Phil L wrote:

Water runs down the inside of your exterior brick/stonework during heavy
rain...

It bloody well should NOT. If it is you have a serious problem.

Or airbricks...

if you could take down the inner leaf and look at the stonework
during a rainstorm you would see a mini waterfall! - this is normal and the
cavity stops anything getting to the inner leaf, this is why brick ties have
a twist or a bump in the centre, to allow water to drain down the cavity.


I've done a diagram and put it up on tinypic:

http://tinypic.com/view/?pic=rw5uyq