There's no rule. Some are separated ("water tank"), some are hyphenated ("5-year-old"), some are combined and written together ("sunrise", "boyfriend"). And again, some exist in all three forms - that is, there is no hard rule and it is the writer's own preference or background that determines the form written.
"Notice board" is one of these. It can be combined, written together or hyphenated.
Showroom, however, is usually written combined - "Showroom".