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Cleaning, Restoration, And Maintenance Masonry Restoration
(Historic Rehabilitation)
1) General
2) Standards For Historic Rehabilitation
3) Moisture Problems in Historic Structures
4) Salvaged Brick
5) Repointing Mortar Joints
6) Cleaning And Water Repellent Coating of Historic Buildings
3. Moisture Problems in Historic Structures
Adapted from a publication under same title by Lee H. Nelson, AlA.
3.1. Introduction
Architects have attempted to control damage to buildings from excessive moisture since ancient times. Vitruvius (1st century B.C.) recommended the use of cavity walls to minimize rain penetration, and natural hydraulic cement stucco to reduce dampness at the base of exterior walls. Renaissance architects likewise understood the value of cavity walls in reducing moisture, but, like the ancients, they showed apparently little recognition of the full range of causes of moisture damage.
In earlier buildings in the U.S. little was done to prevent moisture problems except to deal with certain aspects of site drainage and to provide a sound roof. The great majority of seventeenth and eighteenth century buildings had no protection against the moisture damage caused by ground water, although most urban buildings by the early nineteenth century were fitted with gutters and downspouts to control the rainwater that previously would have dampened walls and flooded basements.
Much of the progress made in controlling moisture damage to buildings in the nineteenth century is attributable to advances in the technology of site drainage systems. In the early and middle years of the century, major east coast cities such as New York, Boston, Philadelphia, and Savannah learned to control excess rain water through drainage ditches or street gutters, and in some places, underground storm sewers. This technology was well developed by mid-century.
Other advances were made in the nineteenth century. Water tables of granite or dense lime stone were increasingly used, especially for public buildings, to prevent the upward rise of moisture within walls, and to direct rainwater away from the bases of buildings. The progress made in understanding and controlling moisture damage can be measured in The Architecture of Country Houses, published in 1850 by Andrew Jackson Downing. Downing was clearly aware of ground water problems when he wrote:
...foundation walls...built of common lime mortar, will always be damp, from capillary attraction.. .common lime mortar offering no impediment to the absorption of the moisture from the soil, or to its gradual passage upwards into the main wall of the house. The remedy for this is to build the foundation walls of hydraulic lime mortar, which completely prevents any such foundation dampness.
Figure 1
Patented cavity wall systems recommended by A. J. Downing in The Architecture of Country House (1850). Since the walls containing such cavities appear outwardly normal, they are difficult to identify without physical probing.
He also noted that "in damp soils, the dampness should be prevented from the soil into the unbuilt wall, by laying one course of slate, or of brick, laid in cement of hydraulic mortar, at the top of the foundation." The detail he recommended is now known as a physical damp proof course (dpc). To prevent problems resulting from rain penetration through walls, Downing recommended cavity wall construction (see figure 1).
Like Downing, the British writer Joseph Gwilt also understood that rising damp resulted from capillary action. Gwilt, however, attributed the problem to damp construction material, as the following glossary definition from An Encyclopedia of Architecture (1842) demonstrates:
Dampness. A moisture generally attendant on buildings finished hastily, on account of materials, not being dry, carrying up the moisture by capillary attraction. A layer of powdered charcoal mixed with pitch or resin and powdered pitcoal laid over one of the courses of the wall near the foundations, will prevent the evil.
While Gwilt failed to realize that porous building materials draw moisture from the earth whether they are dry or wet, he understood that a horizontal damp proof course would arrest the problem.
In the second half of the century a flurry of patented inventions followed, as architects and builders sought to capitalize on the newly discovered virtues of damp proof courses. Some patented courses were made of brick, fired at high temperatures, and often glazed, in order to render them impervious to moisture. Other common damp courses included double courses of slates; a single sheet of lead or zinc set in cement; a layer of tar and sand; or a layer of bituminized building paper.
At the same time, techniques for waterproofing exterior masonry were tested. Some builders impregnated bricks and stone with solutions of animal fat or insoluble silicates of lime. Despite failures of these and other exotic treatments, substantial progress was made by the end of the century, including the incorporation into local building codes of some provisions for protection against ground water. Another practice during this period was to install rain leaders or "hidden downspouts" within masonry walls. However, these were exceedingly difficult to keep clear. When they became clogged, extensive interior and exterior damage resulted.
In the early decades of the twentieth century the building industry devised many new damp proofing treatments. Knapen tubes, shown in figure 2 were invented in 1911. Inserted in a horizontal line within damp walls, they were intended to increase evaporation and thus drying. In these years also a rash of masonry waterproofing treatments and mortar additives were patented. Szerelmey's Stone Liquid, Symentrex, Antihydrine, Liquid Konkerit and Dehydratine are a few of these. Between the wars, large-scale scientific study of moisture problems such as capillarity and efflorescence was undertaken for the first time, yielding a basic understanding of these phenomena.
With the end of World War II, the building industry moved into the era of synthetic materials and high technology. Silicones, initially used in construction to waterproof road surfaces, were used to waterproof masonry walls. In the 1960's silicones and silicone latex mixtures were injected in a horizontal band into buildings in England and Germany to form a continuous dampproof layer. Other new sheet materials like polyethylene, bituminized lead, and asbestos sheets enlarged the choices of materials available as damp courses. Portable chain saws and carborundum blades made cutting mortar joints for the installation of such courses easier. In America, chemicals and some times clay were injected into the soil adjacent to foundations in order to stop the horizontal migration of moisture through the soil and into the foundation walls. Electro-osmotic systems were another European contribution to the waterproofing field. To preclude the upward movement of water, these systems establish an electric field at the point at which a damp course is installed. Both "active" systems that supplied a direct current, and "passive" systems that used the natural electrical potential between the saline saturated wall and the earth were in stalled.
As was the case with many of the products and methods used during the nineteenth century, however, many of the treatments developed in the twentieth century have proven ineffective or inconclusive in treating moisture problems. By the 1970's evidence was clear that Knapen tubes, which had been in fairly wide use since their invention, were not working. Both active and passive electro-osmotic systems have been ineffective. Silicone waterproof coatings have been found in some instances to do more harm than good, and soil injections have generally proven ineffectual.
Figure 2
Knapen Tubes. They are installed every 18 inches in a horizontal line at the top of the foundation wall and are intended to increase evaporation inside the wall and hence, reduce moisture build-up. Their effectiveness in any but dry climates, however is questionable.
Yet the record is far from totally negative: polyethylene sheeting and other "traditional" damp courses, used extensively in Europe, perform as absolute barriers to vertical moisture movement. And a new generation of water repellent coatings known as silane shows great promise. The lesson to be learned from previous attempts to prevent moisture damage is that practitioners should be wary of using methods that have not been extensively tested. This is especially true for historic buildings; they should not be viewed as testing ground for untried methods.
Before undertaking work to correct moisture problems in historic buildings, those involved in designing remedial measures must understand the fundamental types and causes of moisture damage. They must be able to make proper diagnoses and to select appropriate treatments, preferably conservative ones. This report attempts to provide such a framework for approaching moisture problems in historic buildings.
3.2. Sources and remedy of moisture problems in historical masonry buildings. The common sources of excessive building moisture are rain, ground water, and condensation. Building design detects and poor maintenance will exacerbate problems. A familiarity with these sources is important for an accurate understanding of moisture problems and as an aid to proper diagnosis.
3.2.1. Rain and/or ground water. Light rains followed by periods of sun normally do not cause moisture problems in buildings. Both brick and stone is generally identifiable by a horizontal stain or "tide mark" on the wall. The tide mark is formed at the point where equilibrium has been reached between capillarity and evaporation, thus leaving large, visible accumulations of crystallized salts, called "efflorescence". Below the tide mark, moisture is rising through capillarity. This area often appears damp. Efflorescence is unusual below the tide mark because the high moisture content retains the salts in solution, although there is continual evaporation. Above the tide mark, the moisture content in the wall can vary as the equilibrium changes with the weather. In this transition area, the moisture content is sometimes high enough to support capillarity; at other times only water vapor is present. In the transition area, salts are brought to the surface, leaving a band of salt crystals above the tide mark.
The NW Masonry Guide Table of Contents
Masonry Institute of Washington
Washington State Conference of Mason Contractors
