I have a springhouse sitting atop a mostly buried, 14' X 14' open-top concrete cistern. The plywood flooring is rotted and needs to be replaced. Though the cistern has some vents below the floor level, the underside of the floor is essentially a high-humidity environment. What is the best material or construction to use for this floor? Things I have considered:
1) pressure-treated plywood (rejected, since this sits just over our drinking water),2) Marine-grade plywood (very expensive)
3) exterior-grade plywood, installed over a vapor barrier (6 mil poly sheeting?)What will work best?
Thanks,
Kelly