Put on a pair of long pants and a long-sleeve shirt to prevent the mold from coming in contact with your skin. Put on rubber gloves, safety goggles and an N-95 respirator.
Add 1 gallon of lukewarm water into a 5-gallon plastic bucket. Mix 1 cup of chlorine bleach in the water. Alternatively, use 1/4 cup of trisodium phosphate if bleach is not available.
Fill a spray bottle with water and mist the mold growing on the basement rafters. The water will moisten the mold and reduce the amount of spores the fungus releases into the air during the removal process.
Dampen a scrub brush in the diluted chlorine bleach and scrub the wood rafters clean. Continue submerging the scrub brush in the mixture and scrubbing the rafters until you have removed all traces of the mold.
Let the diluted chlorine bleach sit on the rafters for 15 minutes. Rinse the mixture off the wood with cool water and let air-dry.