Well, needless to say, the first step is to repair the roof leak and permit the wood ceiling to completely dry. Then, go to your paint supply company for some wood bleach. This product generally is applied in two parts. The first solution is applied directly to the stained area and left for about five minutes. Then, while the surface is still wet, the second solution is applied. Use a nylon brush or rubber sponge to apply the wood bleach, and separate applicators should be used for each solution. After both solutions have been removed and the area has been permitted to dry, lightly sand to remove any residue that may remain. Most importantly, remember to follow the manufacturer's specifications and always wear rubber gloves and proper eye protection.