Mold or Mildew on Exterior Surfaces

In this article Mr. Geary first gives a list of places mildew and mold may form, and also describes the environmental conditions which contribute to formation of mold. He describes the varying appearances of mold and mildew, reminding the reader that it is a natural occurrence. He gives several procedures used for the removal of mold and mildew and also includes a small section on how these remedies will affect neighboring plant life.





Favorable environmental conditions may encourage mold or mildew to grow on many exterior surfaces, such as:

    Natural stones

    roofs, whether wood or asphalt shingles, roll roofing, asbestos cement shingles, fired clay or cementitious roof tiles

    elevated balcony decks

    horizontal brick or concrete surfaces, such as patios, walks, driveways and decks around swimming pools

    exterior walls such as painted or unpainted wood siding, portland cement plaster (stucco), clay bricks, concrete blocks and other substrates, as well as interior walls of buildings.

Environmental conditions which contribute to formation of mold on roofs, vertical walls and horizontal and sloped surfaces are:

    occasional or frequent presence of moisture, or nearly constant moisture; favorable temperature, plus, of course, presence of spores of mold or mildew. Note that spores of those small plants are always present in the air in temperate climate zones.

I have encountered growth of mold or mildew on many kinds of surfaces in British Columbia, Washington, Oregon and California, down to the Southern border, as well as elsewhere in this and other countries.

It is in that combination of favorable environmental factors that mold and mildew may grow and multiply. A number of different varieties of mold and mildew exist. Some types favor a surface that is nearly neutral, or slightly alkaline, such as concrete, cement plaster and masonry.

Mold or mildew may be apparent as a pale green, greenish-black, yellow or white encrustation. In temperate climates, it may form at any time of the year. During warm and somewhat drier periods, mold is most apt to form on shaded areas of roofs, walls, decks and concrete slabs, if moisture is present.

The presence of mold or mildew does not indicate improper installation of bricks or plaster, placement of concrete, or installation of wood siding or roofing. Instead, existence of mildew under favorable conditions is a natural occurrence.

Mold frequently forms on walls where, for any reason, rain water may drain downward over the wall surface. That may occur because of plugged gutters or downspouts, a break in a downspout, a low point in a gutter where the gutter may have sagged, lack of a gutter, or for some other reason. If continuous or interrmittent drainage of water from a leaking rain gutter, downspout, valley of a roof, or other situation exists, that unsatisfactory condition must be cured, to prevent continuous or sporadic excessive wetting of specific areas of wall surfaces. Again, it is the presence of moisture, warmth and spores in the air that combine to encourage development of the encrustation. Growth of mold or mildew does not necessarily indicate neglect of maintenance by the home owner, but that, sometimes, is a contributing factor.


If it is desired to remove the deposit of growing mold or mildew, the following method is offered for consideration. Diluted household bleach typically kills the undesired growth, which prepares the offensive deposit for removal by high pressure water spray. It should be noted that bleach will gradually attack the cement paste that binds grains of sand together in plaster or mortar, and which binds gravel and sand in concrete. Thus, repetitive use of diluted bleach may cause the surface of a vertical wall or the flat surface of a concrete patio or walk to become somewhat rough, and prone to soiling. If it is deemed necessary to provide repetitive treatment with bleach, the owner might consider application of a clear coating such as acrylic resin or other suitable clear resin, after the first treatment and cleaning.

A method for cleaning mold or mildew from roofs, walls or flat surfaces is offered for consideration. The entire exterior wall surface of plaster, bricks, concrete blocks, natural stone or other material should be washed with a powerful jet stream of water, to remove loose debris. Washing should be done in accordance with procedure provided in my article entitled CLEANING AND CARE OF STUCCO WALLS.

After the clean water wash, the roof, wall or horizontal surface could be sprayed with a mixture of household bleach and clean water.

Spraying of the mixture of bleach and water can be accomplished with a garden type sprayer, an airless sprayer, or other device. The mixture of bleach and clean water should be blended in approximately the following proportions.

About three-quarters (3/4) cup of household bleach should be poured, slowly, into one gallon of clean water. Larger amounts may be mixed but the approximate ratio of bleach to water should be maintained.

When blending fluids, always pour bleach into water, and not the reverse. Add the bleach to water, and stir, outside the building, or in an area where there is good cross ventilation. Safety goggles are appropriate to protect the eyes during blending, stirring, and spraying. Old clothing which will not suffer from being lightened in color by the bleach should be worn, in case of splashing. Plastic aprons can be helpful in protecting clothing.

Do not allow concentrated or dilute bleach to contact any good fabric, rug, or unpainted wood work, to avoid affecting color of the object. If bleach does inadvertently contact any critical object, rinse immediately with clear, cold water, and wipe dry. Avoid prolonged contact of bleach, full strength or diluted, with aluminum window and door trim, metal gutters, or other metal object, because of potential discoloration or corrosion. It is advisable that metal trim or other metal objects be wiped immediately with a clean, damp rag or sponge (a synthetic, not natural, sponge), to remove bleach before discoloration can occur. The rag or sponge should be rinsed regularly in clean water.

After diluted bleach has been sprayed onto exterior walls, allow contact of bleach with walls (or roof or slab) to continue for at least fifteen to twenty minutes. If the growth is thick, longer exposure time may be required. After adequate exposure time to kill the spores, rinse with a blast of water from a hose, using a constricted nozzle to increase pressure of the stream of water exiting the hose. Always start rinsing at the bottom of the wall and progress upward, in accordance with advice provided in my article entitled CLEANING AND CARE OF STUCCO WALLS. Failure to adhere to this advice could result in unsatisfactory stains on the wall, because of absorption of dirty water into pores of the wall surface.

If some mold continues to be evident after the described treatment, repeat as needed to remove all mold.

If it is desired that a clear coating be applied to concrete, plaster, masonry, stone or plaster walls; that additional treatment should be done after a wall has been cleaned and allowed to drain and attain essential dryness. At that time, a coating of clear resin, such as acrylic resin, could be sprayed over the wall surfaces. Avoid application of excess resin, at any one time, which might cause vertical streaks of clear, hardened resin to form on wall surfaces.

If brick or concrete walking surfaces, such as patios, walks, driveways or other exterior slabs have suffered from mold growth, this same treatment may be applied to horizontal surfaces. All precautions related to treatment of vertical walls apply to remediation of horizontal slabs or paved areas. Note that longer drying time is necessary on horizontal surfaces before acrylic resin may be applied to the surface.

Note that sporadic growth of mold on flat surfaces of concrete, accompanied by hosing the slab, gradually etches the surface of the slab, to produce a somewhat roughened surface that is more difficult to keep clean.

After using spray equipment, the tank, hose and nozzle must be flushed well with copious amounts of clean water, following use of bleach, to prevent corrosion of metal components.

Diluted bleach is not harmful to most plants. It should be noted that bleach will acidify the adjacent soil slightly. That should be suitable for acid-loving plants. To maintain the health of certain types of plants which do not thrive in an acidic environment, it may be advisable to sweeten the soil afterward by application of ground limestone.

Tom Geary TCG:mey December 1991 Supersedes March 1983, May 1985, November 1990 editions



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