Most banquet facilities add an 18% gratuity or service charge to their bill but this is not a tip for the server.
We feel calling this 18% charge a gratuity or service charge is misleading so we call that 18% additional charge a "House Charge."
Banquet servers get paid a higher hourly wage than regular servers, so the gratuity, service charge or house charge, as we call it, covers their higher pay plus any expenses for the room such as linens, lighting, extra cooks, etc.
You are more than generous if you would like to leave an extra tip for your server and I'm sure your server would really appreciate your generousity, but that is totally up to how you feel your service was.