On Sat, 15 Oct 2005 13:48:14 +0100, --s-p-o-n-i-x--
wrote:

Depending on the local water. In some conditions the water goes right
through them. Lead isn't actually that stable long-term, you need a
moderately hard water to build up a surface carbonate layer on it.

So how do lead covered roofs work?

A carelessly installed lead gutter may leak in just a year or two.
Properly applied leadwork is treated with "patination oil" after
installation and it's this patina that provides much of the corrosion
resistance. A roof itself isn't that bad, as they tend to dry fairly
quickly after rain.