With stout treads and risers the two strings above mentioned are sufficient for stairs of 3 or 4 feet in width.
For wider stairs, however, the steps require additional support, and this is afforded by means of one or more "rough strings," or "carriages" fixed in the interval between the wall string and outer string, already described.
Two rough strings are shown in Fig. 222, for the sake of illustration, but one only would be necessary in so narrow a stair-ease; one rough string is shown in Fig. 224.
The rough strings sometimes have small notches on their upper surfaces to receive the back edge of the tread, and their ends are attached to trimming joists TJ (see Figs. 221, etc.), or into pitching pieces P (Fig. 228), or trimmers T (Fig. 222), where trimming joists are not available.
Wooden Steps are formed of boards, as shown in Fig. 220.
The risers are united to the treads by joints, which may be grooved and tongued, as in steps 5, 6 - feathered as in step 4 - or rebated, as No. 3 ; in every case the joint is glued. The riser often has only its upper end tongued, the lower butting upon the tread below. This is not so good a construction as that shown at 3. A common practice is to house the lower edge of the riser into the tread below, as at x. The tread is sometimes tongued into the riser, but that is not a good construction.
The joint between the tread and riser is strengthened by small blocks, hi, glued into the inner angle, as shown in steps 3 and 4; these may be either rectangular or triangular in section.
Fig. 220. Scale, ½ inch = l foot.
The inner ends of the treads rest upon the rough strings, RS (if any), and they are frequently further supported by rough bracketa rb, attached to the rough strings or carriages.
These brackets may be pieces nailed alongside the string, as in steps 1, 2, 3, 4, or triangular pieces fixed to its upper surface, as in 5 and 6.
Occasionally vertical brackets are made of a width equal to that of the tread of the step, as at xy in Fig. 223.
The treads project over the risers and are finished with a rounded or a moulded nosing, the projection of the nosing being generally equal to the thickness of the tread. When a moulded nosing is adopted with an open string, the moulding is returned at the end of the step, being mitred at the angle, as shown at Fig. 217.
The mouldings are generally planted on under the rounded nosing of the tread.
The treads should be of oak or other hard wood, and may be 1 /18 inch thick for steps 4 feet long - the thickness being increased by 1/8 inch for every 6 inches added to the length of the step.
In very common stairs the risers are sometimes dispensed with.
In some cases, especially in geometrical stairs of a high class, the upper edges of the risers are dovetailed to the treads, and the back of the treads screwed up to the lower edge of the risers.