On Fri, 19 Aug 2011 01:20:55 -0700, CJB wrote:

tl;dr: there was a lot of rain in a short period, some buildings that
cover a lot of area don't have enough drainage on their roofs to cope.

Roofs are generally designed such that normal levels of rainfall are
carried away by pipes. However, these pipes have a capacity limit, and
can be blocked by all sorts of debris that ends up on the roofs.

When that happens, and you then get an unusually high level of rainfall
over a short period, it's not unusual for certain types of roof to start
letting in water, as it arrives faster than it can be removed.

For example, any tiled type roof with a gully between adjacent slopes
will leak if the water level in the gully rises far enough above the
gully lining.

This is just as true when the "tiles" are 8' x 4' sheets of corrugated
material as it is when they're 18" x 8" of slate.

Rgds

Denis McMahon