You may have some success with a low loss 50 Ohm coaxial cable like LMR-400 to extend the antenna to the roof. It should be fairly easy to find from a local cable supplier with pre-terminated ends or you could always get it in spools if you have the tools/skills to terminate the cables yourself.
It should be noted that adding length to the antenna cable will result in a loss of signal so it might be worth testing first to see if the amount of signal loss is acceptable for your requirements.
