We often are asked questions about how close willow trees (and other vigorous, damp loving species such as Poplars and Alders) can be planted to a house or drain.
The easy answer is "not close at all". A willow may look beautiful (what is more delicate than a golden weeping willow - Salix sepulchralis Chrysocoma) but there is a monster lurking below. In their class, willows have to be the all time root producing world record holders. Their tentacles will sniff out water and - even better - drainage at long range.
I really have no idea what their maximum reach is, but I think it would be wise to leave AT LEAST 10 metres between tree and drain. If it were me, I might up that to 15 metres.
And here is why - a new horror movie available to you only on this screen-
The Willow from the Black Lagoon
More seriously, the point is that this (poor) little grey willow (Salix cinerea) which produces some of the most beautiful catkins in the willow world is between 2-3 years old and was pulled out of a downpipe owned by a friend of Rebecca (who works at Ashridge and brought the creature in. The film does not lie - the roots are well over 3 metres long while the plant is about 1.50 metres. The root mass is perfectly tubular and perfectly SOLID. Apparently it was a real tug of war to get it out and another year would probably have seen it a metre longer and completely immovable.
So keep them away from drains.....