“So, the magic powder is Borasic Acid also known as Borax or Boric Acid or its chemical name H3BO3. Get it in powdered form, not crystal, and sprinkle it everywhere. I know that it sounds like a dangerous chemical, but it has the toxicity of salt to humans and pets. As long as it does not get wet, it keeps working for years. Dust it behind, inside and under cupboards. Dust it in the loft and in wall cavities – anywhere that roaches might go. Health officials in USA are recommending that it is strewn around liberally in attics and voids as houses are being built so as to provide long term resistance.”
This is a quote from a Fethiye Times article, search the full one on their site. We bought the borax up in Fethiye at a Chemist. Think they use it on the boats for the same reason. It seems to work. Hope this helps.