23. One of the simplest forms of stairways in use is a step ladder, fixed or movable, such as is sometimes built from an attic floor to the roof, as is shown in Fig. 18, where <I >(a)</I> is a plan, and <I >(b)</I> an elevation. The top step should always have an increased width, and the sides of the ladder should be at least 6 inches wide by 1 1/8 inches thick. The steps are gained into the sides not less than 3/8 inch and securely nailed or screwed thereto. This ladder rises 8 feet, or 96 inches, which, divided by 10, the number of steps, gives 9 3/5 inches for each riser. The run, or horizontal distance from the face of the beam at the landing to the bottom of the ladder, equals 3 feet 7 inches, less 3 inches for additional width of top step, leaving 3 feet 4 inches, or 40 inches, which, divided by 10, the number of treads, gives 4 inches as the width of each tread from a vertical line dropped from the nosing above.
24. Plank Stairway. — In <I >(a)</I> and <I >(b)</I>, Fig. 19Fig. 19. The plan and elevation of a strong stairway., are shown the plan and elevation of a strong stairway, such as may be required for a shop or factory; a and b are newels, 6 inches