if its all cosmetics to you, you can also do double deep floor tiles so the bottom (seen interior) remains fresh looking while the top one takes rain damage. Assuming you don't live in a valley where you climb the nearby hill/mountain/whatever to look down on your home, you should rarely see the grey 50% wood topside.

the game is not done, so comments about what is available at endgame is premature. But, yes, at this point we have only stone flat 'roof' using lofted floor tiles (and with marble, that includes 1x1 blocks, 1/2 floor tile triangle blocks, 2x2x floor tiles, 2x1 blocks ... all fit as a flat 1m thick stone roof. Marble has some slanted and oddball pieces so you can do something a little more fancy than just flat, mostly spiked corners but you can put those together in interesting ways.