Maintenance of Cedar Siding: Removal of Algae, Mold, and Iron Stains

Wood surfaces

As wood ages, mildew (mold) and algae begin to grow on the surface. This is a normal process; these organisms do not degrade the wood. They cannot break down the structural components of wood. They just live there. They feed off airborne contaminants, extractives, and oils in wood and in some finishes. Algae and molds can be cleaned quite easily and effectively with bleaching agents such as sodium hypochlorite (liquid household bleach) and sodium percarbonate (the active ingredient in some commercial cleaners). Bleaching agents quickly kill mold and algae, but they also can degrade wood. Therefore, mix cleaners as dilutely as possible. The object is to remove the fungi without excessive wood damage.


The cedar siding on this house has become stained. Photo from Flickr:

The authors of the study, Installation, Care, and Maintenance of Wood Shake and Shingle Siding, recommend using commercial cleaners containing sodium percarbonate or other oxygen bleaches because they are more gentle oxidizers than chlorine-containing bleaches such as household bleach. Chlorine bleaches tend to cause excessive pulping of the wood to give a fuzzy surface. However, some commercial cleaners contain strong alkali (sodium hydroxide or potassium hydroxide). These ingredients help to remove residual finishes on the wood surface, but can cause even more surface damage than chlorine bleach. The photo below shows a person cleaning a deck with oxygen bleach and a gentle brush. These same techniques can be used to clean shingles and siding.


When using a cleaner, you might mix the solution weaker than the recommended strength and try it on a small area. If the weaker solution doesn’t work, increase the concentration until you find a concentration that cleans the wood. A fuzzy surface appearance or excessive removal of surface fibers indicates that the solution concentration is too strong. Apply cleaning solutions with a garden-type sprayer, sponge mop, or soft bristle brush and keep the surface wet with the cleaning solution for 12 to 15 minutes. It is best to work on a cool cloudy day or even during a gentle rain so the solution doesn’t evaporate. Aggressive scrubbing shouldn’t be necessary; let the cleaner do the work. Rinse with a garden hose keeping the water-stream pointed down. High pressure shouldn’t be necessary. You do not need a pressure washer! Allow the surface to dry for several days before refinishing.

We recommended using commercial cleaners, but if you prefer to use liquid household bleach, start with a cleaning solution of about five parts warm water mixed with one part bleach) with a small amount of powdered detergent. Do not use detergent that contains ammonia. Ammonia reacts with bleach to form toxic fumes. If the surface mildew is difficult to remove, you should then work with a stronger solution of three or four parts water added to one part bleach and detergent. It should not be necessary to use a concentration stronger than three to one.

If the dilute cleaning solutions described above are not effective, it is probably because the mildew is inter-grown with residual finish on the surface. The residual finish is keeping the cleaning solution away from the mildew. In this situation, it may be necessary to use more aggressive cleaning methods, such as the cleaners containing strong alkali.  In some cases, use a paint stripper to remove the residual finish prior to cleaning.

As with unfinished wood, wood finished with wood repellant preservatives (WRPs) and semitransparent stains degrades as these finishes degrade. Ultraviolet radiation in sunlight degrades lignin at the surface. Lignin is the natural glue in wood that holds the cellulose fibers in place. Degradation of lignin weakens the surface fibers, and strong cleaning solutions and aggressive methods will remove excessive amounts of fiber from the surface. Removing these fibers is detrimental to the performance of subsequent application of WRPs or semitransparent stains. These finishes perform best by penetrating the wood surface and as a wood surface degrades, it becomes more porous. If excessive amounts of fiber are removed during cleaning, the surface will not accept these finishes as well as the porous weathered surface. Again, when cleaning wood, the gentler the better.

Iron reacts with the extractives in cedar redwood to give a dark blue-black stain. This often occurs when the zinc on galvanized fasteners weathers away or from rust washed from other sources such as window screens, failed flashing, or metal ornaments. This blue-black stain can be neutralized with a 5% solution of oxalic acid (usually available at drugstores).

Note: Oxalic acid is toxic. Many commercial wood cleaners contain oxalic acid. Oxalic acid will neutralize the iron stains and will also remove extractive stains. Oxalic acid generally brightens the wood surface, but is not very effective for removing algae or mold, nor will it keep iron stain from reoccurring if the source of the iron is not removed.


Looking into the Future of Wood Preservation

Carol Clausen and her group are serious about wood as a sustainable and versatile building material. Here Amy Blodgett, Rachel Arango, and Bessie Woodword check soil block samples in their ongoing research.


Members of the Durability and Wood Protection Research Unit check soil block samples.

The soil block decay test method determines the minimum amount of preservative that is effective in preventing decay of selected species of wood by selected fungi under optimum laboratory conditions. Conditioned blocks of wood are impregnated with solutions, emulsions, or dispersions of a preservative in water or suitable organic solvent to form one or more series of retentions of the preservative in the blocks. After periods of conditioning or weathering, the impregnated blocks are exposed to recognized destructive species of both brown-rot and white-rot wood-destroying fungi.

Speaking about her work, Clausen says that wood’s “increased use in construction minimizes life-cycle impacts of a structure while maximizing carbon storage for the life of the structure.” An upcoming report by Clausen, Frederick Green III, Grant Kirker, and Stan Lebow summarizes presentations and comments from the inaugural Wood Protection Research Council meeting, where research needs for the wood protection industry were identified and prioritized.

As a part of that conversation, Clausen states that chemicals used to protect wood from deterioration by fungi and insects have generally been broad-spectrum biocides that have been discovered by the traditional screening approach. However, a more logical approach is to develop selective and targeted biocides by defining the target first, characterizing that target, and then designing inhibitors based on the mechanism of action of the defined biotarget.

Despite substantial progress in explaining the biochemistry of wood degradation, biochemical targets for fungal inhibition have seldom been described. Discovering the mechanism of preservative tolerance(s) in economically important fungi and insects will enable researchers to design preservative systems that neutralize, block, prevent, and eliminate the preservative tolerance. Development of a genetic database of microbial activity during the process of wood deterioration will offer a better understanding of precisely how and when decay or insect attack begins. A molecular database would provide myriad of capabilities to the wood preservation community. For example, researchers could characterize decay risks for a particular location, define the prevalence of tolerant fungi on a national and international basis, and determine the influence of preservative exposure on the ecology at test plots.

The next generation of novel wood protection methods may incorporate nanotechnology for design or controlled delivery of biocides for improved durability of building materials, with an emphasis on engineered composites. Nanotechnology may also play a vital role in the development of water-resistant coatings and treatments for the prevention of fire. Results from basic research on genetic analysis, biochemical processes, nanotechnology, and characterization of biotargets will lead to technological developments that extend the service life of wood and wood-based materials in all major end uses emphasizing environmentally friendly methods.


Tips for Protecting Other Wood Structures

According to Build Green: Wood Can Last for Centuries, the same principles of decay that are applicable to buildings also apply to wooden decks, fences, boardwalks, pergolas, gazebos, planters, and playground structures.


Using treated wood for exterior applications will eliminate decay worries in decay-susceptible zones at ground line and the intersection of post and horizontal rail (Photo by Carol Clausen, FPL).

Authors Carol Clausen and Samuel Glass tell us that the greatest decay hazard exists at the ground line. Pressure-treated wood approved for use in the ground is recommended. A lesser, but still important, decay hazard exists where horizontal rails, stringers, or timbers are joined together and where vertical boards fasten to the structure. Both of these locations collect water and dry slowly. Use preservative-treated wood in these areas and keep vertical boards off the ground.

Does Painting Protect from Wood Decay?

Carol Clausen and Samuel Glass tell us about another easy and inexpensive trick for preserving your building projects in Build Green: Your Wood Can Last for Centuries. Coatings that form a moisture-impervious film are effective in preventing decay if the seal where two pieces of wood are joined is kept intact. A well-maintained film of paint over the wood and good paint seals in the joints shed water, thus protecting wood by keeping it dry.

Cracked paint seals permit entry of water and contribute to decay. Water seeping through the broken seal where wood is joined together will be rapidly absorbed at the ends of the wood pieces; its loss by evaporation from the sides of the wood member is retarded by the impervious, intact paint film over the rest of the wood. This keeps the wood moist and favors decay, at least near the joints. The effectiveness of a moisture-impervious paint in protecting wood from decay depends upon the quality of the seal in the joints.


Keeping the paint seal at wood junctures intact is critical. These are the points where moisture can seep into wood.

Keeping Wood Dry Isn’t Always Enough: Beware of Dry Rot

Build Green: Wood Can Last for Centuries by Carol Clausen and Samuel Glass tells us that most wood-decay problems only occur when decay fungi grow in wet wood. But one kind of fungus is uniquely capable of transporting its own water from a source of moisture (usually soil) into wood that is typically too dry to decay. While decay by such water-conducting fungi is uncommon, when it occurs, it is devastating.


Dry rot starts when an infected piece of wood forms a bridge between soil and other wood in a house (Photo by Carol Clausen, FPL).

Large areas of flooring and walls can be destroyed each year unless the fungus is stopped. Ironically, it may be the easiest fungus to prevent or control. Unlike typical decay fungi that start growing from airborne spores, water-conducting fungi usually start growing from previously infected lumber that forms a bridge between the soil and other wood in the house. This can happen if old, discarded beams that have been lying on the ground are used in home repairs or additions; new wood has been improperly stored in contact with soil; or infected wood waste is used as fill under a porch or addition. This type of decay can be stopped by simply breaking the contact between susceptible wood and the source of moisture. Once the water supply is broken and the infected wood dries, the fungus will die.

Good news for the homeowner: a potentially destructive source of rot is easy to control with attention and care.