Set down two sets of numbers, each in arithmetical progression—the first set showing widths of tread, increasing by inches; the other showing heights of riser, decreasing by half-inches.
| Treads, Inches | Risers, Inches |
|---|---|
| 5 | 9 |
| 6 | 8½ |
| 7 | 8 |
| 8 | 7½ |
| 9 | 7 |
| 10 | 6½ |
| 11 | 6 |
| 12 | 5½ |
| 13 | 5 |
| 14 | 4½ |
| 15 | 4 |
| 16 | 3½ |
| 17 | 3 |
| 18 | 2½ |
It will readily be seen that each pair of treads and risers thus obtained is suitably proportioned as to dimensions.
It is seldom, however, that the proportions of treads and risers are entirely a matter of choice. The space allotted to the stairs usually determines this proportion; but the above will be found a useful standard, to which it is desirable to approximate.
In the better class of buildings, the number of steps is considered in the plan, which it is the business of the Architect to arrange; and in such cases, the height of the story rod is simply divided into the number required.
Pitch-Board.
It will now be in order to describe a pitch-board and the manner of using it; no stairs can be properly built without the use of a pitch-board in some form or other. Properly speaking, a pitch-board, as already explained, is a thin piece of material, generally pine or sheet metal, and is a right-angled triangle in outline. One of its sides is made the exact height of the rise; at right angles with this line of rise, the exact width of the tread is measured off; and the material is cut along the hypotenuse of the right-angled triangle thus formed.
The simplest method of making a pitch-board is by using a steel square, which, of course, every carpenter in this country is supposed to possess. By means of this invaluable tool, also, a stair string can be laid out, the square being applied to the string as shown in [Fig. 13]. In the instance here illustrated, the square shows 10 inches for the tread and 7 inches for the rise.