Re: Punishment for this?
I would have two consequences. One for purposely throwing keys on the roof, and one for lying about it.
If he had come and told you he did that, you probably would have been angry, but not AS angry. He made a stupid choice, and didn't think it through first. Because of his stupid choice, Dad had to miss out on overtime, and risk hurting himself to get the keys down.
At the age of 8, he can start "fixing" or making amends for what he's done. He can make a choice, but there's always a consequence that goes along with that choice, and he gets to live with his consequence.
I would suggest having some consequences in mind for the future, so you can make it swift each time, instead of thinking on it for a day or so. In my experience "punishments" don't work. One sweeping punishment like being grounded, or no tv, or something doesn't work as well, as fixing what you did.
In this case, he took the keys from his sister. He didn't want her to get them back, so in a HUGE lack of judgement, he threw them on the roof...then he framed his sister for the crime. He owes his sister, you and his Dad some time, apologies, and maybe a some extra work to pay off what he's done.