What Causes Stains on Cedar Siding?

Mildew

• Mildew is the primary cause of staining on the majority of cedar siding. Mildew forms when the cedar is in a slightly damp, shaded area or when dirt and grime have adhered to the siding, trapping moisture behind them. Mildew stains can often be taken care of with a commercial mildew stain remover, following the manufacturer's directions to avoid destroying or damaging the surface of the wood. A mixture of half water and half household bleach can also be used to remove mildew stains on cedar. Use a nylon brush to gently scrub away the stains and wash off the bleach with a garden hose. Do not use a power sprayer or a metal brush to clean cedar siding as these may damage the surface of the wood.

Mold and Algae

• If the cedar siding is in a location near a water source where there is constant humidity or dampness or if you live in an area with exceptionally high humidity, algae and mold can grow on the surface of the cedar siding and cause unsightly staining. Begin the cleaning process by using a stiff nylon brush and plain water. If this does not remove the stains, add liquid dish soap to the water and scrub it again with the brush.

Moss

• If your cedar siding is shaded by trees or plants and is subjected to constant moisture or if you live in a humid area, the cedar can be stained by the growth of moss. Moss can almost always be removed with a nylon scrub brush and plain tap water. For stubborn areas add liquid dish soap to the water and scrub again. Wash the liquid soap residue off with a garden hose.

Sun and Rain

• Cedar has a tendency to fade over time when exposed to sunlight. Rain can also cause staining, especially as any protective coatings wear or break down with time and exposure to the elements. Remove the old wood coating with a nylon scrub brush and water or add liquid dish soap if necessary. Try not to damage or scour off the surface of the wood. Use sandpaper only as a last resort and rub it lightly across the surface; you only want to remove the old finish and not the surface of the wood itself. Use 100-grit sandpaper then make one last smoothing pass with 150-grit sandpaper.

Extractive Bleeding

• Cedar contains natural chemicals to help it resist staining, but occasionally water will get behind cedar siding planks. This dissolves these natural chemicals and then bleeds them onto the plank below in a process known as extractive bleeding. Generally, stains caused by extractive bleeding can be removed easily with a nylon scrub brush and water or water with a dash of liquid dish soap, then a gentle rinsing with a garden hose. If extractive bleeding is a problem, the source of the water and its path behind the siding must be eliminated, or the problem will reoccur.