Any ideas for replacing existing stucco patio ceiling with wood plank
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Make sure its not stucco on drywall, I had a stucco ceiling on my 1920's sunporch and found prev owners put it up as drywall and the original wood planks where under there. If it is directly on the ceiling you would need to strap the ceiling as if installing drywall and nail in tongue and groove boards.
UPVC is best
removing the plaster is the best but to overlay it you will first have to lathe. I would use 1X4 on 24 inch centers and long screws to hold them in place. before installing the planking shim the lathe so that the final result will not have waves