How invasive are lilac roots?

[Devichan]

On a scale of corn to willow, how invasive are they? I want to put a whole bunch of them near the house. I love the scent of the flowers and once the flowers are done, the foliage is lovely. I am concerned that they'll get into the sewer system. We've already had to root out the system a few times with the maple trees in the front yard.