There are different types of groundsheets. the ones in my synthetic tents are polythylene, same as builders tarpaulins.
then there are heavyweight rubbery type ones you can get
some people use pondliners for groundsheets in canvas frame tents too.
basically they can be anything waterproof.
then there are also breathable ones in rubbery type material with holes for the grass to grow through, usually used in caravan awnings where sites say no other groundsheets except these can be used.
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