Before planning or laying out a stair, the following particulars are required to be known, and are generally determined by circumstances.
2. The position of the first and last risers. These must be conveniently arranged in connection with the approaches, doorways, etc., leading to and from the stairs.
3. The width and length of the staircase available.
These particulars being known, the description of stairs to be adopted can be determined upon, the choice being further influenced by the class of building in which the stairs are to be erected.
Thus, in small common houses or cottages with a very narrow space for the staircase, a straight stair may be necessary. In a slightly better class of house there may be just sufficient width for alternating flights, and a dog-legged stair will be suitable. In buildings on a larger scale, with spacious staircases, a geometrical or open newel stair may be constructed, the well hole increasing in width with that of the staircase.
Circular geometrical stairs with solid or open newels are required in towers, and stairs such as those in Fig. 211 are necessarily made use of in a turret.
The description of stair having been decided upon, no general rule can be laid down for the arrangement of the steps. Some ingenuity and contrivance will be required in order to proportion and arrange them so as to fulfil all the conditions of a good stair fitted to the peculiarities of the position.
Such a stair will consist of flights running alternately in opposite directions, and each containing not more than 10 or 12 steps. All sudden alterations in the length of flights, especially single steps introduced here and there, should be avoided. The landings between the flights should be of a length and width at least equal to the length of the steps. Winders should be avoided as much as possible, and the steps should have the rise and tread carefully proportioned to one another, as directed at page 104. Care must also be taken that when one flight passes under another, or below a landing, there should be plenty of headway; and also that the steps are clear of all doors and windows in the staircase. The stairs should be well lighted throughout their length, more especially at the approaches.
The light may be furnished by windows in the sides of the staircase above the landings, or by a lantern at the top, the latter giving the best and most equally diffused light.