214. Ceiling joists require to be no thicker than is necessary to nail the laths to; and 2 inches is quite sufficient for that purpose.

To find the depth of a ceiling joist, when the length of bearing and breadth are given.

Rule. - Divide the length in feet by the cube root of the breadth in inches; and multiply the quotient by 0 . 64 for fir, or by 0.67 for oak, which will give the depth in inches required.

Example. - Let the bearing be 6 feet, and the breadth 2 inches; to find the depth of a ceiling joist of fir.

The cube root of 2 is nearly 1.26; and the length, 6 feet, divided by this number, that is, ,. 6/1.26= 4.76; which being multiplied by the decimal 0 . 64, gives 3 inches, the depth required.

215. If two inches be fixed upon for the breadth, the rule for ceiling joists of fir becomes very easy; for then half the length in feet is the depth in inches: that is, if the length of bearing be 10 feet, the depth of the joist should be 5 inches. The distance apart in the clear is generally from 10 to 12 inches, according to the length of the laths.

It is better to notch ceiling joists to the under-side of the binding joists, and nail them, than to mortise and chase them in; because less labour is required, the binding joists are not weakened, and the ceiling stands better. Oak is not so good a material for ceiling joists as fir, because it is more liable to warp; particularly if not well seasoned.

* ' Transactions of the Society of Arts,' vol. xii., p. 2G5.