I'm sorry, but fixed headers are almost always a bad idea, because it make assumptions about the clients that may not be valid. The assumption is that the client has the vertical space for a fixed header.
Even the example is shown for a client that has plenty of vertical space. If the user is on something like a 13" laptop, the fixed header reduces the amount of "usable" space on the page.
Unless you're absolutely sure that the client is on a big monitor, and has a browser window that's higher than it is wide, don't use fixed headers.
Interestingly enough you rarely see website/webapps that utilises the fact that most users are on wide-screen monitors (mobile excluded).
Even the example is shown for a client that has plenty of vertical space. If the user is on something like a 13" laptop, the fixed header reduces the amount of "usable" space on the page.
Unless you're absolutely sure that the client is on a big monitor, and has a browser window that's higher than it is wide, don't use fixed headers.
Interestingly enough you rarely see website/webapps that utilises the fact that most users are on wide-screen monitors (mobile excluded).