Hi Dan,
First thing I would suggest. Get that porous stuff tested. It'll cost you very little and and possibly save you a whopper of a bill later. I speak from experience.
That aside, we use an EPDM rubber membrane. It costs a little more but I would say its the best stuff. Lay it and 20...
Hi MPaton2009,
I have to agree with the other posts. One thing that has failed to be mentioned though I think is that those gaps let out heat. It's inevitable. Loss of heat means higher energy bills.
Certainly a good slate roof should last 100 years or so but if it hasn't been maintained...