"Will" wrote
The large ivy all over the wall of a friends house came down in a
recent storm. I have cut it back to about 6 ft from the ground and
noticed that the trunk is about 6 - 8 ins.diameter. My friend wants to
keep it and not let it grow so rampant in the future ......... however
is concerned that as the roots are so near and in fact must be going
under the house would they be likely to cause any structual damage
etc.

The advice to take professional advice is a good one as the situation can
be complicated and it also depends on the type of soil:

IME, structural damage is just as likely to occur from the dewatering of
the ground by trees and shrubs located close to foundations, whatever the
nature of the root system: Certain subsoils (particularly "shrinkable
clays") need a good supply of moisture if they are not to contract and
cause subsidence. This contrasts with the tree damage and accompanying
soil "heave" that most people are familiar with, caused by invasive roots
from larger trees etc. which can physically disrupt the foundations

In the mid-90s, prolonged drought was a problem in (for example) some
soils in London, where reduced moisture in shrinkable clay caused some
Victorian houses to move which had never moved before. In these cases,
any plants that exacerbate the moisture loss can add to the problem and
some large flats have watering systems around their foundations which keep
the subsoil moist and stable when droughts occur.

However, I've also known of cases where movement of soil *after* a large
well-established tree/shrub has been removed (and the soil then suddenly
becomes more saturated where it has previously been largely dry) has
caused expansion and movement of the foundations, where none had occurred
when the tree was still there.


- Tom.