From the ArcaMax Publishing, Gardening Newsletter:
http://www.arcamax.com/news/gardening/s-1898-166893
Question: I have English ivy climbing up the brick wall of my house.
I've heard that it can damage the mortar. Is this true?
Answer: Ivy climbs or clings with little aerial roots that form along
the stems and attach themselves to a rough surface (such as brick)
like tiny suction cups. These roots can work their way into mortar
that is already loose and can worsen the problem. If the mortar is
sound, however, the ivy should not cause problems. To some extent, the
ivy protects the wall because it shields it from the weather. On the
other hand, ivy climbing a building can get out of control and the
little suction cups are all but impossible to remove. Additional
cautions: the weight and roots may possibly damage woodwork and window
frames, and certainly make painting difficult, and the vines can hold
excess moisture against the house.