Removing the Algae
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Begin algae removal by scraping all affected areas of wood with a stiff putty knife or paint scraper. Much care should be taken so as not to gouge or mar the wood. A high-powered pressure washer is not recommended because it may damage wooden stairs. When you begin the scraping process, keep in mind that the idea of the first step is not to remove all the algae growth, just the areas where the algae has built up excessively. This hand scraping process also allows you to closely examine the extent of algae growth on your stairs.
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Continue the algae removal process by scrubbing with coarse steel wool and a mixture of warm water and a mild biodegradable soap and detergent. Use some elbow grease and scrub hard until the algae is completely removed. If large areas of growth are present, a long-handled, plastic scrub brush might be helpful.
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Rinse the stairs with clean water.
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Clean the wooden area with a long-handled scrub brush and a mixture of warm water and an oxygen bleach cleaner. Clean all areas of the wooden stairs completely. Be sure the cross-cut sides of the wooden steps are treated with the oxygen bleach mixture as well as the main surface of the treads and risers (if present).
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Rinse the stairs with a garden hose (if outdoors) or a pail of clear water and a sponge.
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Let stairs dry thoroughly, then seal, paint or stain them to help prevent the algae from returning.