Gravity never goes on vacation which makes a houses’ ceiling joists the most important part of the ceiling. Load from a typical pitched roof will put outward pressure on a structure’s walls and over time cause the walls to lean out. A well designed ceiling prevents this and should be made from quality wood to stand the test of time and adequately bear the load of sheetrock or other material used to enclose the space.
The load bearing capacity of the ceiling joist has got to be taken into consideration during the planning stage because it must be able to handle the weight of the ceiling material with ease. As an example, if heavy 3/4″ plywood is going to be used for the ceiling then the joists have got to be strong enough not to bow over time versus 1/2″ sheetrock which weighs significantly less.
Moving on from the joist strength it is also important to note that the sizing of the ceiling joist directly correlates to the ceiling dimensions that are going to be used. As a case in point, the joists that will be used when it comes to putting up a large 20 x 20 ceiling in a garage will be significantly larger dimensionally when compared to those that will be used in and average bedroom due to the load the ceiling will be expected to carry over a given space. You would not want to use 2×6’s to frame a large garage ceiling out, where as they may be fine in a bedroom.
Finally, when it comes to spacing there are two main options that are available and these are the 16 inch and the 24 inch spacing. The main consideration to be made when it comes to selecting the right spacing is the amount of space to be spanned and your local codes.
Framing the ceiling is relatively straight forward. Joist should be cut to the same length as the outside edge of the top plates if possible on each side. If the roof line interferes with the top corner of the joist it may be necessary to trim that corner to match so it does not protrude out. You should plan on tying in the ceiling joist and roof rafters for added stability. You may need to install stub joist along the sides of the ceiling that run parallel with the joist themselves to give support to those walls. Bridging may also be necessary to stabilize the joist so they stay vertical. Lastly, if this is an attic space you will want to install strong backs ( or rat runs) which are typically 2x6s nailed perpendicular to the joist from one end to the other. One is nailed lying down and the second is nailed to it on its side resting on the joist. These creates additional support and stabilizes the ceiling.
In short, there are several things that ought to be considered with regards to framing ceilings, but the two most important considerations are the area to be covered and the weight the ceiling is expected to bear.