You can't stop a leak from underneath. The slate needs to be reattached or replaced. But from up top.
Casey

I agree with Casey....

Casey

"http://www.driverdb.com/Sean McDonagh"

yeah i was afraid of that :>(
i worry about anyone going up on that slate and maybe damaging more tiles but i guess i'll be putting my 18 yo son up there.

Don't tread on the slate; you should lay a ladder with a "ridge hook" attachment on the roof first. (the ridge hook hooks on the ridge, keeps the ladder from sliding off.) You need a second ladder to get up on the roof, unless you have a convenient dormer or hatch. If you walk on the slates, they can crack, multiplying your woes.
Casey

I'm not sure I would trust this to an 18 year old -- assuming he has no experience. Could you find a roofer to do it. I realize it's a small job -- maybe a good handyman who's worked with slate before.