Q: Should I use bleach to clean mold and mildew off outdoor wood?
A: The answer depends on the kind of bleach you’re using. Whereas chlorine bleach does an excellent job of killing bacteria and viruses, and will remove superficial mold, it has not been proven effective in killing mold on porous surfaces. This is because the mold’s enzyme roots grow inside the pores, and chlorine bleach cannot penetrate the porous material due to its ionic structure. This means the mold problem will likely soon return. Chlorine bleach also damages the lignin in the wood, which is how the wood is bonded together, and this will make it more prone to aging and splintering along the surface. Chlorine bleach can also cause some deck stain/sealers to fail, removing the color tone and even some of the wood’s natural color. A better alternative is to use oxygen bleach. As found in laundry detergent, oxygen bleach cleans fabrics without disrupting the color or damaging the material. Oxygen bleach can usually be combined with water, applied to the wood and allowed to sit for 10 or 15 minutes, then it can be easily rinsed from the surface with a garden hose. Oxygen bleach can clean the wood without damaging the fibers or harming the surrounding vegetation, which eliminates the need to use plastic or drop cloths for protection.