Long story short: got a new roaster, it requires Category III (positive pressure) venting. The roaster itself has the fans. It seems no one in El Salvador has heard of these categories as people don't...
Category 3 is an insulated, multi layer pipe. the whole point is to prevent burning materials in direct contact with the pipe, like roofs or walls. Generally you also want a 1" gap between the pipe and surrounding materials. Your pipe idea shoukd work fine, so long as the pipe does not come into contact with flammable materials.
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