The planning of a straight stair is a very simple matter.
The height of the storey being known, a convenient height for the risers, appropriate to the class of staircase (see page 104), is assumed pro tern.
The total height to be gained, divided by this dimension, gives the number of risers, the number of treads will be one less (see page 109); and the proper width for each tread (in proportion to the height of the riser) will be found in the table, page 104. If there is room in the staircase for the required number of treads of this width, with the necessary landings, well and good; if not, a steeper rise must be assumed, requiring narrower treads and fewer of them, for which there will be room.
Thus, in the staircase, Figs. 198, 199, the height to be gained is 11 feet 8 inches: assume 7 inches for height of risers, 140 inches/7 inches = number of risers = 20. The width of the tread proportionate to such a riser (see page 104) is 9 inches, the number of treads 19. The total length of staircase required for the treads will be 19 x 9 inches =14 feet 3 inches; thus, without a landing, a staircase 14 feet 3 inches long will be sufficient, but with a landing 4 feet wide, substituted for one of the treads (14 feet 3 inches + 4 feet - 9 inches) = 17 feet 6 inches, will be required for the length of the staircase.
The risers must be equal throughout the stairs, none higher or lower than the rest should be introduced to make up an awkward dimension in the vertical distance. Thus, in the case just given, if the height to be gained had been 11 feet 6 inches, the number of 7-inch risers required would have been 138/7 = 19 5/7 : it would not do to have nineteen 7-inch risers, and one 5-inch riser; but 20 risers would be used, each 138/20 = 6 9/10 inches height, thus equally dividing the vertical distance to be gained.