To kill cockroaches with boric acid, spread it where the roaches are likely to walk through it. When they do, the toxic powder adheres to their bodies, legs and antennae. Upon grooming themselves, they ingest the powder and die.
Boric acid is highly toxic to roaches, although it is non-toxic to humans and pets, and is very effective in small amounts. It has no repellency, so roaches keep returning to treated areas until they die.
Spread boric acid powder in a thin, barely visible layer where roaches live, which is near water, food and warmth. In the kitchen, dust boric acid under the sink and under and behind the oven, refrigerator and dishwasher. Sprinkle the powder in the bathroom under the sink and behind the toilet, and dust it behind the washing machine. Plug cracks in the walls and around pipes with caulking or steel wire, or apply boric acid into these cracks.