How to lay decking (with concrete base) around a lilac tree

Hi all, I was looking for some advice on how to lay decking around a lilac tree. Specifically, I'm extending an old shed base by adding concrete around the edges (up to the fence - this is the foundation for the decking), but I want to preserve the lilac tree (shown in the picture) - I'm not sure how close to the tree the concrete can come without causing the tree problems? While digging around within 40cm or so of the tree I've come across some fairly chunky roots (say 3 or 4cm diameter, quite close to the surface) - I'm worried that if I cut these to clear the depth for the concrete it may prove fatal for the tree?

Any advice on the above would be greatly appreciated! Thanks in advance,

Paul