Answer: Any tree's roots can easily range at least as far as the branches spread and usually go about half again as far as the branches, sometimes twice as far. If there is a gap in a foundation or pipe then the roots of any tree could enter it and work their way through. Willows in particular will range great distances to reach water and follow it to a source. Unfortunately, I do not have a "safe" number of feet to give you. If you are thinking of planting in proximity to pipelines and drain fields you might be wise to use a smaller and less adventitious tree.
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