This section is from the book "A Treatise On Architecture And Building Construction Vol3: Stair Building, Ornamental Ironwork, Roofing, Sheet-Metal Work, Electric-Light Wiring And Bellwork", by The Colliery Engineer Co.. Also available from Amazon: A Treatise On Architecture And Building Construction.
8. In trimming joists of staircases, the two cross trimmers which are called headers should be so placed as to allow sufficient headroom to meet all probable requirements of a stairway. The header (which determines the headroom) should be placed so as to secure 7 feet of headroom from the tread vertically beneath it. A careful drawing of the elevation of the stairway will accurately determine this point.
In cases where a number of winders are placed at the bottom of a stairway, and the risers thereby stand one above the other at the center from which they radiate, a considerable rise is produced at the commencement of the run, the amount depending on the number of winders. In such a case, the header is placed at the extreme end of the staircase, and thereby ceases to be a factor in the headroom. This applies especially to boxed stairways, which are enclosed between partitions.
Care should also be taken to make the staircase of the correct width, which should be at least 5 inches wider than the width of the stairway; that is, the distance from the center of the rail of the flight to the facia of the floor landing should be at least 5 inches, to permit the rail to pass clear of the nosing on the landing.