I have tree house with circular porch that serves as roof over goat stable.plywood is bad& needs replacing what can I do with plywood that is economical.

Economical? If you have the money, use pressure treated plywood. If saving, use OSB. If the roof is rotted, you must have a leak. You do have shingles or other roof covering?

Economical? If you have the money, use pressure treated plywood. If saving, use OSB. If the roof is rotted, you must have a leak. You do have shingles or other roof covering?
What about roll roofing?

What about roll roofing?
Roll roofing has to sit on something, over roof decking.