This section is from the book "Cyclopedia Of Architecture, Carpentry, And Building", by James C. et al. Also available from Amazon: Cyclopedia Of Architecture, Carpentry And Building.
Joists are the light pieces which make up the body of the floor frame and to which the flooring is nailed. They are almost always made of spruce, although other woods may be used, and may be found more economical in some localities. They are usually 2 or 3 inches thick, but the depth is varied to suit the conditions. Joists as small as 2X6 inches are sometimes used in very light buildings, but these are too small for any floor. They may sometimes be used for a ceiling where there are no rooms above, and, therefore, no weight on the floor. A very common size for joists is 2X8 inches, and these are probably large enough for any ordinary construction, but joists 2X10 inches make a stiffer floor, and are used in all the best work. Occasionally joists as large as 2X12 inches are used, especially in large city houses, and they make a very stiff floor, but this size is unusual. If a joist deeper than 12 inches is used, the thickness should be increased to 2 1/2 or 3 inches, in order to prevent it from failing by buckling, as explained for girders, P. 95. The size of the joists depends in general upon the span and the spacing. The usual spacing is 16 or 20 inches between centers, and 16 inches makes a better spacing than 20 inches, because the joists can then be placed close against the studding in the outside walls and spiked to this studding. It is generally better to use light joists spaced 16 inches on centers than to use heavier ones spaced 20 inches on centers. The spacing is seldom less than 16 inches and should never be more than 20 inches.