I am building an addition to a house - first floor is
living space, second floor above is to be a greenhouse.

Greenhouse floor is to be redwood decking/grating or
similar. Decking/grating will be on joists, under which
needs to be a leakproof pan to prevent leakage to the
living space below (greenhouses are wet, right?). Pan
will not support load or traffic, but it needs to be
long-lived or permanent, since access would be difficult
if repairs needed. I can slope and drain the pan.

Choices seem to be Chlorinated Polyethylene (CPE)
sheet membrane, copper sheet, roofing/hotmop tar or
(other ????). Usual considerations of cost, installation
labor, durability, etc. Would appreciate recommendations
and comments from anyone with experience ....