Flat roof question

How technical do you need to get? Kind of a silly answer maybe, but I've always used "stairwell," "roof access stair," "service stair," or some variation thereof. It might have a fancier name, but I don't recall hearing it called anything else in school or practice. I'm always happy to be proven wrong, though.

You might be okay just calling it a stairwell that opens onto the roof. The stairwell isn't really a 'hut' per se, but a specialty penetration through the roof structure that is fire-resistant / fireproofed (fire-rated walls, doors, floors, and ceiling) and weathertight according to codes.

Also, the exterior material you see may be brick, but most modern buildings have stairwells constructed of poured concrete or concrete masonry units. Brick is just an exterior finish for the vast majority of construction, not structural.

So, short answer: no.

/more than you ever wanted to know about emergency egress from a recovering architect

Well, here http://www.thehistorybox.com/ny_city/nycity_tenement_legislation_history_pt_XIII_article1402.htm
they keep referring to a 'bulkhead' for stairs leading to the roof... ??

It is generally called something along the lines of what thewakin g self said. I have heard them called "stairheads."

I've seen it called "stair access" or "roof access" in specs. And thewakingself gave you the construction points already.

Don't know if you'd want to use it in a story because it's not the expected connotation, but any structure built on a rooftop is a penthouse. Really. ;o)