Well, on my (1910) roof, where it meets next door (terraced houses) and their roof is higher ...
There is an 'L' shaped bit of zinc plate under every tile. In situ the flat plate is against the wall and comes down and under a tile. Each tile against the wall has one. Each plate is long (wide?) enough to overlap the next so that any water falling off the side of the tile hits the plate and is directed onto the top of the tile below (or the next plate down). Hope this helps
nospam at my-email.org.uk, September 2008