Chimney pipe thickness?

Dear all,
we're having a 8" rocket mass heater project here, with a chimney pipe exhausting from the RMH and distributing heat into benches and up to the roof. We couldn't find indication on the thickness of the metal pipe we should use for that.
I made previously one with a pipe of 1mm (0,04") approx. and it went fine, but as this installation is more permanent, we'd like to know if there are any special counter-indications to use similar pipe.

This 1mm pipe is enamelled (do you say glazed?) and is meant to be use as a chimney for traditional stove heaters.

Thanks for your indications and help,
Samuel Kalika
Porto, Portugal

People have used all sorts of metal pipe or duct for the passage through the thermal mass. 1mm thickness sounds heavier than most, and will probably work fine. The first 5 feet or 1.5 meters of duct after the combustion core/manifold are the most important, as that area sees the most heat stress.