Two grooved strings, OS and WS, Fig. 222, are placed at the required slope, and at a distance apart equal to the length of the steps.
The wall string, WS, is fixed by being plugged to the wall. The ends of the treads and risers are keyed into the housings or grooves worked in the inner and outer strings.
The upper and lower ends of these strings are framed into newel posts, and so are the outer ends of the first and last risers of each flight. When the flight of steps extends uninterruptedly from the lower to the upper floor, these newels are attached to trimming-joists, TJ, provided in the floors to receive them.
When the flight is broken by a landing, additional newel posts are provided on each side of the landing, and extending the full depth between the floors, as in Fig. 221.
To these are secured the trimmers, T, fixed and wedged into the wall, and projecting from it to carry the landing.
As already mentioned, the two strings are sufficient for stairs with stout treads and risers up to a width of 3 or 4 feet.1
For wider stairs, however, additional support to the steps is necessary, and this is afforded by one or more rough strings (RS, Fig. 222) or carriages placed in the interval between the strings already described.
The ends of these rough strings are framed or housed into the trimming-joists provided to receive them in the floors, between which the stairs extend.
When there is a landing the upper ends of the rough strings are fixed to the special trimmers which carry the landing.
The treads are further supported by rough brackets rb, Fig. 221, secured to the rough strings.
The landing itself is formed like a floor, of boards laid upon joists framed in between the trimmers just mentioned.
When there is a wall on each side of the steps, of course the newels are not required.
When the floor is continued under the lowest flight of a stair, the space between the soffit of the stairs and the floor is called the spandril.
1 When the outer string is a cut-string it is never desirable to omit the carriage, however narrow the stair may be, as it would cause creaking, if not positive weakness. Even when both strings are close a carriage is an advantage.
Fig. 221. Section on AB.
Fig. 222. Plan (with treads and boarding removed).
Figs. 221, 222. Straight Stairs. Scale, ½ inch = l foot.