195. In order to make a strong floor with a small quantity of timber, the joists should be thin and deep; but a certain degree of thickness is necessary for the purpose of nailing the boards, and two inches is perhaps as thin as the joists ought to be made, though sometimes they are made thinner.

* ' Encyclopaedia Britannica,' art. Roof.

† 'Transactions, Society of Engineers,' 1863.

To find the depth of a joist when the length of bearing and breadth are given, the distance apart from middle to middle being 12 inches.

Rule. - Divide the square of the length in feet, by the breadth in inches; and the cube root of the quotient multiplied by 2 . 2 for fir, or 2 . 3 for oak, will give the depth in inches.*

Example. - Eequired the proper depth for a fir joist, the bearing being 12 feet and the breadth 2 inches?

12x12/2 = 72, the cube root of which is 4.16; therefore

4.16 x 2.2 = 9.152 inches, the depth required; or 9 1/4 inches nearly.

The scantling of single joists should be increased when they are used for the support of warehouse, factory, or other floors which have to sustain exceptionally heavy loads.

On account of flues, fire-places, and other causes, it often happens that the joists cannot have a bearing on the wall. In such cases a piece of timber, called a trimmer, is framed between two of the nearest joists that have a bearing on the wall. Into this trimmer the ends of the joints to be supported are mortised. This operation is called trimming. The scantlings of trimmers may be found by the same rule as those for binding joists (Case 2, Art. 210), the length of the joists framed into the trimmer being equivalent to the distance apart in binding joists.

* The constant numbers in this, and in all the rules for flooring and roofing, are derived from the scantlings of timbers that were found to be sufficiently strong; this the author considered to be the best method of obtaining those numbers, because it is difficult to calculate the weight that a floor has to support, yet it is easy to ascertain whether a floor be sufficiently stiff or not after it is executed. These comparisons have not been made from single observations, but from various ones on bearings of different lengths. The constant numbers are taken higher for oak, because the oak is seldom straight grained, and very subject to warp.

The two joists which support the trimmer are called trimming joists, and they should be stronger than the common joists. In general it will be sufficient to add one-eighth of an. inch to the thickness of a trimming joist for each joist supported by the trimmer. Thus, if the thickness of the common joists be 2 inches, and a trimmer supports four joists, then add four-eighths, or half an inch; that is, make the trimming joists each 2 1/2 inches in thickness.

When the bearing exceeds 8 feet, single joisting should be strutted between the joists to prevent them turning or twisting sideways, and also to stiffen the floor; when the bearing exceeds 12 feet, two rows of struts will be necessary; and so on, adding another row of struts for each increase of 4 feet in bearing. These struts should be in a continued line across the floor; short ends of boards, put in moderately tight, and nearly of the depth of the joists, are sufficient; such pieces simply nailed are better than keys mortised into the joists, because they require less labour, and do not weaken the joists with mortises. The best method of strutting is that shown in Fig. 60, which is called "Herringbone Strutting." The pieces are usually about 2 inches square, and arc spiked at the ends to the joists; struts of this description do not become loose in case of the shrinking of the joists. The well fitting of the struts is an essential part in making a good ceiling. For common purposes single joisting may be used to any extent where timber can be obtained deep enough; but where it is desirable to have a perfect ceiling, the bearing should not exceed 12 feet.

Fig. 60.

Where it is desirable to prevent the passage of sound, a framed floor is necessary; but in a single-joisted floor it may be reduced by putting strips of list or thin slices of cork between the upper edge of the joists and the floor boards.