how to use parhs properly

Greetings,

the project I'm working is using scons and looks like this tree-wise:

root
   src
     a
     b
     c
     h
   tools
     d

modules a to c are compiled directly using scons (the other are shell cmds or just code folders).
I think I understand how the structure works, e.g.
$O/$mode/a/..., $O/$mode/b/..., $O/$mode/c/... for objects and $T/$mode/a/..., $T/$mode/b/..., $T/$mode/c/... for targets
as I'm working with variantdir.

thing is, I'm not sure how to address the paths, relative to root or absolute.
whenever I use absolute and post here questions, I get a remark that I should use relative.
but if I need to include root/src/h in both a and b modules, I must use absolute path or else scons will generate invalid include path.

so, what is the proper way to work?