I am recently re laying some 2 x 3 slabs and replacing some badly laid
concrete with matching slabs, under the concrete there is a network of
roots, mostly fine stuff but a few at a 1cm thickness. What is the best
way to deal with this, ideally one of the weedkillers that will kill it
off and prevent regrowth for as long as possible. The roots belong to
trees/shrubs/ivy which is on the other side of my wall and the roots
have over the years forced their way through onto my land so I cant
attack by spraying the leaves as the shrubs are not my property.

Any ideas