only one of many environmental factors that may contribute to the deterioration of stone, and its removal will not necessarily halt the process of erosion.

Where control of growths on the surface of stone is considered necessary for reasons of safety, decay or aesthetics, it can often be achieved simply though the control of surface wetness by repairing or improving drainage, or encouraging quicker dry-out by reducing the sheltering effects of closely situated vegetation or other structures.

In recent times, biological growths on stone have been controlled by the application of biocide washes. These are generally effective for a short period of time only, and, as a result of recent health and safety legislation, some of the more effective products have been withdrawn from the market. This has reinforced the need for new research in the field which may lead to new products and methodologies.

This Technical Advice Note aims to provide guidance to practitioners regarding the colonisation of stone by biological growths, the effect of growths on the stone, and the feasibility as well as the means of removing the growth. It also addresses the problem of the reappearance of growth after treatment with biocides. A body of opinion exists which contends that the cleaning of sandstone buildings and monuments helps to promote the development of biological growths on masonry, especially algae, with the consequent aesthetic deterioration of the stone surface. It has been observed that algae can colonise cleaned stone within a few months of cleaning.

The focus of this advice note is on sandstone, because it is the predominant building stone in Scotland. However, since growths are found on other stone types as well, many of the observations made here are to some extent applicable to a whole range of stone types.

Copies of this book are available through the Scottish Conservation Bureau of Historic Scotland at:

Prices, order forms and information on other Historic Scotland publications can be obtained from the above address or on the Historic Scotland web site at: http://www.historic-scotland.gov.uk/

For further information contact Dr. Maureen Young at m.young@rgu.ac.uk