I implore you to read the howto docs here: http://docs.djangoproject.com/en/dev/howto/static-files/
In short: STATIC_ROOT is only used if you call the collectstatic manangement command. It's not needed to add the directory to the STATICFILES_DIRS setting to serve your static files!
During development (when the automatic serving view is used) staticfiles will automatically look for static files in various locations (because you call its "serve" view with a path to a static file). For this search it'll use the so called "finders" (as defined in the STATICFILES_FINDERS setting).
One of the default finders is the AppDirectoriesFinder, which will look in the "/static/" directory of each of the apps of yours INSTALLED_APPS setting.
Another default finder is the FileSystemFinder, which will look in directories you specify in the STATICFILES_DIRS setting.
BTW, both these search patterns are similar to how template loading works.
The same technique will be used when running the collectstatic command, except that it now will collect the files from the various locations (using the same finders as above), putting the found files into STATIC_ROOT, ready for deployment.
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