The Cleaning and Waterproof Coating of Masonry Buildings
Inappropriate cleaning and waterproofing causes deterioration. Here's how to do it right.
Robert C. Mack, A.I.A.Why Clean? ~~ What Is The Dirt? ~~ What Is The Construction Of TheBuilding? ~~ Types Of Cleaning ~~ Planning A Cleaning Project ~~ Potential Problems Of Cleaning ~~ Problems Of WaterRepellent And Waterproof Coatings
The inappropriate cleaning and waterproofing or masonry buildingsis a major cause of deterioration of the Nation's historic resources. While both treatments may be appropriate in some cases, theymay cause serious deterioration in others. The purpose of thisleaflet is to provide guidance on the techniques of cleaning andwaterproofing, and to explain the consequences of their inappropriateuse.
Why Clean?
The reasons for cleaning any building must be considered carefullybefore arriving at a decision to clean.
Is the cleaning being done to improve the appearance of the buildingor to make it look new? The so called "dirt" actuallymay be weathered masonry, not accumulated deposits; a portionof the masonry itself thus will be removed if a "clean"appearance is desired.
Is there any evidence that dirt and pollutants are having a harmfuleffect on the masonry? Improper cleaning can accelerate the deterioratingeffect of pollutants.
Is the cleaning an effort to "get your project started"and improve public relations? Cleaning may help local groups withshort term fund raising, yet cause long term damage to the building.
These concerns may lead to the conclusion that cleaning is notdesirable at least not until further study is made of the buildingits environment and possible cleaning methods.
What Is The Dirt?
The general nature and source of dirt on a building must be determinedin order to remove it in the most effective, yet least harmful,manner. Soot and smoke, for example, may require a different methodof cleaning than oil stains or bird droppings. The "dirt"also may be a weathered or discolored portion of the masonry itselfrather than extraneous materials. Removal of part of the masonrythus would be required to obtain a "clean" appearance,leading to loss of detail and gradual erosion of the masonry.Other common cleaning problems include metal stains such as rustor copper stains, and organic matter such as the tendrils lefton the masonry after removal of ivy. The source of dirt, suchas coal soot, may no longer be a factor in planning for longerterm maintenance, or it may be a continuing source of problems.Full evaluation of dirt and its effect on the building may requireone or several kinds of expertise: consultants may include buildingconservators, geologists, chemists, and preservation architects.Other sources of local experience or information may include buildingowners in the area, local universities, the State Historic PreservationOfficer, and the AIA State Preservation Coordinator.
If the proposed cleaning is to remove paint, it is important ineach case to learn whether or not exposed brick is historicallyappropriate. Many buildings were painted at the time of constructionor shortly thereafter; retention of the paint, therefore, maybe more appropriate historically than exposing the brick, in spiteof current attitudes about "natural" brick. Even incases where unpainted masonry is appropriate, the retention ofthe paint may bc more practical than removal in terms of longrange preservation of the masonry. In some cases, however, removalof the paint may be desirable. For example, the old paint layersmay have built up to such an extent that removal is necessaryprior to repainting. It is essential, however, that research onthe paint type, color, and layering be completed on the entirebuilding before removal.
What Is The Construction Of TheBuilding?
The construction of the building must be considered in developinga cleaning program because inappropriate cleaning can have a corrosiveeffect on both the masonry and the other building materials.
Incorrectly chosen cleaning products can cause damaging chemicalreactions with the masonry itself. For example, the effect ofacidic cleaners on marble and limestone generally is recognized.Other masonry products also are subject to adverse chemical reactionswith incompatible cleaning products. Thorough understanding ofthe physical and chemical properties of the masonry can help youavoid the inadvertent selection of damaging cleaning materials.
Other building materials also may be affected by the cleaningprocess; some chemicals, for example, may have a corrosive effecton paint or glass. The portions of building elements most vulnerableto deterioration may not be visible, such as embedded ends ofiron window bars. Other totally unseen items, such as iron crampsor ties which hold the masonry to the structural frame, also maybe subject to corrosion from the use of chemicals or even fromplain water (Fig. 1). The only way to prevent problems in thesecases is to study the building construction in detail and evaluateproposed cleaning methods with this information in mind.
Previous treatments of the building and its surroundings alsoshould he evaluated, if known. Earlier waterproofing applicationsmay make cleaning difficult. Repairs may have been stained tomatch the building, and cleaning may make these differences apparent.Salts or other snow removal chemicals used near the building mayhave dissolved and been absorbed into the masonry, causing potentiallyserious problems of spalling or efflorescence. Techniques forovercoming each of these problems should be considered prior tothe selection of a cleaning method.
Types Of Cleaning
Cleaning methods generally are divided into three major groups:water, chemical, and mechanical (abrasive). Water methods softenthe dirt and rinse the deposits from the surface. Chemical cleanersreact with the dirt and/or masonry to hasten the removal process;the deposits, reaction products and excess chemicals then arerinsed away with water. Mechanical methods include grit blasting(usually sand blasting), grinders, and sanding discs, which removethe dirt by abrasion and usually are followed by a water rinse.Problems related to each of these cleaning methods will be discussedlater in this leaflet.
Planning A Cleaning Project
Once the existing conditions have been evaluated, including thetype of dirt and the building materials, planning for the cleaningproject can begin.
Environmental concerns: The potential effect of each proposedmethod of cleaning should be evaluated carefully. Chemical cleaners,even though dilute, may damage trees, shrubs, grass, and plants.Animal life, ranging from domestic pets to song birds to earthworms, also may be affected by the runoff. In addition, mechanicalmethods can produce hazards through the creation of airborne dust.
The proposed cleaning project also may cause property damage.Wind drift. for example, may carry cleaning chemicals onto nearbyautomobiles, causing etching of the glass or spotting of the paintfinish. Similarly, airborne dust can enter surrounding buildings,and excess water can collect in nearby yards and basements.
Personal safety: The potential health dangers of each method proposedfor the cleaning project must be considered, and the dangers mustbe avoided. Both acidic and alkaline chemical cleaners can causeserious injury to cleaning operators and passersby; injuries canbe caused by chemicals in both liquid and vapor forms. Mechanicalmethods cause dust which can pose a serious health hazard, particularlyif the abrasive or the masonry contain silica. Steam cleaninghas serious hazards because of high temperatures.
Testing cleaning methods: Several potentially useful cleaningmethods should be tested prior to selecting the one for use onthe building. The simplest and least dangerous methods shouldbe included as well as those more complicated. All too often simplemethods, such as a low pressure water wash, are not even considered,yet they frequently are effective, safe, and least expensive.Water of slightly higher pressure or with a mild non-ionic detergentadditive also may be effective. It is worth repeating that thesemethods should be tested prior to considering harsher methods;they are safer for the building, safer for the environment, andless expensive.
The level of cleanliness desired also should be determined priorto selection of a cleaning method. Obviously, the intent of cleaningis to remove most of the dirt. A "brand new" appearance,however, may be inappropriate for an older building, and may requirean overly harsh cleaning method. It may be wise, therefore, todetermine a lower level of acceptable cleaning. The precise amountof residual dirt considered acceptable would depend upon the typeof masonry and local conditions.
Cleaning tests, whether using simple or complex methods, shouldbe applied to an area of sufficient size to give a true indicationof effectiveness. The test patch should include at least a squareyard, and, with large stones, should include several stones andmortar joints. It should be remembered that a single buildingmay have several types of masonry materials and similar materialsmay have different surface finishes; each of these differing areasshould be tested separately. The results of the tests may wellindicate that several methods of cleaning should be used on asingle building.
The cleaning budget should include money to pay for these tests.Usually contractors are more willing to conduct a variety of testsif they are reimbursed for their time and materials, particularlyif the tests include methods with which the contractor is notfamiliar.
When feasible, test areas should be allowed to weather for anextended period prior to evaluation. A waiting period of a fullyear is not unreasonable in order to expose the masonry to a fullrange of seasons. For any building which is considered historicallyimportant, the delay is insignificant compared to the potentialdamage and disfigurement which may arise from use of an incompletelytested method (Figs. 25).
Potential Problems Of Cleaning
Water Cleaning: Water cleaning methods include: (1) low pressurewash over an extended period, (2) moderate to high pressure wash,and (3) steam. Bristle brushes frequently are used to supplementthe water wash. All joints, including mortar and sealants, mustbe sound in order to minimize water penetration to the interior.
Porous masonry may absorb excess amounts of water during the cleaningprocess and cause damage within the wall or on interior surfaces.Normally, however, water penetrates only part way through evenmoderately absorbent masonry materials.
Excess water also can bring soluble salts from within the masonryto the surface, forming efflorescences (Fig. 2); in dry climates,the water may evaporate inside the masonry, leaving the saltsslightly in back of the surface. The damage which can be causedby soluble salts is explained in more detail later in this leaflet.Efflorescence usually can be traced to a source other than a singlewater wash.
Another source of surface disfigurement is chemicals such as ironand copper in the water supply; even "soft" water maycontain deleterious amounts of these chemicals. Water methodscannot be used during periods of cold weather because water withinthe masonry can freeze, causing spalling and cracking. Since awall may take over a week to dry after cleaning, no water cleaningshould be permitted for several days prior to the first averagefrost date, or even earlier if local forecasts predict cold weather.
In spite of these potential problems, water methods generallyare the simplest to carry out, the safest for the building andthe environment, and the least expensive.
Chemical cleaning: Since most chemical cleaners are water based,they have many of the potential problems of plain water. Additionalproblems of chemical cleaning agents have been mentioned in thediscussion of environmental concerns.
Chemical cleaners have other problems as well. Some types of masonryare subject to direct attack by cleaning chemicals. Marble andlimestone, for example, are dissolved easily by acidic cleaners,even in dilute forms. Another problem may be a change in the colorof the masonry caused by the chemicals, not by removal of dirt;the cleaner also may leave a hazy residue in spite of heavy rinsing(Fig. 3). In addition, chemicals can react with components ofmortar, stone, or brick to create soluble salts which can formefflorescences, as mentioned earlier. Historic brick buildingsare particularly susceptible to damage from hydrochloric (muriatic)acid, although it is, unfortunately, widely used on these structures.
Mechanical cleaning: Grit blasters, grinders, and sanding discsall operate by abrading the dirt off the surface of the masonry,rather than reacting with the dirt and masonry as in water andchemical methods. Since the abrasive do not differentiate betweenthe dirt and the masonry, some erosion of the masonry surfaceis inevitable with mechanical methods, especially blasting. Althougha skilled operator can minimize this erosion, some erosion willstill take place. In the case of brick, soft stone, detailed carvings,or polished surfaces, even minimal erosion is unacceptable (Figs.4 and 5). Brick, a fired product, is hardest on the outside wherethe temperatures were highest; the loss of this "skin"of the brick exposes the softer inner portion to more rapid deterioration.Abrasion of intricate details causes a rounding of sharp cornersand other loss of delicate features, while abrasion of polishedsurfaces removes the polished quality of stone. Mechanical methods,therefore, should never be used on these surfaces and should beused with extreme caution on others.
Grit blasting, unfortunately, still is widely used in spite ofthese serious effects. In most cases, blasting will leave minutepits on the surface of the masonry. This additional roughnessactually increases the surface area on which new dirt can settleand on which pollutants can react.
Mortar joints, especially those with lime mortar, also can beeroded by mechanical cleaning. In some cases, the damage may bevisual, such as loss of joint detail or increased joint shadows.Joints constitute a significant portion of the masonry surface(up to 20% in a brick wall) so this change should not be consideredinsignificant. In other cases, however, the erosion of the mortarjoint may permit increased water penetration, leading to the necessityfor complete repointing.
Other problems of mechanical methods have been mentioned in thediscussion of project planning. In addition, wet blasting or waterrinses can create the potential hazards of water methods.
Problems Of WaterRepellent And Waterproof Coatings
Is waterproofing necessary? Coatings frequently are applied tohistoric buildings without concern for the requirement or theconsequences of the coating. Most historic buildings have survivedfor years without coatings, so why are they needed now? Waterpenetration to the interior usually is not caused by porous masonrybut by deteriorated gutters and down spouts, deteriorated mortar,capillary moisture from the ground (rising damp), or condensation.Coatings will not solve these problems. In the case of risingdamp, in fact, the coatings will allow the water to go even higherbecause of the retarded rate of evaporation. The claim also ismade that coatings keep dirt and pollutants from collecting onthe surface of the building thus reducing the requirement forfuture cleaning. While this at times may be true, at other timesthe coatings actually retain the dirt more than uncoated masonry.More important, however, is the fact that these coatings can causegreater deterioration of the masonry than that caused by pollution,so the treatment may be worse than the problem one is attemptingto solve.
Types of coatings: Masonry coatings are of two types: waterproofcoatings and water repellent coatings. Waterproof coatings sealthe surface from liquid water and from water vapor; they usuallyare opaque, such as bituminous coatings and some paints. Waterrepellents keep liquid water from penetrating the surface butallow water vapor to enter and leave through the "pores"of the masonry. They usually are transparent, such as the siliconecoatings, although they may change the reflective property ofthe masonry, thus changing the appearance.
Waterproof coatings: These coatings usually do not cause problemsas long as they exclude all water from the masonry. If water doesenter the wall, however, the coating can intensify the damagebecause the water will not be able to escape. During cold weatherthis water in the wall can freeze, causing serious mechanicaldisruption, such as spalling. In addition, the water eventuallywill get out by the path of least resistance. If this path istoward the interior, damage to interior finishes can result; ifit is toward exterior cracks in the coating, it can lead to damagefrom the buildup of salts as described below.
Water repellent coatings: These coatings also can cause seriousdamage, but by a somewhat different mechanism. As water repellentcoatings do not seal the surface to water vapor, it can enterthe wall as well as leave the wall. Once inside the wall, thevapor can condense at cold spots, producing liquid water. Waterwithin the wall, whether from condensation, leaking gutters, orother sources, can do damage, as explained earlier.
Further damage can be done by soluble salts. Salts frequentlyare present in the masonry, either from the mortar or from themasonry units themselves. Liquid water can dissolve these saltsand carry them toward the surface. If the water is permitted tocome to the surface, efflorescences appear upon evaporation. Theseare unsightly but usually are easily removed; they often are washedaway by the simple action of the rain.
The presence of a water repellent coating, however, prevents thewater and dissolved salts from coming completely to the surface.The salts then are deposited slightly behind the surface of themasonry as the water evaporates through the pores. Over time,the salt crystals will grow and will develop substantial pressureswhich will spall the masonry, detaching it at the depth of crystalgrowth. This buildup may take several years to cause problems.
Test patches for coatings generally do not allow an adequate evaluationof the treatment, because water may enter and leave through thesurrounding untreated areas, thus flushing away the salt buildup.In addition, salt deposits may not cause visible damage for severalyears, well after the patch has been evaluated.
This is not to suggest that there is never a use for water repellentsor waterproofings. Sandblasted brick, for example, may have becomeso porous that paint or some type of coating is essential. Inother cases, the damage being caused by local pollution may hegreater than the potential damage from the coatings. Generally,coatings are not necessary, however, unless there is a specificproblem which they will help to solve. If the problem occurs ononly a portion of the masonry, it probably is best to treat onlythe problem area rather than the entire building. Extreme exposuressuch as parapets, for example, or portions of the building subjectto driving rains can be treated more effectively and less expensivelythan the entire building. Washington, D.C. November, 1975.
This publication has been prepared pursuant to the National HistoricPreservation Act of 1966, as amended, which directs the Secretary of theInterior to develop and make available information concerning historicproperties. Technical Preservation Services (TPS), Heritage PreservationServices Division, National Park Service prepares standards, guidelines,and other educational materials on responsible historic preservation treatmentsto a broad public.
