Hey old friend, is it roof's dripline or is it
the roofs dripline? I think the correct answer
is the first one. My spell check ruled against
me when I used roof's dripline. What is wrong
with that, its showing position, right? Please
help. Thank you GM, William

Roof's dripline is fine.

Sometimes constructions like these do not require an apostrophe because the word is an adjective in its own right. For example, 'sports page'.

However, roof's is okay. Some are not keen on non-animate objects 'possessing', but they are in the minority (i.e. they may object to the concept that a roof can own a dripline).

Thank you GM. Incidentally, the drip-line is owned by the roof. Thanks again. William