Answer: You can generally expect shrubs and trees to develop roots that extend out about one and one-half times the length of the branches. Lilac roots are not considered invasive, but a shrub that's 10' wide will probably have roots extending out about 15' in all directions.
Lilacs may not be the plant of choice if you're looking for a privacy screen, because they lose their leaves in the fall. Perhaps a flowering evergreen viburnum will appeal to you instead. Excellent choices include Viburnum rhytidophyllum (leatherleaf viburnum), with clusters of yellowish white flowers in spring and clusters of scarlet fruit in autumn, and Viburnum tinus, with dark green foliage, wine red new stems, clusters of fragrant white flowers from winter through spring, and bright metallic blue fruits that last through summer. Both produce dense foliage and make beautiful privacy screens.
Best wishes with your new shrubs!
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