Ok this way you can get on not only the stables roof but the stablles flag too.
Step 1-
this is the easy stuff
Step 2- Slightly harder, what your attempting to do is jump onto the ledge ( it's invisible btw)
Step 3- once you hit the ledge run up and ur there
Now to jump onto the flag, this could be used very well by rouges/druids.
follow all the previous steps and mount up and jump onto the flag.
yet again ther is another invisible barrir making it easy to land on
Enjoy