Boric acid to kill roaches?

Boric Acid doesn't kill roaches by direct poisoning, it works by damaging their exoskeleton, which results in death by dehydration. It will take days rather than hours to work properly. If you wish to kill roaches more quickly, buy some insect foggers and set them up in your house, being sure to turn off the furnace, A/C, gas range pilot lights, etc. before setting them off.