HierarchyId And Multiple-Trees

Hi all,
I leverage HierarchyId data type to implement tree structure in a table. Scenario: We are working on a project management system. First of all, user must define the hierarchial structure of the persons contributing to that new project(i.e defining project manager and subordinate relatioships).
Consequently, We are going to store the hierarchial structure of different projects in one table in order to manage them and each project has its own structure and root. These tree structures are independent of each other and projects are advancing in parallel.
What's Your Solution?
Is it a good idea to define a main root for all projects?
Thanks in advance.

Yes, you can. but I am not sure what exactly you are talking about. (Note: HierarcyId can have a single root only)
for your reference you can also check this: http://msdn.microsoft.com/en-us/magazine/cc794278.aspx
Thank you,
Sandy.