It is important to understand when do we need a web application, site collection and sub site.
In general, you want to see if the data and its usage or functionality fit into any of the exising elements(eg: web application). If so, based on the target audience, you select the hierarchy(site collection or sub site).
Root site mostly contain links to all the subsites. Because it is simpler to go to an area to look for the information.
Reasons to Create Additional Site Collections
- Different featues deployed (e.g. publishing vs.collaboration)
- Different taxonomy features deployed (e.g. site collection specifically for document management or records management)
- To isolate custom applications or deployed features (will help with future migrations)
- Different security roles and isolation levels(SharePoint Groups)
- Size is expected to be greater than 40 GB(usually it would be 100GB)
- Site Collection settings are of value to vary per site collection (e.g. one site collection with auditing turned on,
another with it turned off)
- Aligned to logical information management zones
Everyone gets their own site collection or One big monster site collection would spoil the SharePoint implementation.
Reasons to Create Additional Sub Site
- This site has a different function from the main site.
- The site has a different target audience from the main site.
- The site has information which is not needed elsewhere.
Having more than 2 levels of sub site would complicate the sub-site to site collection conversion.